A ballast layer is a layer of gravel, crushed stone, or other heavy material applied on a flat roof or beneath a foundation to add weight for stability or to keep the roofing membrane in place.
Function and application
The ballast layer serves different purposes depending on where it is used:
On flat roofs, the ballast layer holds the underlying roofing membrane (usually a synthetic or bituminous sheet) in place. Without ballast, the membrane can be lifted or displaced by wind. The gravel layer also protects against UV radiation, which significantly extends the lifespan of the roofing membrane.
Beneath foundations, a ballast layer of crushed stone or gravel is installed as a draining and stabilising sub-base. The material distributes pressure evenly across the subsoil and provides drainage.
Material and thickness
Commonly used materials for a ballast layer include:
- Gravel (16-32 mm) — the most common choice for flat roofs.
- Crushed stone — used primarily beneath foundations.
- Concrete pavers — sometimes used as an alternative to gravel on roofs, with the advantage of making the roof walkable.
The thickness of a ballast layer on a flat roof is typically 50 to 80 millimetres. The exact thickness depends on the local wind load and the weight the roof structure can support.
Key considerations
- Verify that the roof structure can bear the additional weight of the ballast layer.
- Use a filter fabric between the roofing membrane and the gravel layer to prevent blockage of the drainage system.
- Keep roof drains clear of gravel to prevent water accumulation.
Related terms
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