Low-Slope Roofing Systems (Built-Up, Modified Bitumen, Single-Ply)
This domain tests membrane selection and installation for roofs at or below about 3:12: built-up roofing, modified bitumen and single-ply, including attachment methods, lap dimensions, seam quality and wind uplift.
Built-up roofing
BUR is built from a base sheet plus multiple plies of saturated felt set in hot or cold adhesive, finished with a flood coat and gravel, a cap sheet or coating. Four plies is a common specification. Side laps and end laps must be staggered so no four-ply corner coincides. Interply asphalt must be at the mopping temperature range appropriate to its type; too cool and it will not bond, too hot and it damages the felt. Cant strips at vertical transitions prevent sharp bends.
Modified bitumen
SBS and APP membranes come in torch-applied, self-adhered and hot-mop versions. Side laps are typically 75 to 100 mm and end laps about 150 mm, with T-joints offset. Torch work needs a permit, extinguisher and fire watch, and the seam must show a continuous bleed bead of bitumen.
Single-ply
TPO and PVC are hot-air welded; seam quality depends on clean surfaces, correct temperature, roller pressure and speed, and seams are checked with a probe or air lance. EPDM is spliced with splice tape or adhesive and is highly sensitive to contamination. Attachment is fully adhered, mechanically attached or ballasted; perimeter and corner zones need enhanced fastening because wind uplift concentrates there.
Worked example
Twelve squares of 4-ply BUR with a base sheet. Each ply covers 12 squares; a roll of No. 15 felt covers about 4 squares, so each ply takes 12 ÷ 4 = 3 rolls, and four plies take 12 rolls. Add 3 rolls for the base sheet, giving 15 rolls. Add 10 percent for laps and waste, giving 16.5, so order 17 rolls, plus cap sheet and gravel by weight.
Traps that cost marks
Stacking end laps into a continuous vertical seam. Welding TPO when the seam is dirty, cold or wet. Touching an EPDM splice with contaminated gloves. Omitting the base sheet. Under-ballasting a ballasted roof, or fastening perimeters at the same density as the field.