The short answer
There is no single minimum pitch for roof windows. Each manufacturer sets a pitch range for each model, and the flashing kit you choose has its own range too, so the answer is on the data sheet for the exact product. The roof covering around the window has its own minimum pitch as well. On low pitches and flat roofs, different products are made for the job.
Why the answer sits on a data sheet
Roof pitch is how steeply a slope rises, given in degrees above horizontal. A roof window is a tilted piece of glazing, and how well it sheds water depends on that tilt. Too shallow and rain lingers on the glass and at the flashing; too steep and the window may not be designed to open in the way you expect.
Window makers publish the range of pitches for which each model is approved, often with different ranges for the window and for the flashing that finishes it. Those figures are the ones to follow, because the maker has tested the product at those angles and will stand behind it only inside them.
A single figure would be wrong for part of the market. A top-of-the-range model and a basic one can carry different ranges, and a maker can change them between generations. The model reference stamped on the old frame, or the label on the new one, names the exact product to look up, and that is where we start on every survey. Quoting a number from memory is how a window ends up in a roof it was never made for.
The covering has its own limit
A window sits in a roof, and the roof covering around it must also suit the pitch. Tiles and slates are tested for rainproofing at minimum angles, and the figures vary by product. Marley's published minimums show the spread: its Plain tile needs 35 degrees, its Double Roman 22.5 degrees and its Mendip 15 degrees in the smooth finish. For natural slate, BS 5534 indicates a minimum roof pitch of 20 degrees.
These limits are about the covering, not the window, and they are checked separately. A pitch that suits a window can still be too low for the tiles that run up to it, and the other way round. The product data sheet for the tile or slate is the place to look.
On a low roof the flashing carries more water than on a steep one, so the bottom flashing and the apron matter more. It is a poor place to economise. Our answer on roof window flashing kits explains what the surround does and why it must match both the window and the covering.
Options when the pitch is low
Where a roof is shallower than a standard window allows, manufacturers make alternatives. Some offer windows or fittings designed for low pitches, and some reduce the opening angle to suit the roof. The range on offer changes by maker and range, so get the data sheet and check the pitch band stated for each item on your shortlist.
Raising the window is another route. A kerb, a raised upstand fitted on the roof, can lift the frame and tilt it, but that moves the frame above the covering and alters the look from the street. In a conservation area it is worth checking that first, and our answer on conservation roof windows covers the planning side.
A third option is to accept that a conventional roof window is wrong for the slope. A low-pitch roof deserves a calm look at what the room needs: more light, ventilation, or both. Sometimes the right answer is a different kind of rooflight, and sometimes a small change to the roof itself.
When the roof is flat or nearly flat
A flat roof is not zero degrees. BS 6229:2025 is the code of practice for roofs pitched at no more than 10 degrees, and for those roofs the usual choice is a purpose-made flat-roof rooflight on an upstand rather than a standard roof window. These sit above the waterproof membrane, usually on a kerb, so water cannot pool around the frame.
Our page on a flat roof skylight goes into the options. The Part L value for a rooflight is 2.2, which is not the figure asked of a roof window: that is 1.4, with a Window Energy Rating of Band B as the other route.
The pitch of a roof in between is the hardest. On a roof with a gentle slope, a few degrees can be the difference between a standard window being allowed and ruled out. The pitch is measured at survey stage, with a digital level on a rafter, and we explain what the reading means for the products open to you. You can see the full range of work under roof windows and skylights.
