Roof Decks

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Roof Decks

NOTE TO READER: Where applicable, the content of this page is replicated in the RoofStar Guarantee Standards sections of this Manual

1 General

The roof deck must be designed to serve two major functions:

  • As a structural component to transfer the weight of live and dead loads to supporting members. Live loads include construction equipment and workers, rain, snow, and ice. Dead loads include the deck itself, the roofing system, HVAC units and, possibly, landscaping. Most decks must also act as a diaphragm, transmitting wind or seismic lateral forces to the buildings structural framework.
  • As a suitable substrate to which the roof membrane or roof system is attached.


Prior to the application of the roof system, the deck must be smooth, straight, and free of "humps" or depressions. These can counteract any sloped insulation systems installed by the roofing contractor and cause ponding. On new construction projects, this is usually provided by the general contractor with review by the design authority and/or the building authority having jurisdiction.

In addition, the roof deck must comply with several guiding principles, to be acceptable for a RoofStar Guarantee:

  • For low slope (membrane roof or waterproofing systems) it should provide positive slope to drain (required for a RoofStar 5-Year Roofing Guarantee, RoofStar 10-Year Roofing Guarantee, or for a RoofStar 5-Year Waterproofing Guarantee for new construction and for organic felt membrane systems when re-roofing). Positive slope to drain is attained when no standing water remains on the deck within a reasonable amount of time after rainfall stops during days with conditions that permit evaporation. Where required, crickets and saddles should be provided to assist drainage. Slope can be provided by sloping the deck, or using tapered insulation boards or insulating fill (insulating fill is rarely used in B.C.). Tapered slope insulation products can only be used to provide slope if any differences from level in the roof deck are less than the percentage of slope provided by the insulation. A minimum slope of 1:50 (1/4" in 12") is strongly recommended. The exception is often drain wells that are designed to collect water and provide a flat surface for the installation of roof drains. Therefore, standing water may remain in drain wells for a longer period of time after a rainfall stops.
  • It should be dimensionally stable and capable of accommodating roof system component movement.
  • Deck deflections should be limited to 1/240 of the total span, but must conform to the BC Building Code (latest edition). For plywood decks, the RoofStar Guarantee Program limits the allowable deck deflection to 1/360 of the total span to prevent differential edge movement. Concentrated, non-uniform construction loads should be considered during design. Drains should be located at points of maximum deflection whenever possible, not over columns or bearing walls. Building settlement can often cause slopes to change and low spots to appear on the roof surface.

Prior to the commencement of roofing, any curbs, cants and blocking that are to be installed by others, should be in place.


2 Wood Decks

Wood is a common construction material that has been used for many years because of its economy, ease of fabrication, lighter construction, and ready availability. Acceptable wood roof decks may include wood board decks, plywood decks, and, subject to restrictions, non-veneered wood decks (oriented strand board, waferboard, etc.). All types of wood decks should be roofed promptly after installation.

All wood decks shall be properly fastened to the BC Building Code requirements (latest edition). With the exception of standard insulated roof systems, all knots or cracks shall be covered with metal prior to acceptance of deck surface.This work is to be done by others. Non-veneer (O.S.B. or wafer board) panels are acceptable sheathing for decks with low-slope roofing applications, provided the roofing membrane is not fully adhered directly to the panels. The minimum plywood or non-veneer (O.S.B or wafer board) deck thickness for low-slope roofing (i.e., membrane roofing) shall be a minimum 12.7 mm (1/2")and as indicated in the applicable the BC Building Code (latest edition).

All types of wood decks should be roofed promptly after installation.

Differential edge movements or deflection in excess of1/360 of the span in plywood or non-veneer decks in low slope roofing must be prevented by:

  • solid blocking under non-supported edges, and /or
  • use of tongue-and-groove plywood with non-grooved edges supported by joists or solid blocking, and / or
  • when rigid insulation is over the deck, properly sized and installed H-clips must be used on the unsupported edges.

