Slope (or fall) is a slight gradient applied to a surface so that water drains towards a desired point. Without slope, water pools on flat surfaces such as flat roofs, balconies, patios 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/patio | 1-2% |
| Garage floor | 1-1.5% |
| Shower | 1-2% towards the drain |
| Window sill | 3-5% (away from the building) |
| Gutter | 3 mm per running metre |
Creating 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 commonly used in new builds.
- Angled-cut joists — The roof joists are cut at an angle so that the roof deck automatically has fall.
- Sand bed — During renovation: a layer of sand or lightweight concrete with slope under the roof covering.
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 mortar with fall.
On a deck or patio
- Place the support beams at slightly different heights.
Common mistakes
- No slope on a flat roof — Water pools, accelerates wear on EPDM/bitumen, risk of leaks
- Slope in the wrong direction — Water runs towards the wall instead of towards the gutter
- Too little slope — Over time, structural deflection can negate the minimal slope
Related terms
- Flat roof
- EPDM
- Gutter
- PIR insulation
- Expansion joint
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