What Is a Lean-To Roof?
A lean-to roof (German: Pultdach) is a roof with only one sloped surface. Unlike a gable roof with two sloped surfaces, the lean-to roof rises from one eave side to the opposite wall. The result is an asymmetric roof shape reminiscent of a tilted lectern.
Lean-to roofs are very popular in modern architecture. They enable large window areas on the high side, offer excellent opportunities for solar installations, and can be designed to look very striking.
Pitch Calculation for Lean-To Roofs
With a lean-to roof, the rafter spans the entire building width (not just half as with a gable roof). Ridge height above eave is: Ridge height = building width x tan(pitch angle).
For a 10 m wide building at 15 degrees: Ridge height = 10 x tan(15) = 10 x 0.268 = 2.68 m. Rafter length: 10 / cos(15) = 10 / 0.966 = 10.35 m. With 50 cm overhang: (10 + 0.5) / cos(15) = 10.87 m.
Lean-to roof rafters are significantly longer than those of a gable roof with the same width and pitch. This makes the construction more expensive and requires stronger cross-sections or intermediate supports.
Typical Pitches for Lean-To Roofs
Lean-to roofs are built across a wide range of pitches. Shallow lean-to roofs at 5 to 10 degrees are popular for cubic buildings and are frequently covered with standing seam metal or EPDM.
Medium-pitched lean-to roofs at 15 to 25 degrees offer a good compromise: they allow clay tiles (from 22 degrees) and provide enough height for a row of windows on the high side.
Steep lean-to roofs above 25 degrees are rare because the height difference between eave and ridge sides becomes very large. For a 10 m wide building at 30 degrees, the ridge height is already 5.77 m above the eave.
Roof Area and Drainage
Lean-to roof area is calculated simply: rafter length (with overhang) x (building length + 2 x side overhang). For our example (10 x 12 m, 15 degrees, 50 cm overhang): 10.87 x 13 = 141.3 sqm.
Drainage occurs at the eave side (the low side). A single gutter at the eave suffices for the entire roof area. This is an advantage over the gable roof, which requires two gutters.
However, the eave gutter must be larger since it collects water from the entire roof area. For 141 sqm of roof area with a rainfall intensity of 300 l/(s x ha), the gutter must handle approximately 4.2 l/s.
Advantages and Disadvantages of the Lean-To Roof
Advantages: Simple construction, excellent for solar panels (south-facing surface), large windows possible on the high side, modern appearance, only one gutter needed.
Disadvantages: Longer rafters than a gable roof, one-sided wind exposure can be problematic, less attic space than a gable roof of the same footprint, higher stress on the high wall from wind forces.