Why Do They Put Gravel on Flat Roofs?

The direct answer

Gravel is there to protect or to hold down. Built-up roofs use a surfacing of gravel set into hot asphalt as a protective wearing layer. Ballasted roofs use heavier, loose river stone laid over a membrane that isn't fully glued or fastened, so the stone's weight keeps the roof in place against wind uplift.

Built-up roofs (tar and gravel)

A built-up roof (BUR) is made of several layers of roofing felt alternated with asphalt or coal tar, finished with a flood coat into which gravel is embedded. The gravel shields the bitumen from UV, which otherwise dries and cracks it, and it helps resist foot traffic, hail and fire spread.

Over time, wind, water and traffic can scour gravel away and leave bare patches, exposing the asphalt to sunlight. Bare spots, blisters and cracked flashings are typical issues on older built-up roofs.

Ballasted single-ply roofs

On a ballasted roof, a single-ply membrane such as EPDM or TPO is loose-laid over insulation, and larger stone or pavers are spread on top. The ballast resists wind uplift and protects the membrane from UV. The roof deck and structure must be designed for that weight.

Because the stone is doing a structural job, moving it — to shovel snow, reach a leak or install equipment — can leave areas vulnerable to wind. Ballast also hides the membrane, which makes leaks harder to find.

Not all gravel roofs are the same

• Embedded gravel (built-up): small aggregate set in asphalt; part of the waterproofing system's surface.
• Loose ballast (single-ply): larger, round stone or pavers; holds the roof down by weight.
• Gravel stops and edges: the metal edge details that keep gravel on the roof also need to be watertight.
• Modified bitumen cap sheets have factory-applied granules — a different material altogether, not gravel ballast.

How a professional inspects a gravel roof

Inspecting a gravel-surfaced roof takes more care than a bare membrane because the waterproofing is hidden. A roofer will usually start at the details, where most leaks begin: perimeter gravel stops, wall and parapet flashings, drains, and penetrations. On built-up roofs they look for bare spots, blisters, alligatored asphalt and cracked flashings. On ballasted roofs they check for wind-scoured areas where the membrane is exposed, ballast displaced by trades, and drains clogged with stone.

When the source still isn't clear, small areas of ballast may be moved aside, inspected and put back, or a core sample or moisture survey may be used to find wet insulation. That work should be done by people who know which kind of roof it is and how to restore it afterwards.

Gravel also affects drainage. Loose stone can migrate into drains and scuppers, and debris builds up among the aggregate. Strainers and drain guards help, but they need periodic professional clearing — especially before winter in Ottawa.

Why you shouldn't move the gravel yourself

Shifting ballast can expose a membrane to wind and sunlight; scraping embedded gravel can damage the asphalt beneath. Roof access is also dangerous, especially in winter. If water is getting in, protect belongings inside, note where and when it appears, and have a professional inspect the roof. They can move and replace ballast correctly while finding the leak.

Repair or replace a gravel roof?

Localized problems — a cracked flashing, a blister, a bare area — can often be repaired with compatible materials. When an older built-up roof has widespread blistering, wet insulation or recurring leaks, replacement is usually considered, sometimes with a different system. Replacement means removing the gravel, so disposal and weight are part of the scope.