SBS Modified Bitumen - Page 9

7A.27.7 For the top layer, the adhesive is spread along the roof in the same manner used for base and ply felts. For subsequent courses of modified bitumen sheets, spread the adhesive initially from the poured bead toward the laying line of the lower course, developing a smooth and uniform bed of adhesive across the substrate and onto the selvage edge of the lower course sheet. In addition, a continuous 3/8" to 1/2" (10 to 13 mm/m) bead of adhesive should be left immediately above, and adjacent to, the laying line of the lower course. Once this portion of the adhesive bed has been completed, continue spreading the adhesive up the roof until the bed is wide enough to receive the sheet. After the cap sheet has been placed into the bed of adhesive and aligned properly with the adjoining sheet, the side lap seam must be "scuffed in" or rolled down, pressing a small bead of adhesive out of the lap. Roofing granules can be applied to any exposed adhesive if appearance is a factor.

7A.28 One-Part Cold Adhesive Membrane Specifications

7A.28.1 Refer to Specification (Spec ID)-C1 for membrane construction recommendations.

7A.29 Steep Slope Requirements

7A.29.1 Non-Nailable Decks: On decks with a slope over 1/2" per foot (41.6 mm/m), the roofing felts must be installed parallel to the incline and must be nailed. Pressure-treated wood nailers shall be attached to the deck, run perpendicular to the incline, be capable of retaining the nails securing the roofing felts, have the same thickness as the insulation, and be at least 3 1/2" (89 mm) wide. Wood nailers shall be provided at the ridge and at the following approximate intermediate points:

Nailers may also be laid out to conform to the roll length being used. For slopes between 1/2" to 2" per foot (41.6 to 167 mm/m), nailers should be spaced to accommodate full-length modified bitumen rolls. For slopes between 2" to 3" per foot (167 to 250 mm/m), the nailers should be spaced to accommodate half-length rolls.

7A.29.2 Cut the modified bitumen cap sheet to conform to the nailer spacing. Nail the end lap across the width of the sheet, with the first nail spaced 3/4" (19 mm) from the leading edge of the sheet, and the remaining nails spaced approximately 8 1/2" (216 mm) o.c. The nails shall be staggered across the width of the nailer to reduce the  risk of the sheet tearing along the nail line. Nails must have an integral 1" (25 mm) (min.) diameter cap. Where capped nails are not used, fasteners must be driven through caps having a 1" (25 mm) (min.) diameter.

7A.29.3 Nailers must also be used around the roof perimeter, openings and penetrations, for nailing felts, gravel stops, roof fixtures and fascia systems.

7A.29.4 Nailable and Lightweight Concrete Decks: On decks with a slope over 1/2" per foot (41.6 mm/m), the roofing felts must be installed parallel to the incline. Nail the end laps of the modified bitumen cap sheet across the width of the sheet on 8 1/2 " (216 mm) centers. All nails are to be mopped over and covered by the lap of the next sheet. For slopes from 1/2" to 2" per foot (41.6 to 167 mm/m), a full-length sheet can be used. For slopes from 2" to 3" per foot (167 to 250 mm/m), a half-length sheet should be used.

7A.30 Phase Construction

7A.30.1 One of the greatest hazards of roof construction is the application of a roofing system in "phases", where a partially completed roof system is left exposed to the weather overnight or longer. This can lead to entrapped moisture which can cause premature failure of the membrane.