Notes to VRA Standard

Division B - Standards

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Notes to Standard for Vegetated Roof Assemblies

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(Notes are explanatory and non-binding, each provided to support the requirements, guiding principles and recommendations of the Standard.)

Notes to Part 1

A-1.1.1.7.(2) (Responsibility for Design)

Vegetated roof system projects are complex and draw upon many areas of expertise. They require careful coordination of various disciplines and trades, during both the design and construction phases. The person coordinating the design must know and understand all the elements that make a vegetated roofing project a success, or must delegate the responsibility for design to a qualified individual or team. At a minimum, such a person should possess a Green Roof Professional designation or be a Qualified Professional with experience in designing and formulating media for landscape over structures” as defined by the Canadian Landscape Standard, Section 5.2.2, “Properties of Final Over Structure and Green Roof Growing Media Blend”. The elements of a vegetated roofing project include (without limitation)

• structural design, in relation to all loads
• hydrology
• plant science
• climate science
• maintenance requirements
• roof assembly design and materials

A-1.1.1.8. Project Pre-Design It is critical for the success of a vegetated roof system to include installation and maintenance professionals in the pre-design phase of any roofing project. During this phase, project objectives are established and decisions made about the appropriateness of a given design for a particular building in its specific location.

Traditionally, vegetated roofing has been relegated to the specification division for landscaping, and even eliminated from the design when project costs exceed budgets. By integrating vegetated roofing within the scope for roofing and tying its design and construction to other building systems and elements (mechanical systems, and photovoltaics, for example), the vegetated roof system becomes an integral part of the building and may be necessary for operational and aesthetic design parameters.

Using the Critical Path Method (CPM) for designing the vegetated roof system ensures construction tasks are properly considered and sequenced. Working with a green roof design professional (Green Roof Professional is a designation issued by Green Roofs for Healthy Cities), the Design Authority leverages the CPM to ensure that the vegetated roof system design incorporates the necessary elements and planning, including coordination with the roofing contractor, the general contractor, crane and hoisting sub-trades, quality assurance, plant establishment, and supportive maintenance. Using a thoroughly scheduled project, the VRS builder can project order times for plants and build in capacity for unforeseeable delays.

The necessity of organizing the details of a project ahead of time cannot be overstated. For example, derrick cranes that are typical in large Part 3 projects may be in place during most of the construction phase of a vegetated roof system and will subsequently be removed. Removal of the crane, however, will necessitate waterproofing of the penetrations left open by disassembled crane. The approach to these modifications should be planned in detail, in consultation with the green roof design professional, to ensure the viability of the vegetation is preserved and not compromised. Therefore, it is advisable to develop repair and reinstatement procedures before the work is required. This should be coordinated with the general contractor who is responsible for delivering the building to the owner.

Likewise, there may be details in a roof that will have to be dealt with out of sequence, and these may have a disruptive effect on the construction or maintenance of the vegetated roof system. Such disruptions should be anticipated and planned for as part of the project pre-design process.

A-1.1.3.1. Permitted Roof Assemblies When a building is newly constructed, it can be designed from the very beginning to support a vegetated roof assembly. This is not the case for existing buildings. In some instances, an existing building can be improved to make it suitable for a green roof, but often that simply isn’t possible and a green roof is inadvisable. It may be inadvisable because the existing building lacks the structural capacity to resist the expected loads of the green roof. What is more, because a green roof is intended to be wet (plants require water), constant humidity could adversely affect the integrity of other building materials or assemblies. Therefore, the Design Authority must undertake a thorough analysis of conditions and materials.

Assessing loads for existing buildings is challenging and must be undertaken with the utmost care. “It is not simply a matter of replacing certain loads (including that of stone ballast) by the weight of the substrate. Landscaping will change snow accumulation conditions on the roof, water deflections and seismic loads, and must be considered in the structural strength calculation. Existing structural elements, including beams, columns, and decking, will need to be reinforced to be able to support all the additional loads associated with the green roof. It should be noted that, in some cases, the addition of substrate could increase the heat transmission resistance of the existing roof, which could lead to a heavier accumulation of snow than that to which the roof was exposed before addition of the green roof” (Régie du logement du Québec (RBQ), “Critères techniques visant la construction de toits végétalisés”, 2015. P. 21).

If an existing building appears to be a suitable candidate for a green roof, the designer then has to contemplate whether or not to replace all or only some of the existing roof system. A green roof is a significant investment in the life of a building and is both costly and logistically challenging to remove and reinstall. Expect the vegetated roof system to last indefinitely, especially since it will be well-maintained. For that reason, the Standard will not permit a green roof over an existing, aging roof membrane. Ideally, a green roof should be designed for application over a completely new roof system. That said, sometimes it makes sense to leave or reuse serviceable materials, to reduce waste and to economize on roof replacement (for example, leaving dry, new-condition insulation in a conventionally insulated roof). Every design should consider the long-term consequences of these decisions, because even insulation panels age and may not achieve desired thermal performance or load resistance values. For limitations on re-used materials, see Article 4.2.1.1.

