Walk into a hardware store and ask for roofing tar, and you will be handed a can of asphalt-based cement that contains no actual tar at all. True coal-tar pitch and the asphalt products that borrowed its name are different chemicals with different strengths, different temperatures, and very different safety profiles — and knowing which one you are holding determines whether your patch lasts one winter or fifteen years. This guide separates the real tars from the asphalt look-alikes, walks through hot-mop and cold-patch application, and flags the situations where tar is flatly the wrong answer.
Coal-Tar Pitch vs. Asphalt: Two Different Materials
Coal-tar pitch is a byproduct of baking coal into coke for steelmaking. Asphalt (bitumen) is the bottom fraction of petroleum refining. Both are black, thermoplastic, and waterproof, but the resemblance ends there:
- Coal-tar pitch has a low softening point (around 140-155°F for Type III) and remains slightly mobile for its entire service life. That “cold flow” is its superpower — minor cracks and punctures literally self-heal as the pitch slumps back together. It is also nearly immune to standing water, which is why dead-flat roofs with chronic ponding were traditionally coal-tar built-up roofs. Lifespans of 30-50 years are documented.
- Asphalt tolerates slopes and heat better, costs less, and dominates the market. Everything sold in cans and tubes as roof “tar” — plastic roof cement, flashing cement, wet-patch — is asphalt cut with solvents and fibers.
The trade-offs: coal tar’s cold flow means it cannot be used above about 1/2-in-12 slope or it migrates downhill; its fumes are aggressively irritating and classified as carcinogenic, so crews wear respirators and skin protection; and it is incompatible with asphalt — mixing the two creates a soft, unstable mess. Never patch a coal-tar roof with asphalt cement or vice versa. Rub a scrap: coal tar smells sharply of mothballs (naphthalene) and shows a greenish-yellow sheen in sunlight.
Hot-Mop Built-Up Roofing: Where Tar Earned Its Name
The classic “tar and gravel” roof is a built-up roof (BUR): three to five plies of felt bonded with hot-mopped bitumen and finished with a flood coat and gravel. The process on a coal-tar job runs like this: a kettle on the ground heats pitch to its equilibrium viscous temperature — roughly 360-375°F for coal tar, versus 425-475°F for Type III asphalt — and a pump or hoist moves it to the roof. Mopper spreads a 20-25 lb-per-square flood of hot bitumen; felt layers roll into it shingle-fashion with 2 inch laps; the final flood coat of 60-75 lb per square gets 400 lb of pea gravel broadcast into it while molten. The gravel is not decoration — it shields the pitch from UV and foot traffic and provides fire resistance.
Hot-mop coal tar is now a specialty trade. Only a handful of manufacturers (Durapax being the main U.S. supplier of coal-tar products) still produce pitch and coal-tar felts, and many roofers have never mopped it. If you own a coal-tar BUR, find a contractor who works with it before you need one; a sound coal-tar roof restored in kind will outlast almost any replacement.
Patch Repairs with Tar Products
For homeowners, “roofing tar” work usually means cold-applied asphalt cement out of a can. The products, by task:
- Plastic roof cement (ASTM D4586): Trowel-grade, fibered. For flashing repairs, sealing around vents, and bedding metal. Henry 208 or Black Jack All-Weather, about $10-$15 per gallon.
- Wet-patch cement: Formulated to grip damp or even underwater surfaces — the leak-in-progress product. Henry 208R Wet Patch is the standard emergency call.
- Flashing cement in caulk tubes: For nail pops and small punctures; faster than opening a can.
- Fibered aluminum or emulsion coatings: Brushed full-surface coats to extend a tired BUR or modified roof, $30-$60 per 5-gallon pail covering 250-350 sq ft.
A proper patch sequence: dry and brush the area, cut away blistered material, trowel cement 1/8 inch thick extending 2-3 inches past the damage, embed polyester or fiberglass reinforcing fabric, then trowel a second 1/8 inch coat over the fabric. The three-course patch outlasts a glob of goop by years. A common failure is troweling cement an inch thick in one pass — thick masses skin over, never cure through, and crack.
