Template:Part 5 (Waterproofing Roofs)

See more information on Roof Deck Overlays in Section B: Essential Elements.

1 General

1. When a supporting deck structure or wall is unsuitable for the application of roofing materials, it must be covered with an accepted overlay. See 2.1 (3) for supporting deck surface requirements.
2. A roof deck overlay (also called a system underlay) is installed as part of the Roof Assembly, on the top surface of the roof deck but beneath the roofing materials. These products are most commonly affixed to steel decks to provide a level surface for the roof membrane, to support air or vapour control layers, or to serve as a thermal barrier between the roof deck and combustible insulation. Roof deck overlay materials may also be applied to other types of supporting deck structures, depending on the roof design criteria.
3. When a roof design includes any type of overburden, the deck overlay on steel decks, or on wood decks less than 25 mm (1” ) thick, should be at least 16 mm (5/8”) thick, in order to stiffen the supporting surface and reduce deck deflection.

2 Materials

1. Deck and wall overlays must be suitable for, and compatible with, any membrane or panel application. Plywood, measuring at least 12.7 mm (1/2”) in thickness, is acceptable as a deck or wall overlay; for roofs supporting an overburden, the plywood must be at least 16 mm (5/8”) thick.
2. Self-adhering or adhesive-applied materials should be considered as alternatives to torch-applied membranes when the substrate to which they will be applied is combustible, or when nearby structures, openings or materials present a fire hazard. In the alternative, a suitable separation or overlay material as protection from open flame is acceptable. The application of materials to a combustible surface, using a torch, is strictly prohibited.
3. When the Building Code having jurisdiction requires a thermal barrier, an accepted deck overlay must be specified and installed.
4. Walls that require resurfacing for membrane application must be covered with an accepted wall overlay. See Accepted Wall Overlays.
5. Fasteners used to secure deck overlays must be corrosion-resistant or resin-coated, and must be used together with plates when specified or listed in a Tested Assembly report.
6. See also Accepted Roof Deck Overlays.

3 Application

3.1 General

1. Deck overlays must be
   1. fully or intermittently supported along all edges by the deck.
   2. installed in a staggered pattern (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 manufacturer's tolerance of differing board widths and lengths.
   3. affixed to the deck with either or both
      1. mechanical fasteners.
      2. proprietary polyurethane foam adhesives acceptable to the primary roof system manufacturer to meet or exceed the requirements set out in 3 SECURING the ROOF ASSEMBLY.
   4. independently fastened to the deck in keeping with either of the following:
      1. at least four (4) fasteners (with plates) per 1200 x 2400 (4’x8’) sheet, or
      2. as required by a Tested Assembly, when part of a PARS or MARS conventionally insulated roof assembly.
2. Wall overlays
   1. must be applied to existing sheathing, where existing sheathing is not an acceptable substrate.
   2. may be mechanically fastened or adhered.
   3. may be replaced with an accepted material.

3.2 Steel Decks

1. RoofStar-accepted deck overlay boards used as a thermal barrier to achieve a specific fire rating must conform to the applicable code and insurance requirements for the roof assembly.
2. When the roof assembly is uninsulated, the deck must be overlaid with at least one layer of 12.7 mm (1/2”)
   1. moisture resistant gypsum core deck overlay board, or with
   2. plywood.

3.3 Concrete Decks

1. A deck overlay board is not mandatory on a concrete supporting deck.

3.4 Wood Decks

1. A mechanically-fastened overlay board is required for any deck structure that does not meet the deck fastening criteria set out in 2.3.3.3 Wood.
2. Regardless of the type of deck overlay selected by the Design Authority, the overlay boards shall be installed in a staggered pattern (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 manufacturer's tolerance of differing board widths and lengths.
3. Adhering a primary roof membrane directly to a wood deck is not acceptable.
4. Notwithstanding Article 3 above, a self-adhered air or vapour control layer may be applied to a supporting wood deck.
5. Mechanically-fastened or loose-laid membranes applied directly to a supporting wood deck structure do not require a deck overlay board unless specified by the membrane manufacturer.

3.5 Walls

1. Where the wall surface is unsuitable to receive a membrane, it must be covered with an accepted overlay panel material. See also 2.1 General regarding suitable surfaces.
2. Wall overlay panels must be
   1. mechanically fastened with screw fasteners spaced no more than 300 mm (12”) O.C. vertically and horizontally, in alignment with structural supports, and placed
      1. at the perimeters.
      2. at the corners.
      3. in the field.
   2. adhered with a polyurethane adhesive, applied with a continuous z-patterned ribbon spaced no less than 300 mm (12”) apart.