2.1 Wood Board Decks

Wood board decks include tongue-and-groove, shiplapped, or splined boards or planks usually ranging from 19 mm to 89 mm (nominal 1" to 4") thickness. Also 38 mm (nominal 2") by various width boards, placed on end, can be spiked together to form a “mill deck”. The thickness of the boards is determined by the anticipated loads and spacing of roof joists or trusses (consult the BC Building Code - latest edition).

Wood board decks should be of sound seasoned lumber, properly secured to supporting structure. The deck should form a firm smooth surface, free from knotholes, cracks, projections, depressions and other defects. Prior to the roofing contractor's acceptance of the deck, minor knotholes and cracks should be covered with securely nailed sheet metal (performed by others).

Wood board decks, especially mill decks, often have so many surface defects that a plywood overlay is desirable (9.5 mm (3/8") plywood is often acceptable). A plywood overlay is required for asphalt shingle roofs. Where a plywood overlay is not required, a mechanically fastened base sheet or separator sheet is required for built-up roofs or fully-adhered bituminous or modified bituminous flexible roofing membranes. For non-bituminous flexible membranes (elastomeric or plastomeric membranes such as EPDM or PVC), a slip sheet or separator sheet should be installed according to membrane manufacturer's recommendations. Separation sheets, slip sheets, and mechanically fastened base sheets are installed to help relieve stresses in the membrane due to movement of the deck.

2.2 Plywood Decks

Plywood roof decks consist of exterior type plywood mechanically fastened to the roof framing. The plywood panels should conform to CSA 0121, “Douglas Fir Plywood”, CSA 0151, “Canadian Softwood Plywood”, or CSA 0153, “Poplar Plywood”, as per the BC Building Code (latest edition) requirements.

For steep slope roofing (e.g. asphalt shingles), plywood thickness must - as a minimum - conform to the BC Building Code (latest edition); however, the following minimum plywood thickness should be considered:

  • 12.7 mm (1/2") for 400 mm (16") o/c supports
  • 15.9 mm (5/8") for 600 mm (24") o/c supports. Note: Required for asphalt shingle applications when Z-Bars (girts) are used as the attachment method.

These thickness / span ratios will minimize deck deflection, especially with construction loads, and will provide a better surface for nailing as the bouncing effect of thin plywood’s is reduced.

For low slope roofing (e.g. BUR), plywood must be minimum 12.7 mm (1/2") thick unless mechanically attached rigid insulation is installed prior to the roofing membrane. The minimum centres for supporting members are:

  • 400 mm (16") for 12.7 mm (1/2") plywood sheathing
  • 600 mm (24") for 15.9 mm (5/8") plywood sheathing

For low-slope roofing, differential edge movement and deflection in excess of 1/360 of the span must be prevented by:

  • solid blocking under non-supported edges, and / or
  • use of tongue-and-groove plywood with non-grooved edges supported by joists or solid blocking, and / or
  • when rigid insulation is over the deck, properly sized and installed H-clips should be used on the unsupported edges.

Plywood roof sheathing should be installed with the surface grain at right angles to the roof framing, with end joints staggered.

Plywood decks receiving roofing directly applied to the deck should be attached using wood screws or nails which are especially resistant to pull-out or nail popping, particularly when using a single-ply membrane (i.e., use ring-type or ardox; smooth common nails are not acceptable). Nail popping is usually caused by a combination of thermal and structural movement and is especially critical where the membrane is directly adhered to the deck. All fasteners should have a corrosion-resistant coating (such as galvanizing). The structural suitability of the fastener is the responsibility of the design authority.

2.3 Non-veneer Wood Decks

Non-veneer panels are identified as waferboard or oriented strand board (OSB) panels, as opposed to plywood or veneered panels.

Non-veneer (O.S.B. or waferboard) panels are acceptable decks on low-slope roofing applications when an approved separation sheet or rigid insulation is mechanically attached before roofing is applied.

