Roofing cement is a thick, asphalt-based paste used to seal leaks, patch cracks, and bond flashing on a roof. Most products sold as roofing cement are asphalt cutback cements meeting ASTM D4586, troweled 1/8 to 1/4 inch thick, and the Rust-Oleum Plastic Roof Cement data sheet lists a 2-hour dry and a full cure in 24 hours at 70°F and 50% humidity. It is a repair material, not a roofing system: it buys time on a leak or holds a patch, but it does not replace proper flashing, underlayment, or new shingles. A 0.9-gallon can covers about 12.5 square feet at repair thickness, per the same data sheet. Cold weather changes how it goes on, which our guide to roof repair in winter covers.
Reviewed by The Roofing Brief Team. Last reviewed: October 2026.
What is roofing cement?
Roofing cement is a trowel-grade compound of asphalt, mineral fillers, and reinforcing fibers, thinned with a petroleum solvent so it stays workable in a tub. It sets by solvent evaporation, gripping shingles, metal, masonry, and most membranes. The generic product is often labeled “plastic roof cement” or “flashing cement,” and quality grades reference ASTM D4586 (asbestos-free asphalt roof cement).
The nickname “the duct tape of roofing” fits: it is cheap, sticky, and forgiving. The makers price their promises accordingly: Gardner PRO-SEAL Premium Roof Cement carries a 12-month repair warranty that applies only when the repair is reinforced with fabric, excludes flat roofs and ponding areas, and requires a slope of at least 1 inch per foot. Treat it as a bandage, not a cure.
Types of roofing cement and roof sealant cement
Six formulations cover most jobs, and picking the wrong one is the most common mistake. Asphalt (plastic) roof cement such as Rust-Oleum Plastic Roof Cement handles most shingle and flashing repairs. Wet patch cement such as Geocel RP-400 bonds to wet or dry surfaces, even underwater, per its data sheet. Flashing cement such as Karnak 19 Flashing Cement is a heavier trowel grade that stays on vertical faces. Fibered, butyl, and silicone variants trade cost for flexibility or membrane compatibility.
| Type | Base | Best for | Data sheet example and specs | Watch out for |
|---|---|---|---|---|
| Plastic (asphalt) roof cement | Asphalt cutback, fibered | Shingle cracks, flashing beds, nail heads | Rust-Oleum Plastic Roof Cement: 1/8 to 1/4 in thick, 2 h dry, 24 h full cure at 70°F, apply at 40 to 100°F | Rust-Oleum says it is not recommended as a shingle adhesive; check the label |
| Wet patch roofing cement | Asphalt with a wet-application additive | Active leaks, damp or rained-on surfaces | Geocel RP-400: 1/8 in per coat, 1/4 in maximum, extend 3 to 4 in past the damage, 24 h cure | Not for EPDM, PVC or CSPE membranes (Geocel) |
| Fibered / roofing tar cement | Asphalt with glass or cellulose fiber | Larger holes, seams, bridging gaps | Gardner PRO-SEAL Premium (ASTM D4586): 12 sq ft per gallon at 1/8 in, 4 to 8 h to set, apply at 50 to 120°F | Gardner warranty needs reinforcing fabric and a slope of 1 in per foot or more |
| Flashing cement | Asphalt, mineral fibers, solvent, heavy grade | Walls, chimneys, flues, vertical laps | Karnak 19: ASTM D4586 Type I Class I, apply at 40 to 120°F, cured film rated 15 to 180°F | Sold in Summer, Semi and Winter consistencies; match the season |
| Butyl roof cement | Butyl rubber blend | Metal roofs, joints that move | Use the maker’s data sheet for cure and temperature limits | Costlier, slower cure |
| Silicone / polyurethane sealant cement | Silicone or urethane | Flat/low-slope, UV exposure, ponding | Gun or brush grade; see the roof sealant guide below | Poor adhesion over fresh asphalt cement; Gardner says wait 30 to 60 days before coating over its cement |
Asphalt-based cement should not be applied over EPDM, TPO, or PVC single-ply membranes: the solvents degrade the sheet. The data sheets say so directly: Geocel excludes EPDM, PVC and CSPE, Gardner excludes EPDM and Hypalon, and Rust-Oleum excludes rubber and PVC sheeting. For those roofs, use a manufacturer-matched sealant or the compatible seam product. Our EPDM rubber membrane pricing guide covers what actually bonds to rubber. On standing-seam and corrugated panels, a purpose-made product beats asphalt cement; our guide to metal roofing sealant types and how to pick one explains why.
