Asphalt Roof Cement: Uses, Limits, Wet vs Dry Formulas, and Whether It Works on Driveways

Asphalt roof cement (often called plastic roof cement or, loosely, roofing tar) is a thick, trowel-grade mastic made of asphalt, solvents and reinforcing fibers. It's used to seal flashing, fill small holes and seams, bed shingles and seal around vents, chimneys and skylights. "Wet" or "wet-surface" formulas are made to stick to damp surfaces for emergency repairs. It's a patching product, not a roof system; large leaks and worn roofs need proper repair. It's also not a good choice for driveway cracks: it can stay soft and sticky, track onto shoes, and it isn't designed for traffic.

Asphalt crack filler guide →

What's in Roof Cement

• Asphalt for waterproofing
• Solvents that keep it workable and evaporate as it cures
• Fibers and fillers for body, so it stays put on slopes and vertical surfaces

Products are commonly made to an ASTM specification for asphalt roof cement, which includes grades designed for different slopes. Some are labeled for sagging resistance on vertical surfaces such as flashing and walls.

Common Uses

• Sealing step and counter flashing at chimneys and walls
• Sealing around plumbing vents, exhaust vents and skylight flanges
• Patching small punctures or cracks in shingles and roll roofing
• Bedding and sealing lifted shingle tabs
• Seams and blisters on built-up and asphalt-based low-slope roofs
• Gutter seams and small gutter leaks, where labeled

Wet vs Dry Formulas

Type	Surface	Use
Standard (dry-surface)	Clean and dry	Planned repairs in good weather
Wet-surface / wet patch	Damp or wet	Emergency leak repairs in rain
Rubberized or polymer-modified	Usually dry	More flexibility at joints and flashing

Check the label for temperature ranges; cold cement gets stiff and hard to spread. Warming the container indoors before use helps.

How to Apply

1. Clean the area. Remove loose granules, dirt, old failed sealant and debris.
2. Apply with a trowel or putty knife. A margin trowel works well for flashing and small patches. Spread about 1/8 to 1/4 inch thick, or as the label directs.
3. Reinforce larger patches. Embed a piece of polyester roofing fabric in a first layer, then cover it with a second layer.
4. Feather the edges so water runs over the patch rather than catching on it.
5. Let it cure. Solvents evaporate over time; it can stay soft for days to weeks, depending on thickness and weather.

Cartridge versions of asphalt roof cement fit a caulk gun and make small, neat repairs easier.

Where Not to Use It

• Single-ply membranes such as EPDM rubber, TPO and PVC roofs. Solvents and asphalt can be incompatible with these materials; use the membrane maker's products.
• Large failed areas where shingles are worn out or decking is soft.
• Over wet, rotting wood as a permanent fix.
• Visible areas where black smears matter; it's messy and hard to clean.

Can You Use Roof Cement on a Driveway?

People sometimes try it on driveway cracks because it's asphalt-based and handy. It's generally a poor choice:

• It can stay soft and tacky for a long time and pick up on tires and shoes
• It isn't formulated to handle traffic or the movement of driveway cracks
• Solvents can soften surrounding asphalt
• It can bleed through sealcoat applied on top

Use a driveway crack filler instead; our asphalt crack filler guide explains the types. For holes, use cold patch or hot mix. If you're planning to seal the whole driveway afterward, figure gallons with our sealer calculator.

How Long Does a Roof Cement Patch Last?

It depends on exposure, movement and how well the area was prepared. A small, well-bonded patch around a vent boot might hold for years, while a smear over a cracked shingle in full sun may crack or shrink sooner as the asphalt ages. Patches at flashing that moves with temperature are often the first to fail. Inspect repairs each spring and fall, and plan a proper fix, such as replacing a vent boot or flashing, when you have the chance.

Common Mistakes

• Applying too thick. Heavy globs skin over and stay soft underneath, and can crack as they shrink.
• Skipping cleanup of loose granules. The cement bonds to the loose material instead of the roof.
• Covering the real problem. A leak that shows up at a ceiling may start far upslope; cement at the wrong spot does nothing.
• Using it as a gutter liner over large areas, where a proper coating or replacement is needed.

Safety and Cleanup

• Work safely on roofs: use fall protection, stable ladders and avoid wet or icy roofs
• Solvents are flammable; keep away from flames and ventilate enclosed spaces
• Wear gloves; roof cement is hard to get off skin
• Clean tools with mineral spirits or a citrus cleaner while the cement is still wet, and dispose of rags safely

Roof Cement vs Other Roof Sealants

Roof cement is thick and fibered for troweling. Asphalt roof coatings are thinner liquids brushed or rolled over large areas. Sealant caulks in tubes, including polyurethane and hybrid products, are cleaner for small joints and some are compatible with more materials. Choose based on the roof type and the size of the repair. Our overview of asphalt roofs covers the materials these products are designed to work with.

Bottom Line on Asphalt Roof Cement

Asphalt roof cement is a thick, fibered asphalt mastic that's great for sealing flashing, small holes and roof penetrations on asphalt-compatible roofs, with wet-surface versions for emergencies. It's a patch, not a roof replacement, it doesn't belong on single-ply membranes, and it's the wrong product for driveway cracks.