There is no single minimum pitch: it depends on the rib height, the slope length, whether there is a transverse overlap between sheets, the climate zone and the exposure of the building. In France, these values are set by the DTU 40.35 standard. In practice, a slope covered by a single sheet, without overlap or penetration, accepts the lowest pitches; as soon as there is an overlap, a roof window or a translucent sheet, the minimum pitch increases. For our products, the sheet of each panel states its minimum pitch: for example 8° (14%) for the T 26.
Why pitch matters so much
A trapezoidal metal sheet roof is not watertight because of its joints: it is watertight because water runs off faster than it can rise. If the pitch is too low, wind drives rain into the overlaps, water stagnates against the ribs and ends up passing under the sheet. The first signs are damp marks at the overlaps and fixings.
The 5 criteria that set the minimum pitch
- Rib height: the higher the rib, the more water the sheet can drain and the lower the pitch it tolerates.
- Slope length: the longer the slope, the greater the volume of water to drain at the bottom.
- Transverse overlap: when two sheets overlap along the slope, the joint is a weak point and the pitch must be steeper.
- Climate zone and exposure: a windy region or an exposed site (seaside, open plateau) needs more pitch than a sheltered site.
- Elements in the roof: roof windows, translucent sheets, vent outlets or chimneys require a steeper pitch.
What the French DTU 40.35 says
In France, DTU 40.35 (standard NF P34-205) covers roofing with ribbed sheets made from coated steel. Its tables give the minimum pitch according to the climate zone and the configuration: one table for the usual case, and another, more favourable one for a slope without transverse overlap, penetrations or translucent sheets. These values apply within a slope length limit: 40 m for ribs of at least 35 mm, 30 m below that.
Some DTU benchmarks: below a 7% pitch (about 4°), ridge tiles require additional sealing; between 5 and 7%, foam fillers are added; below 10% (about 5.7°), the top row of sheets has its edges turned up under a ridge with cut edges. With additional sealing, the transverse overlap is between 150 and 200 mm.
Pitch in % or in degrees?
Product sheets and drawings use both units. Pitch in % is the height gained per 100 cm of horizontal distance. Useful benchmarks: 7% ≈ 4°; 10% ≈ 5.7°; 14% ≈ 8°; 15% ≈ 8.5°; 27% ≈ 15°; 47% ≈ 25°.
And for metal roof tiles?
Roof metal roof tiles require steeper pitches, because water must cross each module step: from 9° to 25° depending on the model and on the made-to-measure or modular version. The exact value is given on the sheet of each model and in our general catalogue.
Our advice
- Check the actual pitch of the roof structure before choosing the sheet.
- If the pitch is low, prefer a higher rib and a single sheet over the whole slope.
- Beyond the maximum length of a sheet, plan an overlap that complies with the DTU, with additional sealing if necessary.
- If in doubt, send us the pitch, the slope length and the location of the site: we will guide you to the right profile.
Frequently asked questions
What is the minimum pitch of the T 26?
The minimum pitch of the T 26 (TP26 SUPER) is 8°, i.e. about 14%.
Can a trapezoidal metal sheet be laid at less than 5% pitch?
This is outside the usual scope of DTU 40.35. Such a flat roof calls for other techniques, for example waterproofing on a steel deck.
Does the pitch change with an overlap between sheets?
Yes. A transverse overlap requires a steeper pitch than a slope covered by a single sheet. To avoid it, our sheets are cut to the length of your slope, within the maximum length of the profile.