Partial replacement roofs present certain challenges to the designer. Because this Standard requires conventionally insulated roofs to be fully adhered, the designer must consider the strength of adhesive bonds between in situ materials, and their ability to withstand the expected wind loads during the service life of the roof, especially as climate continues to change and wind loads may increase. If those bonds are tenuous (a weak link in the “chain of connectivity”), new materials adhered to those left in place may not be able to resist wind loads. In that case, consider replacing all the materials rather than chancing the efficacy of the connections.

Recovered roofs are not permitted because recovering requires the mechanical fastening of a new cover board to which the new membrane will be affixed, and mechanical fasteners above the insulation overlay (cover board) are not permitted on any roof assembly that will support overburden (See the waterproofing roof system Standards in Division B, Article 9.1.4.1.). Water-shedding roof systems are not permissible because the weatherproofing beneath a vegetated roof system must be absolutely impervious to water (Ref. “Vegetative Roof Systems Manual”, Second Edition. National Roofing Contractors Association, 2009). Insulated roof systems are not permitted on slopes greater than 1:6 because vegetated roof systems installed on slopes must be engineered for anti-shear anchoring, and that anchoring is not compatible with an insulated roof system.

A-1.1.3.2. Permissible Vegetated Roof Systems Vegetated roof systems are classified in this Standard by one of three categories: (1) Extensive, (2) Semi-intensive, and (3) Intensive. The table below, adapted from the “Green Roof Installation and Maintenance Professional Resource Manual” (GRHC and RCABC, 2022), sets out the basic criteria of each. Note that while there are overlapping ranges in system depth, depth is only one criterion that distinguishes one system type from the others.

Extensive green roof systems are typically installed and maintained without the use of penetrating hand tools. They are commonly constructed on a shallow growing media support ranging from a minimum of 152.4 mm (6”) to a generally accepted maximum 203.2 mm (8”) in depth, consist of plants commonly referred to as “ornamental” or “succulents” that are selected from only a few genera (i.e., Sedum, Allium, Sempervivum, Euphorbia, and Delosperma), and are often constructed with pre-grown trays, boxes, or mats (sometimes referred to as ‘modular’). Irrigation is often used primarily to establish the plants, although these plants may require periodic irrigation during extremely dry periods. Plants in an extensive green roof are commonly well suited to coping with extreme weather conditions and have the ability to regenerate easily. Some of these plants may be native to the region for which the green roof system is designed, though indigeneity is not required or necessarily advisable, especially where plant roots may be aggressive and hazardous to the supporting roof assembly. Extensive green roof systems are generally self-sustaining and evolving, using plants that tend to conserve water (drought resistant) and are especially adapted to extreme site conditions (FLL 2018, p. 22). These plants typically require a relatively short period of time for establishment and therefore are suitable for establishing a vegetated profile that can eventually morph into a more varied system.

An extensive green roof is attractive to designers because of the speed with which the system can be constructed and established, and because extensive green roofs require the least maintenance of any vegetated roofing. However, maintenance is still required, particularly to control the inevitable growth of plant species that voluntarily take root in the green roof (See Note A-11.3.2.1.). Semi-intensive green roofs trend toward the more complex in their plant varieties, incorporating ground covers, grasses, perennials, and small shrubs. These plants tend to make lower demands on the soil, compared to an intensive system. Soil depth can vary but is typically limited to a range between 152.4 mm (6”) and 355.6 mm (14”) (Canadian Landscape Standard, Section 5). Maintenance costs vary depending on the types of plants selected, and how much care each species requires.

Semi-intensive green roofs can be designed either as a modular system or may be built in place. Modular systems generally are comprised of trays, mats, or blankets of pre-grown vegetation, installed over a design-specific arrangement of materials required to manage drainage, retain moisture, and protect the waterproofing membrane from both mechanical and root damage. Intensive green roof systems consist of perennials, grasses, flowering bulbs, shrubs, and even trees or lawns. Soil depth also varies but can be quite deep (necessary to support trees) and is usually greater than 355.6 mm (14”). Establishment may take several years, and because the mass of some plants, like trees, will increase substantially with maturity, the system demands on the growing media are significant. Maintenance requirements will be similar to those for gardens on the ground; costs reflect this level of complexity and depth (ASTM E2777-20, 3.2.18.1). Intensive care is required to ensure this type of vegetated roof system grows and sustains life (FLL 2018, p. 21).

