Tar-and-Gravel Roofs: Repair, Re-cover or Replace?

What a tar-and-gravel roof is

A built-up roof is made of several layers of roofing felt, bonded together on site with hot bitumen. Four plies is the most common build. The bitumen is either asphalt, made from petroleum, or coal-tar pitch, made from coal. A heavy top coat of hot bitumen, called the flood coat, goes on last, and gravel is pressed into it while it is still hot.

The gravel is not decoration. It shields the bitumen from the sun, protects the felts from foot traffic and impacts, helps the roof resist fire, and adds weight that holds the felts down in high winds.

How long they last, and how they fail

There is no single number. A 1985 National Research Council Canada paper put the expected life of a flat roof at 20 years, and noted that a substantial number fail before they get there. A 1961 NRC digest on built-up roofing adds that regular inspection, repair and resurfacing can add many years. What wears these roofs out is sun and heat, which slowly harden the bitumen until it turns brittle. Standing water speeds that up: the digest says water or ice plus sunshine does far more harm than either alone. That matters on a flat roof that spends months under snow and ice. Watch for:

• Blisters. Raised, spongy spots where air or moisture is trapped between layers. They swell on hot, sunny days. Many small ones never leak.
• Alligatoring. The sun hardens the bitumen until it cracks into a pattern like an alligator’s hide. Left alone, it curls up and leaves the felts bare.
• Bare spots. Wind and water wash the gravel away and leave the bitumen and felts exposed to the sun.
• Ponding. Water that sits after a rain or a thaw speeds up every other kind of wear.
• Splits. Cracks and tears that open when the deck or insulation moves more than an aging, stiffer membrane can take.
• Flashing failures. Where the roof turns up a parapet, curb or wall, or meets a drain, the flashing can come loose, wrinkle or crack, and water gets in behind it.

When a repair makes sense

A repair is worth doing when the damage is local and the rest of the roof still has life in it. The Canadian Roofing Contractors Association (CRCA) guidance is practical:

• Small blisters that are keeping water out are best left alone. A 1996 CRCA bulletin suggests marking one once it reaches about 2 feet across, and repairing it at about 5 feet, or sooner if it sits in a traffic path or has cracked.
• Bare spots are cleaned and new gravel is set in hot bitumen or roof cement.
• Alligatoring needs primer, reinforcing felts and a coating. More hot bitumen on top rarely helps, because the old layer keeps shrinking and cracking underneath.
• Loose flashings are cut back to solid material and rebuilt.

Each repair starts by scraping the gravel off at least 18 inches past the damage, so the patch bonds to clean membrane. If water has got in, the wet insulation underneath comes out too. Your plans matter as well: an NRC paper on reroofing notes that patching a poor roof can make sense if you will sell or demolish soon, but warns against spending heavily on repairs that only buy short-term relief.

What a re-cover involves on a gravel roof

A re-cover puts a new roof over the old one. It avoids tear-off and disposal, keeps the old insulation, and lowers the risk of leaks during the work. On a tar-and-gravel roof, it comes with conditions:

• The gravel has to go. CRCA says old gravel roofs should be scraped until the surface is smooth, with no sharp stones left to cut the new membrane.
• The old roof has to be dry. CRCA says new roofing should never go over wet insulation, or over a membrane that is badly blistered, buckled or water-soaked. Finding out takes a moisture survey confirmed by test cuts.
• The structure has to carry it. Gravel is heavy: CRCA specifies at least 400 pounds per 100 square feet, about 4 pounds on every square foot, and the 1961 NRC digest’s double-pour specification called for about 500. The NRC reroofing paper says a re-cover only works if the deck and framing can take the added load, and suggests checking the original design against today’s snow loads.
• The two roofs have to be compatible. CRCA warns that most thermoplastic membranes, the family TPO belongs to, break down in contact with bitumen, so a separation layer such as a thin insulation board goes between them.

Coal tar and asbestos: find out before anyone tears it off

Two things in an old built-up roof change how the job has to be done, and neither can be confirmed by looking.

Coal-tar pitch. Asphalt and pitch do not mix. The Roofing Contractors Association of British Columbia explains that asphalt laid over pitch softens and flows off while the pitch hardens and cracks, so re-roofs over pitch often use a separation layer. Pitch is also a health hazard. The International Agency for Research on Cancer classes coal tar and coal-tar pitch as known human carcinogens, and CAREX Canada reports that roofers and shinglers are the largest group of Canadian workers exposed to them. Industry guidance from NRCA and the asphalt roofing manufacturers warns that coal tar is easy to mistake for asphalt, and tells contractors to identify what is in a roof before removing it.

Asbestos. An NRC reroofing paper notes that built-up roofs installed before September 1990 often used asbestos felts in the membrane or the base flashings, and that roof cement of that era usually contained it too. Manitoba’s Asbestos Management Guide lists roofing felts and mastics among known asbestos-containing materials, and says suspect materials must be handled as asbestos until a lab test shows otherwise.

Under Manitoba’s rules, owners, employers and contractors share responsibility. Before work that could disturb suspect material, a qualified person has to prepare an asbestos inventory. Since December 5, 2025, the province must be notified five to ten days before work to alter, renovate or demolish a building that contains asbestos. From June 1, 2027, asbestos workers will need certification and their employers will need to be registered.

What replaces a tar-and-gravel roof

A full tear-off is the cleanest fix. CRCA notes that stripping the old roof and flashings lets the crew inspect the deck and parapets and correct any damage. Today’s common replacements are:

• Modified bitumen (SBS). Asphalt made more flexible and weather-resistant with polymers, usually installed as two plies: a base sheet and a cap sheet.
• TPO. A single-ply thermoplastic membrane with heat-welded seams. With PVC, it is the most widely available thermoplastic roof membrane in Canada.
• EPDM. A single-ply synthetic rubber membrane, with seams joined by adhesive or tape.

Oakwood does flat roof repairs and installs all three of these systems, and our sheet metal shop makes the flashings, parapet caps and scuppers that finish the edges. In Winnipeg, plan a tear-off for the warm months: CRCA notes that cold-weather roofing needs extra care because days are shorter and hot asphalt cools faster. Get the asbestos inventory done well before then.

Repair, re-cover or replace: a quick guide

1. Repair when the damage is in one area — a blister, a split, a bare patch, a loose flashing — and the rest of the roof is dry and holding up.
2. Consider a re-cover when the old roof is dry, the membrane is sound, the structure can carry the new load, and the gravel can be fully scraped off.
3. Replace when the insulation is wet across large areas, the membrane is badly blistered or cracked, the deck needs work, or repairs keep coming back.
4. Before any tear-off, get an asbestos inventory if the roof may be older than 1990, and find out whether the bitumen is asphalt or coal-tar pitch.
5. Whatever you choose, find out why the old roof failed. The NRC reroofing paper warns that a new roof that does not deal with the cause may fail early too.