Before single-ply membranes took over, most flat roofs in Pittsburgh were built-up roofs: layers of felt and hot asphalt or coal tar, topped with gravel. Plenty of churches, schools, storefronts, apartment buildings and row houses still have one, sometimes under a newer roof. They can last a long time, but when one is near the end, the choice between repair, a recover and a tear-off matters a lot to the cost.
How a built-up roof works
A built-up roof is made of several plies of roofing felt, each laid in hot bitumen, with a flood coat on top and gravel embedded in it. The plies give it redundancy: one cracked ply doesn't mean a leak. The gravel protects the bitumen from sun and foot traffic. The weak points are the same as on any flat roof: flashings at walls and penetrations, drains, and anywhere water sits.
Signs a built-up roof is wearing out
- Bare spots where the gravel has washed or blown away, exposing the bitumen.
- Blisters and ridges in the surface.
- Cracking or alligatoring in exposed areas.
- Flashings at walls, curbs and drains splitting or pulling away.
- Soft, spongy areas underfoot: wet insulation or deck.
- Leaks in several places at once.
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Option 1: Repair
If the problems are at a few details and the roof is mostly dry, repair it. The gravel is scraped back, damaged plies are cut out, new plies are installed, and the area is resurfaced. Flashings are often rebuilt with modified bitumen. Blisters are cut, dried and patched. This is the right call for a roof with years left in it.
Option 2: Recover
A recover, also called an overlay, puts a new roof over the old one. It saves the cost of tear-off and disposal. Codes based on the International Building Code, which Pennsylvania's Uniform Construction Code adopts, generally allow it only when:
- there is only one existing roof covering;
- the existing roof and its insulation are not water-soaked or deteriorated;
- the deck is sound and can carry the added weight.
On a gravel roof, the loose gravel generally has to be removed, and a cover board is usually installed before the new membrane. Core cuts or a moisture survey decide whether the roof qualifies. A recover over wet insulation traps water and leads to early failure.
Option 3: Tear-off and replace
If the roof is wet across large areas, already has two layers, or the deck is damaged, it has to come off. Tear-off is also the time to add insulation to current code, fix slope and drainage, and rebuild every flashing. Built-up roofs are heavy and messy to remove, which shows up in the price. If the roof is old enough that its felts or cements might contain asbestos, test before tear-off.
What replaces it
Most built-up roofs are replaced with two-ply modified bitumen, which works on the same principle, or with EPDM or TPO single-ply. See EPDM vs. TPO vs. modified bitumen and what each costs.
The bottom line
Repair a built-up roof that is mostly dry, recover one that is a single dry layer on a sound deck, and tear off one that is wet or already recovered. Core cuts make that decision for you. See commercial roofing and flat roof replacement.