Why Every Terrace Needs a Slope
Water is the greatest enemy of any terrace. Without sufficient slope, puddles form that lead to algae growth in summer and frost damage in winter. Standing water penetrates joints and pores of the surface material, freezes, and breaks the stone or concrete from the inside. Repair costs quickly exceed the cost of correct installation.
According to DIN 18195, a terrace must have a minimum slope of 1.5%. Most professional contractors recommend 2% as the ideal compromise between reliable drainage and comfort. More than 2.5% is usually unnecessary and can cause garden furniture to slide on smooth surfaces.
How to Calculate Terrace Slope
The calculation is straightforward: Slope (%) = Height Difference (cm) / Distance (m). For a 5 m deep terrace at 2% slope, you need 10 cm of height difference. This means: the terrace edge at the house is 10 cm higher than the edge facing the garden.
During planning, use the door threshold as your starting point. The terrace must be at least 15 cm below the door threshold (splash water protection). From this point, the terrace drops at 2% toward the garden.
Practical example: Door threshold at 100 cm above ground level. Terrace at house: 85 cm (15 cm splash protection). Terrace at garden edge (5 m away): 85 - 10 = 75 cm. Total height difference from threshold to terrace edge: 25 cm.
Which Surface Material Requires Which Slope?
Not every terrace surface has the same requirements. Concrete slabs and natural stone with closed surfaces need the full 2% slope, as water stands on the surface and drains only across the slabs.
Timber and WPC decking on subframes need less slope (1.5% is often sufficient) because most water drains through the gaps between boards. The slope is created in the subframe, not in the boards themselves.
Gravel and crushed stone beddings need the slope in the subbase (load-bearing layer), not in the bedding material. The crushed stone itself is drainable, but without slope in the subbase, water pools beneath the surface.
Pedestal systems allow the subbase slope to remain while laying slabs perfectly level. This is visually appealing but more expensive and technically complex.
Common Mistakes in Terrace Drainage
The most common mistake: building the slope toward the house wall instead of away from it. This causes rainwater to run along the house wall and penetrate the basement.
Another mistake: the slope is planned correctly but not checked during installation. Irregularities in the substrate can create local depressions where water collects, even though the overall slope is correct.
Third mistake: no drainage at the lower edge of the terrace. Water flows off the terrace but backs up against the curb or lawn edge. A gravel channel or drainage pipe at the lowest point solves the problem.
Checking Slope: The Spirit Level Method
To check the slope of an existing terrace, place a 2 m spirit level on the surface. Lift the higher end (toward the house) until the bubble is perfectly centered. Then measure the gap between the lifted end and the terrace surface.
At 2% slope, the gap is exactly 40 mm over 2 m length. If the gap is less than 30 mm, the slope is too shallow. If it exceeds 50 mm, the slope is unnecessarily steep.
Check at multiple points across the terrace, not just one. The slope must be consistent. Local depressions or rises indicate problems with the subbase.