Template:All systems - ROOF DECKS

1 General

The roof deck must comply with several guiding principles, to be suitable for a RoofStar Guarantee:

• For low slope (membrane roof or waterproofing systems) it should provide positive slope to drain [required for a Five (5) or Ten (10) year RoofStar 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 Building Code. For plywood decks, RGC 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 Concrete

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 hot asphalt over a primed deck. 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, 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 formwork containing reinforcing steel bars or welded steel mesh. The surface of deck should be screeded and trowelled 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.

Pre-cast 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 pre-cast panels) that is pre-stressed using integral steel tendon reinforcement. The unit is compressed to counteract flexural 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.

3 Wood

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 B.C.Building Code requirements. 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 Building Code.

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.

3.1 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 Building Code).

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.

3.2 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 Building Code requirements.

For steep slope roofing (e.g. asphalt shingles), plywood thickness must - as a minimum - conform to Building Code requirements; 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. For deck sheathing, refer to RGC Guarantee Standard A2.4.3 Section 2.1.1.

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.

3.3 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 RGC, as there can be an increased chance of deck deflection between supporting trusses or rafters.

4 Special Circumstances: Electrical Conduit

Conduit, EMT, cable and/or piping applied on the top of roof deck surfaces shall not be acceptable on new construction projects. If conduit is present on decks in re-roofing projects, the deck must be made smooth and even to the satisfaction of the accepted roof inspection firm. The upper layer of insulation must cover the conduit, EMT, cable and/or piping. Detailed as-built drawings should be made to prevent conduit damage and safety risks to roofers, should the roof require replacement at a future date.