There are advantages and disadvantages to each system type, and much has been published on this subject by various organizations, including the Landscape Development and Landscaping Research Society e. V. (FLL), Green Roofs for Healthy Cities, and the City of Vancouver (See Article 1.1.1.3.). Those are not addressed here. To read more about the efficacy of green roofs, refer to the “Information Centre” or consult the Bibliography, in Division E of the Roofing Practices Manual.

Table A-1.1.3.2. Characteristics of Vegetated Roof Systems, by Type

[Insert Table A-1.1.3.2.]

• Vegetation support refers to growing media which may be homogeneous or stratified, depending on overall depth (See Article 7.1.5.1.)
  • Maintenance costs will also be driven by roof accessibility and continuity or discontinuity between roof areas.

† Media saturation weights in Table A-1.1.3.2. are adapted from the Green Roof Installation and Maintenance Professional Resource Manual, developed jointly by the RCABC and Green Roofs for Healthy Cities, 2022. ††Plant diversity refers to genera and, within that, variety in species and habit. For example, an extensive green roof is typified by few genera. By contrast, an intensive green roof may be typified by a wide variety of genera, and within each genus, a wide range of species.

As stated in the Preface to this Standard, this document is “focused on the basics of design and constructability of green roofs as rainwater management tools, each design (specifically, those that are classified as semi-intensive or intensive) should aim to balance the limitations of vegetation on a roof with a diversity of plants, and their appropriate support (growing media), to mimic naturally biodiverse habitats.” A green roof that includes a diverse range of plant types (including sizes) provides necessary habitat for a diverse range of insects, birds, and other fauna, and may by virtue of its biodiversity reduce certain maintenance requirements by repelling undesirable plant species. Furthermore, some research shows that biodiversity may be a key strategy for moderating climate change; this can not only benefit those who own or occupy the building beneath it but may also add benefits to the wider community. For more on the subject of biodiversity, refer to the FLL Standard, p. 62, and resources published in the Living Architecture Monitor (including the article, “The Art and Science of Maintaining Biodiverse Green Roofs”, Summer 2022).

A-1.1.3.2.(1)(9) Roof environment, irrigation needs, and growing media depth The roof environment is more than just the climatic zone for the building (in British Columbia, there are six climate zones, ranging from 4 through 8). Wind exposure, sun exposure, and exposure to natural precipitation shape the needs and design of a vegetated roof system. For example, a roof at a high elevation may be susceptible to significant winds, but so may a vegetated roof at lower elevations, where the building is situated among other structures that induce wind tunneling. In densely constructed areas, adjacent buildings may shade a vegetated roof for many hours of a day. Shade, and exposure to sunlight, are critical design considerations for a green roof; the plant profile must reflect the expected hours of sun the roof will be exposed to. In addition, reflected light of adjacent building glazing may adversely affect vegetation. Expect higher than usual roof-level temperatures from reflected light, especially from mullions between glazing panes. For more on this, see the study on reflected light and its impacts on roof systems in Roofing BC, Vol. 18, Number 2.

[Insert Figure A-1.1.3.2.(1)(9)

Figure A-1.1.3.2.(1)(9) Climate Zones of British Columbia (Click to expand illustration) (https://www.betterhomesbc.ca/faqs/climate-zone/)

A-1.1.3.3. Establishment and Maintenance The establishment period of a vegetated roof system is generally defined the first full summer growing season following planting. This is the time when plant roots work their way into the soil and hold the plant in place, and foliage begins to develop. During this period, development care of the green roof is critical to its survival. This includes irrigation, removal of unwanted seedlings, and fertilization.

[Insert Table A-1.1.3.3.-1]

The table above provides a simplified, broad framework for understanding the concept of plant establishment, but proper establishment may take two years or longer, depending on the method of planting, the choice of plants, the type of vegetated roof system (modular systems for extensive green roofs that utilize pre-grown mats and trays may establish more quickly), and regional or even local climate and weather conditions (severe heat, for example, may slow growth in some plants that were selected for cool winters). As Table 1.1.3.3.-1 suggests, spring planting is optimal for early establishment. Table 1.1.3.3.-2 below provides a somewhat more detailed approach to the establishment period, but again, the period is subject to variables that cannot possibly be accounted for in the table.

[Insert Table A-1.1.3.3.-2]

• Modified from Table 5.7.4, “Green Roof Design and Installation, Vol. 2”, GRHC

Accessibility for maintenance is critical if the roof is to thrive. Because extensive vegetated roof assemblies generally require less maintenance than either intensive or semi-intensive assemblies, an access hatch is acceptable though not necessarily desirable. Maintenance work requires tools and inevitably involves the removal of vegetation cuttings, weeds, and debris from the roof, which can be made more difficult if roof access is restricted to a hatch. To ensure the survivability of the green roof, make access as easy as possible by designing access through a doorway or elevator.

