What Is Slope and How Is It Measured?
Slope (also called gradient, incline, or pitch) describes how much a surface or pipe is tilted relative to the horizontal. In construction, the correct slope is essential for draining terraces, roofs, and wet areas, as well as for properly laying pipes.
In German-speaking countries, slope is typically expressed as a percentage. It represents the ratio of height difference to horizontal distance. A 2% slope means: over 100 cm of horizontal distance, the surface drops 2 cm. This sounds minimal, but it is entirely sufficient for reliable rainwater drainage.
Besides percentages, two other common notations exist: degrees (the actual angle to the horizontal) and ratios (e.g., 1:50). For the small slopes common in building construction, percent and degrees are nearly identical: 2% equals 1.15 degrees.
Calculate Slope: The Formula
The basic formula could not be simpler:
Slope (%) = (Height Difference / Horizontal Distance) x 100
Both values must be in the same unit. In practice, distance is often measured in meters and height difference in centimeters, so the adjusted formula is:
Slope (%) = (Height Difference in cm / (Distance in m x 100)) x 100
Or simplified: Slope (%) = Height Difference (cm) / Distance (m)
Example: A terrace is 5 m long. At the far end, it should be 10 cm lower than at the house connection. The slope is: 10 cm / 5 m = 2%.
To calculate the required height difference, rearrange the formula:
Height Difference (cm) = Slope (%) x Distance (m)
At 2% slope over 8 m distance: 2 x 8 = 16 cm height difference.
And for distance calculation given height and slope:
Distance (m) = Height Difference (cm) / Slope (%)
Slope Standards by Application (DIN)
In Germany, various DIN standards regulate minimum and maximum slopes for different applications. Compliance with these standards is not only technically sensible but often legally required under building codes.
DIN 18195 covers building waterproofing and requires a minimum slope of 1.5% for terraces and balconies. The recommended ideal value is 2%. Below 1.5%, standing water can form, which damages waterproofing over time and leads to frost damage.
DIN 18534 addresses interior waterproofing and specifies a slope of 1.5% to 2% for walk-in showers. Less than 1.5% leads to standing water on tiles; more than 2% can make standing in the shower uncomfortable.
DIN EN 12056 covers gravity drainage systems within buildings. The minimum slope for drainage pipes depends on the nominal diameter: DN50 requires 2.5%, DN100 (the household standard) requires 1.5%, DN200 needs only 0.5%. Too little slope causes deposits and blockages; too much slope lets water flow too fast, leaving solids behind.
DIN 4095 addresses drainage for building protection and recommends a slope of 0.5% to 2% for drainage pipes.
DIN 18040-1 for accessible construction limits wheelchair ramp gradients to a maximum of 6%. Outdoors, many planners recommend a maximum of 4%. The maximum run length is 6 m, after which an intermediate landing of at least 1.50 m is required.
DIN 18531 covers flat roof waterproofing and requires a minimum slope of 2%, maximum 5%.
Terrace and Balcony: Planning Drainage Slope
When planning a terrace, slope is one of the most important factors. It must direct away from the building so that rainwater drains reliably. Standing water on a terrace leads to algae growth, frost damage in winter, and long-term destruction of the surface material.
For concrete slabs and natural stone, 2% slope is recommended. For WPC decking or timber terraces on subframes, 1.5% is often sufficient since water drains through the gaps between boards. For gravel or crushed stone bedding, the slope should be created in the subbase, not in the surface material.
Practical tip: For a 4 m deep terrace at 2% slope, the height difference is only 8 cm. This is barely visible to the naked eye but entirely sufficient for drainage. For longer terraces (e.g., 8 m), the height difference increases to 16 cm, creating a noticeable step.
Pipes: Minimum Slope by Diameter
When laying drainage pipes, the correct slope is critical. Too little slope causes solids to remain in the pipe, forming deposits and eventually causing blockages. Too much slope lets water flow so fast that it cannot carry solids along. There is an optimal window.
The most common household pipe diameters are DN50 (kitchen sink, washbasin), DN75 (bathtub), DN100 (toilet connection), and DN150 (collecting pipe). Minimum slopes are: DN50: 2.5%, DN75: 2.0%, DN100: 1.5%, DN125: 1.0%, DN150: 0.8%, DN200: 0.5%.
In practice, this means: A DN100 pipe must drop at least 1.5 cm per meter of length. Over a pipe run of 10 m, this yields a minimum height difference of 15 cm. This must be considered when planning basement and ground-level pipe runs.
Measuring Slope: Spirit Level and Straightedge
The most reliable method for measuring slope on site is combining a spirit level (or straightedge) with a measuring stick.
Step by step: Place a 2 m spirit level on the surface. Lift the higher end until the bubble is centered (the spirit level is now horizontal). Measure the gap between the lifted end and the surface in millimeters.
Then calculate: Slope (%) = Gap (mm) / Length of spirit level (mm) x 100. With a 40 mm gap on a 2 m spirit level: 40 / 2000 x 100 = 2%.
Conversely, you can pre-calculate the required gap: Gap (mm) = Slope (%) / 100 x Length (mm). For 2% on 2 m: 2/100 x 2000 = 40 mm.
Modern digital spirit levels display the slope directly in degrees or percent. This is more convenient, but the classic method with straightedge and measuring stick is cheaper and available on every construction site.