Gravel-Surfaced Roof (Flood Coat and Gravel)

A gravel-surfaced roof is the classic "tar and gravel" finish on built-up roofing. After the last ply is laid, the roofer pours a heavier layer of hot asphalt or coal-tar pitch, called the flood coat, and immediately broadcasts gravel or slag into it so the aggregate sticks.

Purpose

• UV protection: bitumen hardens and cracks in sunlight; the aggregate shades it.
• Mechanical protection: gravel takes the wear from hail, foot traffic and debris.
• Wind: the embedded aggregate helps the surface resist uplift.
• Weight: gravel surfacing typically adds about 15 to 20 kg/m2 (3 to 4 lb/sq ft).

Edges and gravel stops

At roof edges, sheet-metal gravel stops hold the aggregate in place. These edges are a known weak point: felts stripped in over gravel stops often split because they cannot absorb the movement of the metal, and modified bitumen strip-in membranes are often used instead.

Problems

Loose gravel can be moved by wind, water flow or foot traffic, leaving bare bitumen that then cracks or blisters. Wind scour at corners and perimeters is very common. Gravel also hides the membrane, making leaks harder to trace, and must be removed for repairs or reroofing.

Cool-roof variants

Where reflectivity matters, light-coloured aggregate such as marble chips or grey slag can replace dark gravel, or a reflective coating or cap sheet can be used instead of a flood coat.

Related systems

Loose aggregate is also used as ballast on single-ply and inverted roofs, but there the stones are loose-laid on top rather than embedded in bitumen. Spray polyurethane foam can also be protected with aggregate, which adds about 49 kg/m2 (10 lb/sq ft).