A-1.1.3.4. Electronic Leak Detection Electronic Leak Detection (ELD) utilizes low-voltage electrical current, typically conducted through wires installed in a grid pattern. ELD technologies are used in response to a leak, to isolate its location in order to minimize investigation time and material removal. This can benefit a building owner who will have to bear the costs of demolition or overburden removal when the standard limits of coverage afforded by the RoofStar Guarantee are exceeded by the Project design and construction. ELD technologies may be passive (installed but not monitored) or actively monitored (by the installer, through real-time data collection).

A-1.1.4.3. Protection of Vegetated Assemblies Vegetation will thrive and grow if the conditions on a roof promote it, and that requires the careful coordination of design disciplines, including the structural team and those who are designing mechanical systems and glazing. By approaching the building design holistically, common issues can be worked out before the project is tendered. Too little roof slope can lead to poor drainage and saturated growing media, resulting in root rot (this can happen despite the use of drainage materials in the green roof system). By designing the roof with adequate slope for drainage (usually 2%, or 0.25” in 12”), water will move through the vegetated system and into the building’s storm water drainage system. Wind displaces seedlings and plugs and can adversely affect plant habit (shape). Protection from wind (perhaps with higher parapets, or with wind screens) will protect establishing plants and allow mature plants to attain the shape and height they were selected for (these strategies may also reduce suction wind loads). Pay attention to the structural mounting of screens; attaching them on the inside face of a parapet, or on the vertical face of curbs, reduces the possibility of leakage around bolts and may also deliver better resistance to lateral wind loads. Emissions from vents and fans can stunt plant growth by drying out the growing media, coating plants with frost (from moist exhaust air on sub-zero days), emitting hot air, or exhausting damaging gases (for example, sulphur dioxide). Consider isolating the exhaust fans with larger vegetated free zones, relocating them to other parts of the roof, or employing pavers to deflect exhaust fumes. Vegetation installed on slopes can lead to slumping (displacement when the slope exceeds the angle of repose (internal angle of friction) for bulk materials), and slopes can inadvertently promote erosion (by wind and water). Design slopes with a view to minimizing slumping and, where anti-shear devices are necessary to hold materials in place, design the roof assembly together with a green roof professional to ensure devices are anchored to the structure and properly waterproofed (See Article 3.2.1.1. and Article 3.3.4.1.) Reflected light from glazing and (especially) glazing mullions can severely damage (burn) vegetation (see the study funded by the RCABC and published in Volume 18, No. 2 of Roofing BC, “The Impact of Solar Reflectivity of Glazing Adjacent Roofs”). Employ wider (deeper) vegetation free zones, or leverage other building strategies such as awnings, sun shades, or light shelfs, to shield glazing and mullions from direct exposure to the sun. Finally, be cognizant of the risks posed by human access to the green roof. While the vegetated roof may be largely inaccessible to the public or building occupants, the vegetated roof system may be traversed by service personnel to reach mechanical fans, makeup air units, or air conditioners; often, service calls require repeated trips and will involve water hoses and boxes of filters that may be dragged across the roof. To prevent damage from walking and abrasion, consider locating all serviceable mechanical units in a dedicated area (they can even be visually isolated from the rest of the roof with a solid or vented screen), and make them accessible by a clear pathway (perhaps even with guardrails to prevent the curious from wandering across vegetation).

Figure A-1.1.4.3. Exhaust air literally scoured away the vegetated roof system, including the growing media. A wider vegetation free zone meeting the minimum requirements of the Standard would have prevented this damage. (Click to expand illustration) (RCABC Photo, Image Library)

Notes to Part 2

A-2.1.3.1. General Requirements for Roof Slope The subject of slope and drainage is largely governed by the Standard having jurisdiction over the supporting roof assembly. For more about slope and drainage requirements applicable to those assemblies, see Article 2.1.3.1. in the Standard for the supporting roof assembly, together with the accompanying note, A-2.1.3.1.(4) (Sufficient Slope). Achieving a minimum slope (fall) of 2 per cent is critical for vegetation thrive, especially for extensive vegetated systems. Unintended water accumulation can lead to vegetation die-off. Where the slope is marginal (expect variations in the sloped plane of the exposed membrane, especially when the roof is conventionally insulated), plan for plant failures and the establishment of volunteer vegetation (i.e., woody seedlings). Standing water will not necessarily be mitigated by thicker drainage layers. Sufficient slope is perhaps the most reliable method for breaking the capillary action of water to ensure it flows beneath the green roof system toward the building’s plumbed drainage system. In some instances, ponding from irrigation may be desirable (for example, on some intensive systems). In those instances, the roof system manufacturer must provide written consent, as ponding may adversely affect the membrane (See Article 1.1.1.9. and Article 2.1.3.1.). As slope increases, so does the rate of drainage. Above 5° (approximately 8.8%) slope, the vegetated roof system should incorporate a “layer structure with higher water storage capacity and lower drainage capacity”, or utilize plants with lower water requirements. Additionally, as slope increases, the green roof design must incorporate measures to prevent material displacement (adapted from FLL 2018, p. 29).

