Roof Cement vs Roof Sealant: The Best Type for Each Leak

Plastic roof cement, wet-surface patch, sealant cartridges and butyl tape compared by the leak, the roof and the weather each one is made for.

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Roof cement and roof sealant are not the same product. Roof cement (often sold as plastic roof cement) is a thick, asphalt-based paste spread with a trowel; it beds flashing, fills splits and covers nail heads on shingle and built-up roofs. Roof sealant comes in a caulk cartridge, is thinner and more elastic, and seals joints that move: around vent pipes, along counter-flashing and over metal seams. A third group, wet-surface patch, is made to bond through water when the leak cannot wait for a dry day.

Below, each type is compared by the job it does, the specification to read before buying, and its honest limits. None of them replaces a failed flashing or a worn-out roof — they seal a detail. To size the flashing itself, use the flashing calculator; to judge whether a repair is worth it, see the roof leak repair guide.

Compared by type

Pros and cons describe each type from its specifications; this is not a hands-on product test. As an Amazon Associate we earn from qualifying purchases.

Plastic (asphalt) roof cement

Best for: Bedding step and counter-flashing, sealing shingle tabs and nail heads on asphalt roofs. Check: Asphalt-based and fibered, the temperature range, and that the surface must be dry.

Pros

Thick enough to fill gaps and bed flashing in one pass
Bonds well to asphalt shingles, felt and built-up roofing
Cheap per job in a tub or cartridge

Cons

Needs a dry surface on most formulas
Turns brittle with sun exposure and should be covered where possible
Attacks EPDM, TPO and PVC membranes

Wet-surface roof patch

Best for: Emergency repairs on a wet or actively leaking roof. Check: The label must say it can be applied to wet surfaces or in rain.

Pros

Displaces water and bonds to a damp surface
Stops a live leak until a proper repair
Same trowel work as ordinary roof cement

Cons

A stopgap; the detail underneath still needs fixing
Costs more than standard roof cement
Still asphalt-based, so not for single-ply membranes

Roof sealant in a cartridge (rubberized, tripolymer or polyurethane)

Best for: Pipe boots, counter-flashing reglets, metal seams and fastener heads that move. Check: Listed substrates (asphalt, metal, membrane), paintability and whether it tolerates damp surfaces.

Pros

Stays elastic, so it follows joints that expand and contract
Precise bead from a caulk gun with little mess
Many grades can be painted to match flashing

Cons

Will not fill wide gaps or rebuild a missing detail
Silicone grades cannot be painted and are hard to re-seal over later
Needs a clean joint; it does not bond to old caulk

Butyl or aluminum-backed seal tape

Best for: Pinholes, split flashing and seams on metal roofs, gutters and skylight frames. Check: Butyl adhesive (not acrylic), backing material and minimum application temperature.

Pros

Instant repair with no cure time
Bridges a crack that sealant would sink into
Clean to apply on a ladder

Cons

Surface must be clean, dry and warm enough for the adhesive
Edges lift if the surface was dusty
Visible, so not a cosmetic fix

Reinforcing fabric (polyester mesh)

Best for: Embedding in cement or coating over splits, seams and flat-roof blisters. Check: Roll width to suit the seam and compatibility with the cement or coating you use.

Pros

Stops the repair cracking along the joint
Spreads stress over a wider area
Cheap per foot

Cons

Adds a step: base coat, fabric, top coat
Only as good as the material it is embedded in

Which product for which leak

Matching the repair product to the leak
Leak	Use	Why
Gap behind step or counter-flashing on a shingle roof	Plastic roof cement (or sealant at a reglet)	Thick paste beds the flashing; sealant suits a narrow reglet joint
Active drip during a storm	Wet-surface roof patch	The only type designed to bond through water
Cracked collar around a vent pipe	Roof sealant cartridge — then replace the boot	Elastic bead follows the pipe's movement; a perished boot needs replacing
Exposed nail heads and lifted tabs	Plastic roof cement	A dab under the tab and over the head holds the shingle down
Pinhole or split in metal roofing or flashing	Butyl seal tape or a metal-rated sealant	Bridges the hole; asphalt cement stains and cracks on metal
Split seam on built-up or modified-bitumen roofing	Roof cement with reinforcing fabric	Fabric stops the patch cracking along the seam
EPDM, TPO or PVC membrane	The membrane maker's own sealant or patch kit	Asphalt products degrade single-ply membranes

If the same flashing leaks again after a patch, the flashing is the problem, not the sealant. The chimney flashing calculator and the flashing calculator size a proper replacement.

Roof cement vs roof sealant at a glance

Roof cement compared with cartridge sealant
	Roof cement	Roof sealant
Form	Thick paste, trowel or putty knife	Cartridge for a caulk gun
Base	Asphalt (bitumen) with fibers	Rubberized asphalt, tripolymer, polyurethane or silicone
Flexibility once cured	Low to moderate	High
Fills wide gaps	Yes	No — narrow joints only
Wet surfaces	Only wet-patch grades	Only grades labelled for damp surfaces
Single-ply membranes	No	Only products listed for that membrane

How to make a patch last

Clean first: brush off granules, dirt and old flaking cement. New material does not bond to loose material.
Dry the area unless you are using a wet-surface product; a heat gun or a sunny afternoon both help.
Spread cement about 1/8 in thick and feather the edges. A thick heap cures slowly inside and cracks as it shrinks.
On seams and splits, embed reinforcing fabric between two thin coats.
Pull and reseat popped nails before sealing over them — cement over a raised nail head leaks again.
Write the date on the patch in your maintenance notes and check it every spring.

Working out how much flashing a valley or wall needs instead? See the step flashing calculator and the drip edge calculator.

Safety notes

Do not go onto a wet or icy roof to patch a leak; contain the water from inside (bucket, tarp in the attic) and repair when it is dry, or use a wet-surface product from a secured ladder.
Wear gloves and eye protection — asphalt cement is hard to remove from skin, and solvent-based products give off fumes in an enclosed attic.
Use fall protection on any roof steeper than about 6/12 or higher than one storey; see the roof pitch calculator to check the slope.

Frequently asked questions

What is the difference between roof cement and roof sealant?

Roof cement is a thick asphalt paste applied with a trowel to bed flashing and fill gaps on shingle and built-up roofs. Roof sealant comes in a caulk cartridge, is thinner and stays more elastic, so it suits narrow joints that move, such as around vent pipes and along counter-flashing.

Plastic roof cement vs wet patch — which should I buy?

Plastic roof cement for planned repairs on a dry roof; it is cheaper and is the standard product. A wet-surface patch is for emergencies: it is formulated to bond to a damp or wet surface, so it can stop a live leak until a proper repair.

Can I use roof cement on a wet roof?

Only a product labelled for wet surfaces. Standard plastic roof cement needs a dry surface and will not bond properly to a wet one.

Will roof cement work on a rubber roof?

No. Asphalt-based cement attacks EPDM and most single-ply membranes. Use a sealant or patch kit made for that membrane.

How long does a roof cement repair last?

It depends on sun exposure, movement and how well the surface was prepared. A clean, thin, fabric-reinforced patch at a joint lasts far longer than a thick smear over a failed detail; check patches every spring and redo any that have cracked.

What is the best sealant around a roof vent pipe?

A flexible roof sealant from a cartridge, applied to a clean collar, as a temporary fix. Rubber pipe boots perish with age, so the lasting repair is a new boot or a lead or metal flashing sleeve.