Roofing Cement, Tar, Caulk and Sealants: What to Use Where
The tube-and-trowel aisle causes more bad roof repairs than any product category, because the right sealant in the right place is craft, and the wrong one is a leak with a warranty problem. The field guide.
The first rule of roof goop
Sealants in roofing are gaskets, not structure: they finish and weatherproof details, nail heads, counter-flashing terminations, boot collars, small junctions, inside systems that shed or seal water by geometry, metal laps, membranes, shingle courses. The recurring disaster of the category is inversion: goop asked to do the roof’s job, cement smeared over a failed flashing instead of rebuilding it, tar troweled across a worn valley, caulk crowning a dead pipe boot. Those repairs work for a season, trap water behind themselves, and make the eventual real fix harder, the signature of the previous bad roofer on half the roofs we open. Everything below serves that rule: right product, right detail, never as a substitute for the roofing itself.
The product families
Plastic roofing cement (the black trowel goop)
Asphalt cutback mastic, the classic “roof tar”: thick, cheap, sticks to asphalt surfaces, embeds fabric for small mod-bit and shingle patches, and hardens brittle within a few years. Emergency and asphalt-family patching, never a finish material, never on metal seams or membranes.
Polyurethane sealant
The quality tube for roofing joints: tough, flexible, paintable, tenacious on metal, masonry and wood, the flashing-termination and coping-joint standard. NP1-class products are the trade staple.
MS-polymer / hybrid
The modern premium: urethane toughness with silicone-class weathering, adheres to nearly everything including damp surfaces, stays paintable, the best all-rounder for exposed exterior joints and the metal-roofing tube of choice.
Silicone, butyl and specialty
Pure silicone for glass and some boots (never under paint or future coatings, per the coating rules); butyl tape doing the hidden sealing inside metal laps and terminations; and the self-leveling lap sealants of the RV world.
What to use where
Exposed nail heads (ridge caps, final courses, exposed-fastener trim): a dab of urethane or MS-polymer, the legitimate tube use every roof has a few of. Counter-flashing reglets and terminations: urethane, tooled into the joint the metal already made, sealant finishing flashing, never replacing it. Metal laps and trim: butyl tape inside the joint at assembly; MS-polymer at cut ends and odd junctions, per the metal repair guide. Pipe boots: a failing boot gets a new boot; sealant only bridges the wait. Small punctures in asphalt surfaces: cement with fabric reinforcement as honest temporary or sacrificial-area patching, the flat-repair triage deciding what follows. Masonry-to-roof junctions: urethane at the counter-flashing, mortar repair for the masonry itself, chimney law from our chimney guide.
The misuse hall of fame
The patterns to refuse, on your roof or in a quote: tar over tar, recoating failed cement with more cement, each layer trapping more water and telegraphing the same crack; caulked valleys and flashings, geometry problems answered with chemistry, the most common disguise on storm-chaser “repairs”; silicone under future work, contaminating surfaces nothing will ever bond to again; cement on EPDM or TPO, petroleum attacking membrane chemistry, per the compatibility rules of the membrane guides; and the sealed-shut weep and vent, goop closing openings that existed to drain and breathe. A tube in honest hands finishes details; in dishonest ones it is the cheapest way to make a roof look repaired, which is why “they just caulked it” is the autopsy line of so many leak investigations. Ours carry the right chemistry for each surface, and the judgment about when the answer is not a tube at all, across our service areas.
Roofing sealant FAQs
What is the best roofing sealant?
By joint: polyurethane (NP1-class) for flashing terminations, copings and general roofing joints; MS-polymer hybrids as the premium all-rounder, especially on metal; butyl tape inside metal laps; plastic roofing cement only for asphalt-family patching with fabric; silicone only where nothing will ever need to bond afterward. Surface chemistry picks the tube, not brand loyalty.
Is roofing cement a permanent repair?
No, it is emergency and sacrificial-area patching for asphalt surfaces: it sticks well, then hardens and cracks within a few years. Permanent repairs rebuild the detail, new flashing, new boot, plied patch, with sealant in its gasket role. Layers of old cement on a roof are the fingerprint of deferred real repair, and they make the eventual fix harder.
Can I caulk a roof leak?
Only if the leak is genuinely a failed small joint, an exposed nail head, a dried termination bead, which a minority are. Most leaks are failed flashings, boots, seams or coverings, geometry problems that caulk disguises for a season while water reroutes behind it. Find the actual source (the leak guide craft), fix the detail, and let sealant finish, not perform, the repair.
What sealant works on metal roofs?
Butyl tape inside laps and seams at assembly, and MS-polymer or polyurethane at exposed cut ends, junctions and fastener heads, both tenacious on painted metal and tolerant of its thermal movement. Avoid plastic roof cement (corrosive mess on metal) and bargain silicones that peel and contaminate. Color-matched MS-polymer is the metal-roofing standard tube.
Why should silicone be used carefully on roofs?
Because nothing bonds to cured silicone, not paint, not coatings, not even fresh sealant other than more silicone, so a bead in the wrong place contaminates every future repair and restoration on that surface. It has legitimate lanes (glass, certain boots, silicone coating systems), but the default exterior tube for roofing joints is urethane or MS-polymer.
Roof full of old tar and mystery caulk?
We open the goop, find the details underneath, and rebuild them properly, sealant in its gasket role. Get a free assessment.
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