A-2.1.4.1. Structural Loads When considering structural loads, it is prudent to consider worst case scenarios where drainage systems fail and the roof cannot drain. Therefore, consider the possibility that roof drains may become blocked and that in a large rain event the vegetated roof system will become saturated and flooded to the level of peripheral overflows. If overflows are not part of the roof assembly design, consider adding them even when parapet heights do not exceed 150 mm (See Article 2.4.10.4., Division B, Book II, British Columbia Plumbing Code).

Live loads must include all loads related to the green roof, including the weight of equipment used to build the green roof, staging of materials during construction, snow loads resulting from snow captured by vegetation, water intentionally captured by and detained in growing media and other roof system components, and the weight of mature vegetation.

ASTM E2777 (“Standard Guide for Vegetative (Green) Roof Systems”) addresses live load concerns when vegetated roofs are subject to freeze-thaw cycling (5.3.13). Winter phenomena like this must be considered when designing the load capacities of the roof structure; water that cannot drain off the roof because it is captured as ice may excessively load a roof beyond what might otherwise be considered normal in milder climate zones. Therefore, the design team should review the entire context for a green roof inasmuch as climate and weather may impact the building’s structural limitations.

A vegetated roof system, unlike the waterproofing roof system beneath it, changes in mass over time because of vegetation growth. This is particularly pronounced in semi-intensive and (especially) intensive green roofs; trees may be planted as relatively light-weight saplings, but at maturity may weigh thousands of kilograms. Furthermore, other biomass may accumulate on the roof (layers of thatch, for example, that are left either out of neglect or intentionally, to contribute to habitat for birds and insects. Point loading of mature elements, such as trees, must also be considered. Their loads will be distributed over smaller areas of the roof system and may have unintended consequences for insulation that is unable to support them. In addition, mature trees may introduce imbalanced (asymmetrical) loads on the roof, which both the supporting roof assembly and the building’s structural design must accommodate.

The effects of wind on taller vegetation may also stress the supporting roof assembly membrane differently than, for example, a low-profile extensive green roof. The potential for shear loads from wind or seismic events, and suction loads from wind uplift, must all be accounted for. In short, design the underlying roof system to support the fully mature state of the green roof system, under all expected conditions.

A-2.1.5.1. Suitability of Roof Deck The roof membrane, when installed to the applicable RGC Standard, will repel water and keep the structural roof deck dry, but if the membrane is damaged and water leaks through undetected, untreated wood decks exposed to water for a prolonged period may succumb to decay, which can compromise deck integrity. To protect against accidental leakage, wood decks and adjoining wall or curb surfaces should be constructed with pressure-treated plywood. Many membrane roof assemblies tested on plywood stipulate a minimum deck thickness of 15.88 mm (5/8”) (See Article 3.1.3.3. on the subject of Tested Assemblies). When the existing wood deck is untreated, it should be covered over with a layer of treated tongue-and-groove plywood. To ensure that water does not leak through the joints, tape the joints with a self-adhered material that is compatible with, and will adhere to, the treated plywood.

Properly securing decking or a deck overlay will require the help of a registered professional engineer, who will also ensure that fasteners will be compatible with the plywood treatment. Deck overlays form part of the roof assembly and so their securement is typically governed by what has been tested.

For more about pressure-treated plywood, refer to the Canadian Wood Council. For a helpful guide on fastener compatibility, see the BC Housing Builder Insight document, “Compatibility of Fasteners and Connectors with Residential Pressure Treated Wood”.

Notes to Part 3

Note A-3.1.3.3. Resistance to Specified Wind Loads Wind resistance of a vegetated roof assembly should be patterned after the design approach for the supporting roof assembly. In other words, expect that wind loads will exert the highest negative pressures in the corner zone, followed by the edge zone and (lastly) the field zone. When possible, select a vegetated roof system that has been tested using the test method CSA-A123.24. When a tested assembly is not available, wind resistance may be determined using ANSI/SPRI RP-14, “Wind Design Standard for Vegetative Roofing Systems”, but this work must be performed or at least reviewed and stamped by a registered professional, to demonstrate compliance with the Building Code and this Standard.

