What is a slope (fall)?
A slope (or fall) is a slight gradient applied to a surface so that water drains toward a desired point. Without proper slope, water pools on flat surfaces such as flat roofs, balconies, terraces and garage floors. Standing water causes leaks, algae growth and accelerated wear.
How much slope is needed?
| Surface | Minimum slope |
|---|---|
| Flat roof | 1.5-2% (1.5-2 cm per metre) |
| Balcony/terrace | 1-2% |
| Garage floor | 1-1.5% |
| Shower | 1-2% toward the drain |
| Window sill | 3-5% (away from the building) |
| Gutter | 3 mm per running metre |
Creating a slope
On a flat roof
- Tapered insulation boards (PIR) — Insulation boards with a built-in gradient. The thickness varies across the board (thick on one side, thin on the other). Most common in new builds.
- Tapered rafters — The roof joists are cut at an angle so the roof deck automatically has fall.
- Sand bed — During renovation: a layer of sand or lightweight concrete with fall beneath the roofing membrane.
On a concrete floor (garage)
- The slope is cast into the concrete slab: the formwork on the door side is set slightly lower.
- Or afterwards: a layer of levelling screed applied with fall.
On a deck or terrace
- Place the support joists at slightly different heights.
Common mistakes
- No fall on a flat roof — Water pools, accelerating EPDM/bitumen wear and increasing the risk of leaks
- Slope in the wrong direction — Water runs toward the wall instead of toward the gutter
- Too little slope — Over time, deflection of the structure can negate the minimal fall
Related terms
- Flat roof
- EPDM
- Gutter
- PIR insulation
- Expansion joint