Best roofing cement for asphalt shingles: what ASTM D4586 compliant means
For asphalt shingles, the best roofing cement is a plastic asphalt roof cement labeled as meeting ASTM D4586, the grade shingle makers name in their own instructions. Choose Class I for a dry roof and Class II if the surface is damp. Use it in small dabs: Owens Corning hand-seals each laminate shingle with four 1-inch dabs and warns that excess cement can blister shingles.
ASTM D4586 covers asbestos-free asphalt roof cement made for trowel application to roofing and flashing. A tub marked “ASTM D4586 compliant” has been made to one of two types and one of two classes in that specification (ASTM D4586/D4586M):
| Label on the tub | What ASTM D4586 says it is | Pick it when |
|---|---|---|
| Type I | Made from asphalts characterized as self-healing, adhesive, and ductile | You want a pliable bed under shingle tabs and flashing |
| Type II | Made from asphalt with a high softening point and relatively low ductility | The label or the shingle maker calls for it |
| Class I | For application to essentially dry surfaces | The roof is dry: most planned shingle repairs |
| Class II | For application to damp, wet, or underwater surfaces | You are patching a wet roof (the wet patch grade) |
Shingle manufacturers write this standard into their instructions. Owens Corning’s cold weather installation bulletin calls for hand sealing shingles along the whole roof perimeter with “ASTM D4586-compliant asphalt roof cement” when the factory sealant strip cannot activate: four evenly spaced 1-inch dabs about 1 inch from the leading edge of each laminate (architectural) shingle, or two 1-inch dabs under each tab of a three-tab shingle (Owens Corning TB-10022497). The same bulletin warns against letting cement squeeze out past the leading edge, and that excessive amounts can cause shingles to blister. That is the same rule as the 1/8 inch guidance below: thin and small beats thick. Read the tub before using it as a shingle adhesive: the Rust-Oleum Plastic Roof Cement data sheet says that product is not recommended for it, so pick a cement whose label names shingle tab sealing.
What is roofing cement used for?
Roofing cement seals penetrations and patches small breaches. The five jobs it does well are sealing around flashing, chimneys, and vents; re-bedding lifted shingle tabs; filling nail-hole and hairline cracks; bridging small membrane splits on low-slope roofs; and gluing down loose ridge caps. Each is a spot repair measured in inches, not a coating for the whole roof: Gardner tells users to work the cement 2 to 3 inches beyond the problem area, and Geocel 3 to 4 inches.
- Flashing seals: a bead of a heavy flashing-grade cement such as Karnak 19 behind step or counter flashing stops the wall-to-roof junction from wicking water. It backs up metal, it does not replace it.
- Vent and pipe boots: Rust-Oleum lists stacks and vents among its plastic roof cement’s uses; a ring around a cracked boot buys time, but the boot itself is the fix.
- Shingle cracks and lifted tabs: a dab under the tab and a smear over the crack, pressed flat, reseals wind-loosened shingles.
- Emergency leak patch: troweled over a tarp seam or split, wet-patch grade holds through a storm until a real repair.
What it is not for: sealing whole valleys, coating an aging roof to “extend” it, or standing in for flashing on a new install. Those uses fail because cement stiffens once cured and cracks as the roof moves, and Gardner’s warranty on its premium roof cement excludes flat roofs and ponding areas outright. If the goal is a whole-surface coating on a low-slope roof, a purpose-made product such as a polyurethane roof coating is built for that job. When the problem is a leak with an unknown source, start with our roof leak repair guide to find it before you seal blind.
How to apply roofing cement
Apply roofing cement to a clean, dry surface with a trowel or putty knife, spread about 1/8 inch thick, work bottom to top on vertical surfaces, and let it cure about 24 hours at 70°F (the Rust-Oleum and Geocel data sheets both say 24 hours). Thickness is the detail most people get wrong: piling it on does not make a stronger seal, it makes one that skins over on top and stays soft underneath, then cracks. Thin, pressed, and feathered at the edges lasts longer.
- Clean the area. Remove debris, granules, old failed cement, and loose material. Adhesion is only as good as the surface under it.
- Dry it (or buy wet-grade). Standard plastic cement wants a dry substrate (ASTM D4586 Class I). If the surface is damp and the leak is active, use a Class II wet patch cement such as Geocel RP-400, which its data sheet says bonds even underwater.