Temperatures, Cure, and Working Conditions
Cold-applied asphalt cements are solvent-cured: they skin in 2-8 hours, resist rain within a day, but take weeks to months to fully harden, and they shrink slightly as solvent leaves. Apply between 40°F and 90°F for workable trowel consistency; in cold weather, store the can indoors overnight so it spreads instead of tearing the felt. Hot-applied bitumens cure instantly on cooling, which is why hot work produces stronger laminations — but kettle temperatures demand training. Overheating asphalt past its flash point (about 550°F) or letting a kettle run unattended causes fires; hot bitumen burns are deep and the material sticks to skin. OSHA expects gauntlets, face shields, long sleeves, and a kettle attendant at all times.
Coal-tar fume exposure deserves its own line: volatiles cause photosensitized “tar burns” on skin exposed to sunlight, and long-term exposure is linked to skin and lung cancer. Crews mop pitch in full cover-up even in July.
Safety, Mess, and Cleanup
Even the homeowner-grade products are punishing to work with. Wear nitrile gloves you can throw away, dedicated clothes, and kneepads; asphalt cement does not wash out of anything. Clean tools and skin smears with mineral spirits or a citrus degreaser, never gasoline. Keep solvent-based cement away from open flame — the fumes are flammable until cured. On the roof itself, remember that fresh cement stays soft: do not walk a patch for at least a day, and never smear cement across a walkway area where it will track on shoes into the house. Foil the mess in advance: cheap chip brushes and margin trowels you discard afterward beat cleaning good tools.
When Tar Is the Wrong Answer
- On single-ply membranes. Asphalt cement chemically attacks EPDM rubber and does not bond to TPO or PVC. Patching a rubber roof with roof tar ruins the membrane and voids any warranty; use manufacturer seam tape or weldable patches.
- On shingle field surfaces. Smearing tar over a leaking shingle area traps water beneath and telegraphs black stains. Replace the shingles; reserve cement for flashing details.
- As a substitute for flashing. Tar bridging a wall-to-roof joint fails in 2-5 years as it embrittles in UV. Metal flashing is the repair; cement is the gasket behind it.
- Mixing chemistries. Asphalt products on coal-tar roofs (and the reverse) soften both materials.
- In ponding water on asphalt systems. Standing water degrades asphalt cement; use wet-patch only as a stopgap, then fix the drainage.
How Long Tar Repairs Actually Last
Set expectations by product and detail. A three-course fabric-reinforced cement patch on a BUR or modified bitumen field is a genuine 5-10 year repair, often outliving the surrounding roof. A caulk-tube dab on a nail pop lasts 2-4 years before UV embrittles it — put those details on a recheck calendar. Fibered aluminum coatings buy 4-6 years per coat on a tired flat roof and drop surface temperatures 20-40°F, which slows the aging of everything beneath. Emergency wet-patch smears should be considered 1-year bridges to a proper repair, not repairs themselves. And a full hot-mopped flood coat with new gravel on a structurally sound BUR is a legitimate 10-15 year restoration at a third the price of replacement — the option most homeowners with old tar-and-gravel roofs never hear about because so few local contractors still run kettles. Ask specifically for a BUR restoration quote before accepting a tear-off number.
The Bottom Line
Roofing tar covers two families: coal-tar pitch, a self-healing, ponding-proof material now confined to specialty flat-roof work, and the asphalt cements that inherited the name and handle 95% of everyday sealing and patching. Match chemistry to chemistry, use the three-course fabric method instead of thick blobs, respect the temperatures — 360-375°F mopping for coal tar, 40-90°F ambient for cold cement — and treat every tar product as the skin-and-lung hazard it is. Used within those lanes, a $12 can of roof cement is still one of the highest-value repair materials in the aisle.