Consider specifying taller parapets, which introduce turbulence and consequently lower suction loads on the roof surface in the corners and at the edges. Parapets that are sufficiently tall can also serve as fall protection guards where a roof is accessible and will support the requirement in this Standard for accessibility to maintenance (See Article 1.1.3.3.; also refer to the British Columbia Building Code, Div. B, Part 3, and to the WorkSafeBC Regulations).

Taller parapets will also ensure that when loose stone is used for separation zones at the roof edge, wind will not ‘scour’ it from the roof surface and propel the stone off the roof, where it can become a public safety hazard.

Notes to Part 4

Notes to Part 5

A-5.1.3.3. Separation of Vegetation from Roof Details Separation zones should be designed to follow the latest edition of general rules and guidelines published by the Landscape Development and Landscaping Research Society e. V. (FLL), “Green Roof Guidelines: Guidelines for the Planning, Construction and Maintenance of Green Roofs”. The FLL refers to separation zones as “marginal strips” (8.6.6.2, FLL 2018). Although FLL allows that some marginal strips may be “mostly vegetation-free”, this Standard requires that separation zones be completely vegetation-free. These zones protect vulnerable membrane detailing from plants, permit simplified inspection of roofing details, and allow for ease of maintenance, particularly at drains.

Although some vegetated roof systems pose a low root threat to the roof membrane, invasive weeds are a threat, and when the roof is left unmaintained, vegetated free zones protect vulnerable details from possible damage by unmitigated weeds.

Figure A-5.1.3.3. - 1 Maintained separation zones Forming Part of Note A-5.1.3.3. (Click to expand illustration) (RCABC Photo, Image Library)

Figure A-5.1.3.3. - 2 Unmaintained separation zones Forming Part of Note A-5.1.3.3. (Click to expand illustration) (RCABC Photo, Image Library)

  Figure A-5.2.1.2. - 1 Root damage to membrane around penetration (Click to expand illustration) (RCABC Photo, Image Library)

Figure A-5.2.1.2. - 2 Aggressive penetration of roof membrane by roots (Click to expand illustration) (RCABC Photo, Image Library)

A-5.2.1.2. Root Barriers Some roof membranes are particularly vulnerable to root penetration and require a separate root barrier that is installed above the membrane. However, some sheet and liquid-applied membranes are manufactured to resist penetration by roots. Use of these membranes without a separate, additional root barrier is acceptable in Article 5.2.1.2. provided the membrane meets the requirements of the Article and the roof system manufacturer has expressly consented to this. Expressed consent may be obtained through a letter from the roof system manufacturer’s Technical Department manager, or through the roof system manufacturer’s published technical information about the membrane.

A-5.2.1.3.(1)(6) Edge Curbing Height Edge curbing should terminate flush with the top of the bulk material it contains. If the curbing is taller, it may present a tripping hazard for maintenance workers. It may also become distorted over time, and its exposure to the elements may result in movement of the curbing due to thermocycling.

Notes to Part 6

A-6.1.3.1. Drainage (See related Note A-2.1.3.1.) Managing water drainage is critical to the success of a vegetated roof system. If water cannot adequately drain out of every area of the system, plants growth will be stunted, moulds will flourish, and plantings will die back. Incorporating the 2018 FLL Green Roof Guidelines, drainage should be designed around several criteria: 1) The entire area of a green roof must be drained. This includes boundary areas; where edging materials are employed, they should not inhibit the free flow of water to drainage outlets located in separation zones, including overflows. 2) Distinct areas of vegetation must be drained with separate systems for drainage. 3) Drainage calculations must be made to ensure that drain and gutter sizes are adequate to manage the anticipated flow. 4) Where membrane gutters are employed, they must be designed according to the requirements in the applicable Standard for the supporting roof assembly published in Division B of the RCABC Roofing Practices Manual. 5) Drains must be located in serviceable areas and must be protected with separation zones and ballast guards or perforated inspection covers.

The Standard permits (with consent from the vegetated roof system manufacturer) the use of flow control drains because they may retain water on the roof unnecessarily and may in fact leave the growing media in a state of permanent saturation, which will both compromise the media’s capacity for ongoing stormwater management and may also result in damage to materials that are in continuous contact with water (RBQ, p. 13).

To prevent oversaturation, overflow drains are required. Their placement within the roof perimeter is critical; whereas the British Columbia Plumbing Code (Book II) does not provide direction on where to locate overflows in relation to the drainage plane, the RGC Standard requires that they be no higher than 101.6 mm (4”) above the drainage plane. Since the drainage plane for a conventionally insulated roof may be different than that for an ‘inverted’ roof assembly, the Design Authority must give this proper consideration when designing the entire roof drainage system. For more on drainage for the waterproofing (membrane) roof assembly, refer to Part 11 (Drainage) in the related RGC Standard.