- Load the trowel. Scoop a golf-ball amount. Work small; the solvent flashes off and the tub skins over if you leave it open.
- Spread at 1/8 inch. Fill the crack or hole, then feather the cement 2 to 3 inches past the edge (Gardner) or 3 to 4 inches (Geocel) so water cannot track under the patch. Geocel caps RP-400 at 1/4 inch total.
- Embed reinforcement for larger patches. Rust-Oleum calls this a 3-course repair: a coat of cement, an embedded strip of patching fabric or mesh, then a final coat. Gardner’s 12-month warranty applies only to fabric-reinforced repairs.
- Work bottom to top on vertical faces. Around chimneys and walls, this keeps the cement from sliding and shingles the overlaps against gravity.
- Cure before rain. Rust-Oleum lists 2 hours to dry and 24 hours to full cure at 70°F and 50% humidity; Gardner PRO-SEAL sets in 4 to 8 hours. Application windows run 40 to 100°F (Rust-Oleum), 40 to 120°F (Karnak 19) and 50 to 120°F (Gardner). Below 50°F, store the can at room temperature for 24 hours first, as Geocel and Gardner direct.
For flashing work specifically, cement is the backup bead, not the seal itself. Get the metal detail right first: our guide to roof flashing types and installation shows where each piece goes and why flashing, not cement, is what stops most leaks.
Flashing cement vs roof cement
Flashing cement is a heavier-bodied version of the same asphalt roof cement, made to stay put on a wall or chimney. Karnak describes its 19 Flashing Cement as a heavy trowel consistency that spreads on vertical and horizontal surfaces without sagging and resists flow at high temperatures, and it is certified to the same ASTM D4586 Type I Class I grade as a standard plastic roof cement. Use flashing cement where the bead hangs on a vertical face, and plastic roof cement on the roof plane.
Roofing cement vs roof sealant vs roofing tar
These three terms get used interchangeably but describe different products. Roofing cement is a thick trowel-grade asphalt paste for patching (ASTM D4586). Roof sealant is a thinner gun-applied caulk (often silicone or polyurethane) for narrow joints. Roofing tar is hot or cold asphalt or coal-tar pitch used mostly on built-up flat roofs, made to different standards: built-up roofing asphalt to ASTM D312, per the Asphalt Roofing Manufacturers Association, and coal-tar roofing pitch to ASTM D450 in the NRCA glossary.
| Product | Consistency | Applied with | Best joint width | Flexibility once cured |
|---|---|---|---|---|
| Roofing cement | Thick paste | Trowel / putty knife | Gaps, holes, wide beds | Low, becomes rigid |
| Roof sealant | Gun-grade caulk | Caulk gun | Narrow seams under 1/4 inch | High, stays elastic |
| Roofing tar (mastic) | Heavy liquid | Mop, brush, or trowel | Full-surface built-up work | Low, brittle in cold |
Rule of thumb: if the gap moves or is under 1/4 inch wide, use an elastic sealant; if it is a hole or a bed for flashing, use cement; if you are laying or repairing a built-up flat roof, that is tar or mastic territory. Do not layer a coating or sealant over fresh asphalt cement: Gardner says to let its roof cement cure 30 to 60 days before a reflective coating goes on.
| If the problem is | Use | Spec or limit and source |
|---|---|---|
| A cracked shingle or lifted tab on a dry day | Plastic roof cement meeting ASTM D4586 Class I, in small dabs | Owens Corning: four 1-inch dabs per laminate shingle |
| An active leak on a wet roof | Wet patch cement (ASTM D4586 Class II), such as Geocel RP-400 | Bonds wet or underwater; 1/8 in per coat, 1/4 in maximum (Geocel) |
| A bead on a chimney, wall or other vertical face | Flashing cement, such as Karnak 19 | Heavy grade, no sag; apply at 40 to 120°F (Karnak) |
| A split, blister or flashing lap wider than a crack | 3-course repair: cement, fabric, cement | Rust-Oleum data sheet; Gardner warranty requires fabric |
| A joint under 1/4 inch that moves | Elastic gun-grade sealant, not cement | Cement stiffens once cured |
| An EPDM, TPO, PVC or CSPE membrane | The membrane maker’s sealant; no asphalt cement | Geocel, Gardner and Rust-Oleum exclusions |
| A built-up flat roof | Built-up roofing asphalt or coal-tar pitch | ASTM D312 (ARMA) or ASTM D450 (NRCA) |
| The same spot leaking again after a season | Replace the flashing, boot or shingle | Gardner’s premium cement warranty runs 12 months |
When roofing cement fails (and what to do instead)
Roofing cement fails when it is used as a permanent fix, applied too thick, put over an incompatible membrane, or asked to seal a joint that flexes. Cured asphalt cement stiffens, so a roof that expands, contracts, and vibrates in wind eventually opens hairline cracks in the patch, and the leak returns. The data sheets set the limits: Karnak rates its cured 19 Flashing Cement film for 15 to 180°F, and Geocel rates RP-400 for -40 to 150°F in service. Our guide to tar roof repair covers which roofs tar suits and how to apply a patch step by step.