The use of non-veneered panels in steep slope applications with asphalt shingles applied directly to the deck is also not recommended by the RoofStar Guarantee Program, as there can be an increased chance of deck deflection between supporting trusses or rafters.


3 Concrete Decks

Concrete roof decks include decks that are cast-in-place or pre-cast, and structural or non-structural in nature. Although this roof deck category includes numerous variations (discussed later), many of the precautions involved with roofing over them are similar.

The finished surface of the deck should be smooth, level, and free of moisture or frost. All ridges or projections should be removed; and curbs, cants, blocking, and nailing strips should be installed where required prior to the roofing contractor's acceptance of the deck. With cast-in-place decks, special care must be taken to ensure that the deck is cured for 28 days and that the surface is dry (A simple test for determining whether the deck is dry enough for roofing is to apply hot asphalt to the deck; if it foams or is easily peeled off after it cools, the deck is too wet.). Pre-cast decks may require joints to be “taped” or stripped-in with felts, and the joints, weld plates, and elevation differences to be feathered with grout.

Concrete decks, curbs and equipment pads must be clean, dry, and smooth prior to commencement of roofing.

Poured concrete or lightweight concrete decks are not to be roofed for a minimum of 28 days after pouring unless expressly instructed in writing by the Project Structural Engineer.

Although some concrete decks are considered “nailable”, insulation is most commonly attached by using proprietary adhesives, or by applying hot asphalt over a primed deck. In the latter case, the asphalt is mopped to the deck and allowed to cool to the point where the insulation is not melted, but is still sufficiently bonded to the deck.

Built-up roof membranes can be directly adhered to both cast-in-place decks and, provided precautions are taken, to pre-cast decks. Vented base sheets may be used to provide semi-adherence to the roof deck.

Concrete roof decks include:

  • Cast-in-place reinforced concrete
  • Pre-cast concrete panels
  • Pre-stressed concrete
  • Lightweight insulating concrete
  • Poured gypsum concrete


A cast-in-place reinforced concrete roof deck is produced by pouring concrete into form work containing reinforcing steel bars or welded steel mesh. The surface of deck should be screeded and troweled to provide a smooth, level surface. Caution should be exercised when using certain curing agents and techniques. Curing agents may not be compatible with roofing materials and some curing techniques do not allow the deck to dry sufficiently.

Precast concrete-panel roof decks consist of manufactured panels designed to span between beams or load-bearing walls. They are available in the following cross-sectional shapes: single “T”, double “T”, solid slabs, hollow-core slabs, inverted channels, and tongue-and-groove planks. Metal plates may be installed to provide a welded structural connection and lateral bracing.

Pre-stressed concrete roof decks consist of concrete (usually precast panels) that is pre-stressed using integral steel tendon reinforcement. The unit is compressed to counteract flexural (bend) tensile stresses which usually result in a cambered unit (a slight curve that results in the centre of the unit being higher than the supporting ends). The camber will usually flatten once the structure is loaded.

Lightweight insulating concrete roof decks and fills consist of concrete mixtures that have vermiculite or perlite as insulating aggregates, or they may also be produced by mixing pre-generated foam with portland cement and water. This type of deck is usually designed for application over steel decks, corrugated metal forms, or bulb-tee or formboard systems. The concrete mixture is placed and screeded on site. Usually these mixes use a very high water / cement ratio and may require a lengthy cure time and a vented base sheet because of high moisture content. Vented base sheets provide a path for moisture vapour to vent to an appropriate point where the vapour may be released.

Poured gypsum concrete roof decks consist of a mixture of gypsum concrete, wood fibres or mineral aggregate, and water. The mixture is poured into formboards that contain welded or woven galvanized wire fabric. The formboards are left in place and may provide the finished interior surface.


4 Steel Decks

Steel roof decks are constructed of light gauge (usually 22, 20, or 18 gauge) cold-rolled steel sections (panels) that are usually galvanized. In cross-section the panels are ribbed, with the ribs usually spaced at 150 mm (6") on centre. The ribs provide the strength and rigidity of the panels. Steel decks are generally supported by open-web steel joist framing and are welded or mechanically-fastened to the framework. Steel deck panels must be properly aligned, squarely intersect walls, and provide a smooth level surface for roofing.

