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Roof cement — thick asphalt mastic, troweled on — for holes, flashing and loose shingle tabs on asphalt roofs. Roof sealant — flexible polyurethane, silicone or rubber caulk in a tube — for joints that move: flashing edges, vent boots, skylights, gutter seams and metal-roof screws. Neither goes on EPDM or TPO.
Roof cement is a thick, fibre-reinforced asphalt mastic (ASTM D4586) you trowel 1/8 to 1/4 in thick to patch holes, bed flashing and stick down shingle tabs on asphalt roofs; it stiffens with age. Roof sealant is a flexible gun-grade elastomer — polyurethane, silicone or rubber-based — for joints that move: flashing edges, vent boots, skylights, gutter seams and screws on metal roofs. "Wet patch" is roof cement made to bond to wet surfaces for emergency repairs. Neither asphalt product belongs on EPDM, TPO or PVC. A gallon of roof cement (231 cu in) covers about 12.8 sq ft at 1/8 in; a 10.1 oz cartridge lays about 31 ft of ¼ in bead.
Short answer: use roof cement — a thick asphalt mastic you trowel on — to patch holes, bed flashing and stick down shingle tabs on asphalt roofs. Use roof sealant — a flexible gun-grade polyurethane, silicone or rubber-based caulk — on joints that move: flashing edges, vent boots, skylights, gutter seams and screws on metal roofs. Neither asphalt product goes on EPDM, TPO or PVC.
| Roof cement | Roof sealant | |
|---|---|---|
| What it is | Thick asphalt mastic cut with solvent and reinforced with fibres (ASTM D4586, asbestos-free roof cement) | Gun-grade elastomer: polyurethane, silicone, solvent-based rubber (SEBS/butyl) or hybrid polymer |
| Comes in | 1-gallon cans, 5-gallon pails and 10.1 oz cartridges | 10.1 oz cartridges (some in 28 oz and 1-gallon) |
| Applied with | Trowel or putty knife, 1/8 to 1/4 in thick | Caulk gun, as a bead |
| Best at | Patching holes and splits, bedding and covering flashing, sealing down loose shingle tabs, valleys and around chimneys on asphalt roofs | Sealing joints that move: flashing edges, vent boots, skylight frames, gutter seams, exposed screws on metal roofs |
| Flexibility | Stiffens with age and heat; cracks at moving joints | Stays flexible; made to stretch with expansion joints |
| Wet surfaces | "Wet patch" grades are made to go on damp or wet roofs for emergency repairs | Most need a dry, clean joint; a few are labelled for damp surfaces |
| Looks | Black, shows on coloured shingles and metal | Clear, black or colour-matched; many are paintable (silicone is not) |
| Do not use on | EPDM, TPO and PVC single-ply membranes — asphalt is incompatible with them | Silicone over asphalt cement or on asphalt you will later patch: little else sticks to cured silicone |
| Typical life | A temporary-to-medium repair; recheck every year | Depends on the polymer; polyurethane and hybrid sealants outlast asphalt mastic at moving joints |
| Size | Roof cement per gallon | Sealant per 10.1 oz tube |
|---|---|---|
| 1/8 in cement · ¼ in bead | 12.8 sq ft | 30.9 ft |
| 3/16 in cement · 3/8 in bead | 8.6 sq ft | 13.8 ft |
| ¼ in cement · ½ in bead | 6.4 sq ft | 7.7 ft |
Roof cement is a thick asphalt mastic you trowel on to patch holes and bed flashing on asphalt roofs; it stiffens as it ages. Roof sealant is a flexible gun-grade elastomer — polyurethane, silicone or rubber-based — for joints that move: flashing edges, vent boots, gutter seams and screws on metal roofs.
For a joint or a nail head, yes — a flexible sealant is usually the better choice. For a hole, a split shingle or under a flashing, use roof cement, which fills and bridges at 1/8–1/4 in thick where a bead of sealant would only skin over.
Both are asphalt roof cement. "Wet patch" grades are formulated to bond to damp or wet surfaces so you can stop a leak in the rain; standard plastic roof cement wants a dry, clean surface. Treat either as a repair to recheck, not a permanent fix.
One gallon is 231 cu in, so it covers about 12.8 sq ft at 1/8 in thick or 6.4 sq ft at 1/4 in, before waste. A 10.1 oz cartridge lays about 31 ft of ¼ in bead or 14 ft of 3/8 in bead.
No. Asphalt roof cement attacks EPDM and is incompatible with TPO and PVC membranes. Repair single-ply roofs with the membrane maker's own primer, seam tape and lap sealant.
Comparing products? See the best roof cements and sealants.
Need the whole roof at once? The free roofing calculator works out roof area, squares, shingle bundles, underlayment, drip edge, ridge cap and installed cost from one set of measurements.