Template:Deck Overlay - SBS

1 General

A roof deck overlay (also called a system underlay) is installed under the roof system and over the roof deck assembly. These products are most commonly placed over steel decks to provide a level surface for the membrane, or as a thermal barrier between the roof deck and combustible insulation.

2 Material Selection

Regardless of the substrate to which roof membranes are applied, installation of roof membrane to the surface must be governed by the membrane manufacturer's requirements for that substrate.

Gypsum boards on vertical surfaces must be a RoofStar-accepted overlay boards with water resistant properties.

On steel framed walls, it is acceptable to use gypsum overlay boards accepted for the Guarantee Program, provided the gypsum board has a minimum thickness of 12 mm (½") or fibre-mat reinforced cement boards with a minimum thickness of 9.5 mm (⅜") that conform to ASTM C1325-04.

See Accepted Deck Overlay Boards for RoofStar Guaranteed roof systems.

3 Application: Decks

3.1 Concrete

• 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.
• All concrete decks to receive adhered asphaltic roofing shall be primed.
• A vented base sheet is required under all asphalt-adhered (adhesive, mopped or torched) roofing membranes that are adhered directly to concrete decks unless insulation is installed directly above or over the membrane.

Pre-cast panels, post-stressed beams, or “T” and cavity decks shall must have a separation layer such as fibre board, gypsum board, plywood, proprietary overlay board etc. applied over the deck prior to the application of the roof membrane. Where a difference exists in the levels of adjacent surfaces, the general contractor or others shall install grouting. This grouting shall be feathered on the lower surface a minimum distance of twelve times the amount of the vertical differential.

3.2 Steel

1. When a membrane is specified for application directly over a steel deck, the RoofStar Guarantee Standards require a steel deck overlay of gypsum with fibreglass facers and a silicone treated gypsum core (see min. standards below), or plywood having a minimum thickness of 12.7 mm (1/2"). Where an underlay is used as a thermal barrier, it must conform to the applicable code and insurance requirements for the roof assembly.

2. Gypsum boards installed over steel decks as substrates for primary roof membranes must be a minimum thickness of 12.7 mm (1/2") RGC accepted overlay boards. All edges of the gypsum board are to be fully or intermittently supported by the deck and end joints to be staggered (offset) 300 mm (12") from adjacent board rows. A minus offset tolerance of 50 mm (2") maximum will be permitted to compensate for variance in the manufacturers’ tolerance of differing board widths and lengths.

3. When gypsum board is required on steel decks as a substrate overlay, the gypsum board can be fully mechanically fastened to the deck or adhered with proprietary polyurethane foam adhesives acceptable to the primary roof system manufacturer. Attachment must meet or exceed the CSA A123.21-14 requirements for the building dimensions and geographical location.

RGC maximum deck flute width and minimum fastener requirements for gypsum board in a deck overlay application are as follows:

Gypsum Board Thickness	Deck Flute Width	Gypsum Board Size	Screws & Metal or Plastic Stress Plates	Bugle Head Drywall Screws
12.7mm (½")	up to 125mm (5")	1200 mm x 2400 mm (4' x 8’)	12	32
15.9mm (5/8")	up to 200mm (8")	1200 mm x 2400 mm (4’ x 8’)	12	32

4. When gypsum board is used under mechanically fastened insulation, the gypsum board must be fastened independently from the insulation. The minimum number of fasteners(screws or screws and plate) is four (4) per sheet. Adhesive systems must meet or exceed the CSA A123.21-14 requirements for the building dimensions and geographical location.

3.3 Wood

1. An RGC approved separation sheet, minimum 40-lb asphalt coated fibre glass base sheet conforming to CAN/ CSA A123.16 - M88 (R1996), Type II or manufacturer’s accepted base sheet of equal or greater weight and reinforcement, or RGC accepted rigid insulation / overlay board mechanically attached or semi-adhered membrane (spot or strips) is required over non-veneer panels before fully adhered roofing membrane is applied.

2. Direct torch (open flame) membrane applications to wood decks and wood surfaces is NOT permitted. For further development of this policy, refer to SBS Application & Safety: Best Practices.

3. Fully adhering modified bituminous membranes directly to wood decks is not permitted.