The honest limit is the one the makers write down: Gardner warrants a fabric-reinforced repair with its premium roof cement for 12 months, on roofs sloped at least 1 inch per foot. If you are re-cementing the same spot each year, the underlying flashing, boot, or shingle has failed and needs replacing, not re-sealing.
Roofing cement: frequently asked questions
What is roofing cement used for?
Roofing cement seals and patches small roof problems: it beds flashing around chimneys and vents, reseals lifted or cracked shingles, fills nail holes and hairline cracks, and bridges small splits on low-slope roofs. It is a spot repair material measured in inches: Gardner says to work it 2 to 3 inches beyond the problem area. It is not a coating for a whole roof, and it backs up flashing rather than replacing it.
What is the difference between plastic roof cement and wet patch roofing cement?
Plastic (asphalt) roof cement is the standard fibered paste for dry-surface repairs like shingle cracks and flashing beds. Wet patch roofing cement uses a formula that bonds to damp or rained-on surfaces, making it the choice for active leaks and storm emergencies. ASTM D4586 draws the same line: Class I cement is for essentially dry surfaces and Class II for damp, wet, or underwater surfaces.
How long does roofing cement take to dry?
Rust-Oleum Plastic Roof Cement dries in 2 hours and reaches full cure in 24 hours at 70°F and 50% humidity, Geocel RP-400 also lists 24 hours, and Gardner PRO-SEAL sets in 4 to 8 hours. Apply within the label window, such as 40 to 100°F for Rust-Oleum or 50 to 120°F for Gardner. Applying it too thick traps solvent underneath, so the surface skins while the interior stays soft and later cracks.
Is roofing cement a permanent fix?
No. Roofing cement is a temporary repair. Cured asphalt cement is rigid and cracks as the roof moves and heats, and Gardner warrants a fabric-reinforced repair with its premium roof cement for only 12 months. If you re-cement the same spot yearly, the underlying flashing, boot, or shingle has failed and needs replacing.
What is the difference between flashing cement and roof cement?
Flashing cement is a heavier-bodied asphalt cement made to stay on vertical surfaces. Karnak 19 Flashing Cement, for example, is a heavy trowel grade that spreads on walls and chimneys without sagging and is certified to ASTM D4586 Type I Class I, the same grade as a standard plastic roof cement. Use flashing cement on vertical faces and plastic roof cement on the roof plane.
Is roof cement the same as tar?
No. Roof cement is a trowel-grade asphalt paste made to ASTM D4586 for spot repairs. Roofing tar usually means built-up roofing asphalt, made to ASTM D312, or coal-tar pitch, made to ASTM D450, which are mopped or applied over whole low-slope roofs.
Can you put roofing cement on a wet roof?
Only with wet patch roofing cement, which is formulated to bond to damp surfaces. Standard plastic roof cement (ASTM D4586 Class I) needs a clean, dry substrate; Class II wet patch products such as Geocel RP-400 are rated to bond to wet surfaces and even underwater. For an active leak during rain, a wet-grade product troweled over the split holds through the storm until a proper dry repair can be made.
Can I use roofing cement on a rubber or TPO membrane?
No. The solvents in asphalt roofing cement degrade single-ply membranes like EPDM, TPO, and PVC. Geocel, Gardner and Rust-Oleum all exclude rubber or PVC membranes on their roof cement data sheets. Use a manufacturer-matched sealant or the compatible seam product for those roofs. Applying asphalt cement to a rubber or thermoplastic membrane can shorten its life and void the membrane warranty.