For insulated roof assemblies, a thermal barrier may be required to conform with Building Code or fire insurance-rated assemblies. This is usually required where the insulation is classified as combustible. Uninsulated systems require a leveling underlay for the membrane. Gypsum board may serve both purposes (thermal barrier & level surface).

Minimum thickness of insulations should be checked carefully to ensure that the material can safely span the flutes. The insulation or insulation overlay must be mechanically-fastened to the deck, unless it is a ballasted system.

Corrosion Protection For New Steel Roof Decks

When steel roof decks are specified, the RoofStar Guarantee Program recommends that the design authority specify the decking be factory-galvanized or factory-coated with aluminum zinc alloy for corrosion protection.

When specifying galvanized protection (in the section of the specifications addressing metal decking), the design authority should specify a coating that complies with ASTM A 525 Class G-60 or Class G-90. A G-90 coating provides greater corrosion protection than a G-60 coating (the ASTM specification previously identified G-60 as “light commercial” and G-90 as “commercial”).

When specifying aluminium zinc alloy protection (in the section of the specification addressing metal decking), the design authority should specify a coating that complies with ASTM A 792. An aluminum zinc alloy coating provides greater corrosion protection than a G-90 galvanized coating.

NOTE: The above recommendation applies to most buildings. However, in highly corrosive or chemical atmospheres, special care in specifying protective finishes should be taken, and individual deck manufacturers should be contacted.


5 Roof Expansion Joints

(See Construction Details to view the illustrated application of RoofStar Guarantee Standards for various membrane assemblies)

Roof expansion joints are designed to prevent membrane splitting and ridging (caused by either movement of the building, or movement of the roof assembly due to thermal expansion and contraction).

Roof expansion joints should be considered wherever:

  • the type of deck changes
  • additions connect to existing buildings
  • expansion and control joints exist in the structural system
  • separate wings of a building join (e.g. “L” or “T” configurations)
  • interior heating conditions change
  • differential movement may occur (e.g. parapet detail where the deck is not supported by the wall)

The location of roof expansion joints must be clearly indicated on the drawings and in the specifications, as the installation of wood blocking and cants is the responsibility of “others” and must be in place prior to the roofing contractor's acceptance of the deck.


6 Roof Divider / Control Joints

(See Construction Details to view the illustrated application of RoofStar Guarantee Standards for various membrane assemblies)

Roof dividers (sometimes referred to as control joints or area dividers) are built on-site. They are designed to help control thermal expansion and contraction stresses in the roof system where no structural expansion joint has been provided in the building design. Roof dividers are especially useful for controlling stress concentration in the membrane, and should be considered (and may be required) on all fully-adhered roof membrane systems.

A roof divider is usually designed as two raised wood members attached to a wood base-plate that is securely anchored to the roof deck and then properly flashed with membrane and metal. Roof dividers should divide the roof into approximately equal, regularly-shaped areas. Ideally, roof dividers should be located at the high points of the roof, with drainage away from the divider on both sides, but DRAINAGE MUST NOT BE IMPEDED BY THE ROOF DIVIDER.

RoofStar Guarantee Standards for a five (5) and ten (10) year guaranteed built-up roof membrane requires roof dividers / control joints “to divide roofs into areas approximately 45 m x 45 m (150' x 150'), at changes of deck material and decking direction, at re-entrant corners, and at junctions or additions to existing structure.

The location of roof dividers must be clearly indicated on the drawings and in the specifications, as the installation of wood blocking and cants is the responsibility of “others” and must be in place prior to the roofing contractor's acceptance of the deck.

NOTE that the requirement for roof divider / control joints for flexible membranes differs, and must be evaluated on a product performance basis.




Roof Deck Overlay

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