For more about RGC requirements for roof drains and overflows, see Part 11 of the Standard for the supporting roof assembly.

A-6.2.1.2. Drainage Layers “Drain layers may be simple, consisting of a single component, or complex, combining multiple components including geosynthetics, Geocomposite, and coarse mineral aggregate” (ASTM E2777-20, 3.2.6.1 Discussion). The type of material component or system that is suitable for a given design will depend on the dynamic interplay of climate behaviour, drainage slope, growing media composition, and plant characteristics.

Geocomposite drain layers may include absorptive mats which function primarily to drain water but also retain water for plant absorption. They may also incorporate reservoirs on their upper surface, to retain water.

Granular drainage media (coarse aggregate) may be used directly above a reservoir-designed Geocomposite drainage layer to promote the free drainage of the vegetated roof system (ASTM E2777-20, 3.2.14).

A-6.2.1.3.(1)(10) Filtration fabric and resistance to flotation In a protected membrane roof assembly, where the insulation is located above the waterproofing materials, filtration fabric plays a dual role. First, it keeps fine materials away from the drainage plane where they may plug up the dedicated drainage course or silt up the waterproofing membrane. Secondly, they keep insulation boards from becoming displaced by foot traffic or by water, which could accumulate on the waterproofing surface. Extruded polystyrene insulation is highly buoyant, and although the accumulation of water on the roof membrane in a protected roof assembly is rare (largely because much of the water runs above the insulation toward the roof drains), the Standard contemplates those rare occasions when drainage layers may become blocked, or severe rainfall may simply overwhelm the primary drains. To retain the overlapped and staggered configuration of the insulation panels and prevent panels from subducting each other, filtration fabric is stretched across the insulation assembly and held in place, usually by tucking it behind perimeter metal flashings.

Although properly installed filtration fabric provides some protection against insulation displacement, it is really secondary to the protection from overflow drains. For more about drainage requirements and the design and placement of overflow drains, read the requirements in Part 11 of the Standard for the supporting roof assembly. For more about the securement of protected roof assemblies, see also Note A-3.1.6.1. (Securement of Ballasted Roof Systems) in the Standard for the supporting roof assembly.

Notes to Part 7

A-7.2.1.1. General Requirements for Growing Media The only blend of growing media that is permissible under this Standard is the media specified by the vegetated roof system manufacturer, unless the Standard permits customization.

A-7.1.5.1. General Requirements for Growing Media From the 2018 edition of the German standard, FLL: “when using a single layer construction for intensive green roofs it is necessary to consider the lower water and nutrient storage capacity of the substrate. Either plants that have lower water and nutrient requirements are to be used or extra maintenance procedures are to be planned to ensure the needs of the vegetation are met.”

Notes to Part 8

A-8.1.3.2. General Requirements for Irrigation Carefully matching irrigation needs to both the growing media composition and local climatic conditions cannot be overemphasised. According to ASTM E2777-20, “The introduction of irrigation in arid and semi-arid climates may...promote biodegradation. Once settled on a geotextile, organic fines may decompose and promote the development of a bioslime and impede drainage.”

Notes to Part 9

Notes to Part 10

A-10.1.4.1.(3) Root Barriers and Drainage Mat inside planters As planter depth increases, so does the potential for hydrostatic pressure on the inside face of planter walls, especially when the growing media is saturated after a heavy rain. Because concrete is prone to crack, hydrostatic pressure against a planter wall that is bare concrete can drive solutes out of the concrete, forming efflorescence on the exterior face. Even when roofing membrane is carried up the full height of the inside planters, hydrostatic pressure can prove detrimental to waterproofing details. For these reasons, and because separation zones are not required inside planters, it is necessary to carry both the root barrier and drainage mat materials all the way up to the finished surface of growing media, to protect the planter from damage.

A-10.1.4.1.(5) Required Overflows Because planters are restricted in size and often are deep, heavy rains (1 in 50-year events, for example) can rapidly oversaturate the growing media and overwhelm the drainage mat and any mechanical drains. As a result, overflow drains are required, to relieve the planter of excess volume.

