Roof Calculator 2026

Roof pitch describes the angle of the roof surface relative to the horizontal. It is expressed in degrees, percent or as a ratio. 45 degrees equals 100% slope (ratio 1:1). In Germany, pitches between 20 and 45 degrees are common.

Rafter length is calculated from half the building width (for gable roofs) and the pitch angle: rafter length = half width / cos(angle). With overhang: rafter length = (half width + overhang) / cos(angle). For a 10 m wide building at 35 degrees with 50 cm overhang: (5 + 0.5) / cos(35) = 6.71 m.

For a gable roof: roof area = 2 x rafter length x (building length + 2 x overhang). The rafter length includes the eave overhang. Roof area is always larger than floor area since the sloped surface is longer than the horizontal projection.

Germany is divided into 5 snow load zones (Zone 1 to 3). Northern German lowlands are typically in Zone 1, central mountain ranges in Zone 2, Alpine foothills in Zone 2a or 3. The exact assignment follows DIN EN 1991-1-3 Annex NA. Your local building authority can provide guidance.

Clay and concrete roof tiles require a minimum pitch of 22 degrees. Slate needs 25 degrees, asphalt shingles 15 degrees, standing seam metal just 7 degrees and trapezoidal metal 5 degrees. EPDM membranes can be used from 2 degrees. Below the prescribed minimum, leaks may occur.

In a gable roof, the roof surfaces end vertically at the gable sides, creating triangular gable walls. In a hip roof, the gable sides are also sloped (hipped), so no vertical gable walls exist. A hip roof therefore has more roof area but less wall area and is more wind-resistant.

Roof overhang protects the facade and base area from driving rain and direct sunlight. Typical overhangs are 30 to 80 cm. A larger overhang provides more protection but requires longer rafters. On the gable side, the overhang is called the rake.

A roof is considered flat at a pitch below 5 degrees (approx. 8.7%). Most flat roofs have a pitch of 2 to 3 degrees for drainage. Between 5 and 20 degrees, it is called a low-slope roof.

Easiest with a digital spirit level on a rafter or roof tile. Alternatively, measure the horizontal run and the rise in the attic and calculate the angle: arctan(rise / run). Smartphone apps with inclinometers also work.

Costs vary greatly by material: concrete tiles 20-35 EUR/sqm, clay tiles 25-50 EUR/sqm, asphalt shingles 15-30 EUR/sqm, standing seam metal 50-90 EUR/sqm, slate 80-140 EUR/sqm. These prices include material and installation but not battens and underlay.