The short answer
There is no single figure for every roof window. Each model has its own pitch range on the manufacturer's data sheet, and the roof covering around it has a minimum pitch too, so the shallower of the two limits decides. Natural slate, for example, needs 20 degrees under BS 5534. Below the window's stated range, a flat rooflight or a different design is the usual answer.
How to find the pitch range of a roof window, why the tiles around it matter, and what to do if your roof is too shallow.
Why one number does not cover every window
Pitch means how steeply a slope rises from level, counted in degrees. A roof window is built to shed water at a certain range of angles, and the range depends on the model. A standard pivoting window, a top-hung window and a window made for very shallow roofs are not interchangeable, so a quoted minimum for one tells you nothing about another.
The place to look is the manufacturer's data sheet or installation instructions for the exact model and size. That sheet gives the angles the window is approved for, and the flashing kit that goes with it has to suit that range as well. Fitting one outside that range is a gamble with water, because rain will not clear the frame fast enough and the flashing may lift in wind-driven weather.
Tell us the model if you already own one, or the make and size if you are shopping. We measure the actual pitch on site rather than relying on the drawing, since an old roof can differ from the plan. A digital spirit level held along a rafter, or along the top edge of a batten, gives a more reliable reading than a guess.
How the roof covering sets its own limit
The window is only half of the detail. The tiles or slates around it must also be laid within their own minimum pitch, and that figure comes from the covering's maker. Marley's published table lists its Mendip 12.5 concrete tile at 12.5 degrees, its Mendip at 15 degrees and its Plain tile at 35 degrees. For natural slate, BS 5534 indicates a minimum roof pitch of 20 degrees,, with hook fixings ruled out on anything shallower than 25 degrees.
So a roof can be perfectly legitimate for its tiles and still be too shallow for a particular window, or the other way round. We look at both numbers together. If the covering is already doing its job and sits comfortably above its minimum, the window range becomes the limit.
Flashing is the metal or lead strip that closes the joint where frame meets covering, and the kit is chosen for the covering as well as the window. Our answer on Velux flashing kit types explains how the kits differ. Slate roofs need particular care, which we cover in fitting a Velux in a slate roof.
What to do when the roof is too shallow
Low-pitched roofs on extensions, porches and garage conversions often fall below a standard roof window's range. There are a few honest routes. Some manufacturers make models for lower angles, and the data sheet will say so. Some builders fit the window on a raised kerb, a small upstand frame that tilts the glass and gives rainwater a better run-off, which suits a flat or almost flat roof.
On a truly flat roof the right product is a flat rooflight, which sits on its own upstand instead of in a pitched frame. We compare the two in flat rooflight vs roof lantern.
The last route is to change the roof angle itself, which means altering the structure and is a bigger job. It makes sense only if the roof is being rebuilt anyway. Ask whoever quotes to say which route they propose, and why, so you can see the choice is made from the data sheet and not from convenience.
Whatever you choose, new openings in a roof are building work. Ask your local building control team whether the job needs notice before it starts.
Checking your own roof before you order
You can get a rough idea of pitch before anyone visits. If you have a loft hatch, a straight batten or a plank placed against a rafter with a phone level app on top will show the angle in degrees. Photograph the roof from the ground, side-on, and note the covering. If you are unsure whether you have concrete, clay or slate, a close photo of one tile helps.
Do not buy a window before the pitch is known, because returning a made-to-order frame is not always possible. Check the rafter size and spacing too, since the opening has to fit between them or the rafters have to be trimmed.
When we survey a roof, we measure the pitch, name the covering, and write the window model and flashing kit into one total price. You see photos of the slope and the rafters before anything is ordered. Send us the roof photos and any model label, and we will say whether your pitch suits the window you have in mind. For the whole picture of what we do with these openings, start from our roof windows and skylights hub.
More on this in Velux Roof Windows & Skylights in Crawley
Two pitch questions
Will a roof window go on a very shallow slope?
It depends on the model, not on a general rule. Some windows are made for lower angles and their data sheets say so, and some low roofs take a raised kerb or a flat rooflight instead. Check the stated pitch range for the exact window first, then the roof covering's own minimum, and only order once both are confirmed.
Does a steeper roof cause problems for a roof window?
It can. Very steep pitches have an upper limit too, and the window may need a different position so the sash can open and rotate. Reaching the handle or cleaning the glass gets awkward as a slope steepens. The data sheet states the upper range, so the same check on the model applies to both ends of the scale.