A-10.3.2.1. Coordination of Work Installation of the first drainage course, insulation, and filtration material is usually the work of the Contractor, but that is realistic when the roof is completed with ballast (which the Contractor also installs). However, when the project will qualify for a RoofStar Vegetated Roof Assembly Guarantee, a seamless shift from waterproofing to green roof installation is critical. Insulation and its associated materials cannot be left exposed and unballasted. Therefore, it is permissible (since the qualified green roof installer is a member of the RCABC and under contract to the roofing contractor) for this aspect of the roofing contractor’s work to be undertaken by the qualified green roof installer, provided the work normal to the Contractor is directly supervised by a journeyperson-certified established employee of the Contractor to ensure the installation complies with the Standard.

Water supply to the roof under construction is essential for proper establishment of the planted vegetation. If water is not provided to the roof before construction, the Guarantee may be amended with restrictions on coverage for the vegetation.

The roofing contractor (Contractor) is responsible for the proper execution of the entire project because the quality assurance RoofStar Guarantee certificate is furnished by the Guarantor through the Contractor.

A-10.3.2.2. Protection of Installed Roofing Materials Protecting the waterproofing roof system is, of course, paramount to maintaining the watertightness of the roofing, but it is also necessary to protect other vulnerable materials from damage. For example, on large green roof systems, a network of perforated drain piping may be needed to aid in draining the system. When this is done, the pipes are laid inside or immediately below the growing media. However, the pipes are then vulnerable to crushing, especially from heavy equipment used to move growing media around, or for loading heavy root-balled trees onto the assembly. If pipes are crushed during the initial construction phase, the pipes must be replaced before vegetation is planted.

A-10.3.2.6. Installation of Drainage Courses and Water Retention Layers The use of drainage pipes to assist in moving large volumes of water to roof drains is not common and is usually specified when particularly heavy rainfall conditions are common. This type of drainage system can be designed with either solid or perforated pipes. When drainage pipes are installed as part of the drainage system for the vegetated roof system, they must be connected together, shall be leak-free, and shall be terminated at or near roof drains in a separation zone using a screen suitable to prevent blockage by the gravel. In addition, drain pipes should 1) be kept clear on the inside of construction debris, 2) be installed with adequate clearances (between pipes, or between a pipe and a curb, wall, or other obstruction), 3) utilize only fittings approved by the pipe manufacturer, and 4) be covered with enough growing media to distribute compressive loads by any heavy equipment or other superimposed materials.

A-10.3.2.16. Planting Shrubs and Trees Securing shrubs and trees against the forces of wind and gravity is critical, but bracing must be attached at points well away from where water will drain. This could be on top of a curb or on a wall, or with frames that are ballasted and superimposed on the green roof assembly. Structural connection for bracing must be located outside of the plane of drainage because any movement of the structural connection point may compromise the waterproofing (movement can occur for any number of reasons – expansion and contraction due to heat or cold, building movement or settling, or because of seismic activity).

Notes to Part 11

A-11.3.2.1. General Requirements for Care and Maintenance Growing media must be tested annually, and its pH amended as needed to maintain a pH range between 6.5 and 7.5. The upper end of this range will allow for the gradual acidification of the media from acidic precipitation, but the principle of pH balance in the growing media remains the same: growing media with a pH between 6.5 and 7.5 will promote the absorption of micronutrients by vegetation. Media pH can be amended as pH declines, using additives that are appropriate for the green roof design.

Figure A-11.3.2.1. – 1 Absence of maintenance (RCABC Photo, Image Library)  

Figure A-11.3.2.1. - 2 Absence of maintenance (RCABC Photo, Image Library)

Over time, the media must be amended with any organic nutrients, perhaps in the form of compost teas, to promote plant growth without adding unnecessary organic matter that can adversely affect the engineered structure of the soil. Where the vegetation was pre-grown when supplied for construction, the grower should be consulted for advice on appropriate amendments. Proper irrigation is critical and should be scheduled to meet the specific water needs of the established vegetation system. Shrinkage, die-back, or browning of species like Sedum album will indicate that the green roof may require irrigation; this should be referred to the vegetated roof system manufacturer for comment and instructions. Others may go dormant and shrink in size if left unirrigated during hot, dry weather. This may leave soil exposed and vulnerable to volunteer plant species (weeds) that compete for nutrients. Consequently, a weedy green roof will require additional maintenance to remove unwanted plants. Irrigation volumes should be planned around the requirements or recommendations from the grower but in the absence of any prescribed levels, irrigation should be considered sufficient when water can no longer be absorbed by the vegetated roof system.

Avoid commercially produced pre-emergent chemical herbicides, as they can damage roof assembly components and adhesives, and or destroy desirable organisms within the growing media that are necessary for the biodiversity of the green roof system. Furthermore, herbicides will contaminate runoff, which may be collected within the building for treatment or non-potable uses. Woody plants should be pruned seasonally in keeping with best practices, and should in any event promote an open, natural plant habit.

Notes to Part 12

Notes to Part 13

Notes to Part 14

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