Low-Slope and Built-Up Roofing Systems (Mod-Bit, EPDM, TPO, PVC, Foam)
The families
Built-up roofing (BUR) is layers of felt or ply sheet set in hot or cold asphalt; four plies means four reinforcing layers plus surfacing. Modified bitumen comes in torch-applied, self-adhered, and cold-adhesive forms, usually as a base sheet plus a granulated cap sheet with staggered side and end laps. Single plies - EPDM, TPO, PVC - go down fully adhered, mechanically attached, or ballasted (ballast generally 10 lb/sf minimum). Spray polyurethane foam is applied in thin passes and must be coated for UV and traffic protection.
Seams are the test
The roof lives or dies at the seam. EPDM uses primer plus a splice tape or adhesive, rolled for full contact. TPO and PVC seams are hot-air welded at controlled temperature and speed, then probed and pull-tested. Any seam that opens, or any fishmouth, is cut out and repaired, not just sealed over.
Compatibility
PVC is incompatible with asphalt and polystyrene insulation and needs a separator. TPO over aged modified bitumen also needs a separator. Solvent adhesives attack foam insulation. Torching near combustible decking, wood curbs, or dried vegetation is a fire hazard and often prohibited.
Details and slope
Low-slope roofs need at least 1/4 in. per foot of slope to drains or scuppers, with crickets and saddles to eliminate ponding. Terminations at walls, curbs, and edges take a base flashing and counterflashing; field seams run with the slope, not across it.
Traps
Questions offer "just seal the seam" and "ballast with any available gravel." Both are wrong.
Worked example
A 2% slope roof with a 40 ft run drops about 9.6 in.; check that tapered insulation and drain saddles deliver it.