01The number
Where the minimum pitch for a roof window sits
For most pitched rooflights and skylights the floor is about 15 degrees, but that is a starting point, not a rule that covers every product.
Pitch is the angle of your roof slope measured from the horizontal. A flat roof is nearly 0 degrees. A steep Victorian slate roof can be 40 degrees or more. Most standard roof windows are designed to sit in a slope somewhere between about 15 degrees and a near-vertical 90, which covers the great majority of pitched roofs on houses and bungalows around Basildon.
We say "about" for a good reason. The limit is set by the window maker and the flashing, which is the metal and rubber surround that seals the frame to the roof covering. Change the flashing kit and the allowed range can change with it. Some makers publish a lower limit for certain models when you add a low-pitch kit. Others stop at 15 and say nothing below. So the answer to the question is a range and a check, not one number.
- 15degrees, the figure pitched roof windows commonly start from
- 90degrees, a vertical wall, the top of the usual range
- 0degrees, a true flat roof, which needs a different product
02Why it matters
What goes wrong if the slope is too shallow
The pitch decides how fast rain and melting snow leave the roof, and the flashing is only designed to cope with a set speed.
On a steep slope, water runs straight past the window and off the roof. On a shallow one it moves slowly and can sit against the frame edges. The flashing for a standard roof window assumes it will run off. Put that window on a slope below its stated minimum and the joints are being asked to do a job they were not built for.
There is a comfort side too, and it is our side of the job. A shallow roof has less depth for insulation around the opening, and the glass sits closer to flat, so it faces more open air above. That is why we pay attention to how the frame meets the roof build-up and the lining, so the room under the glass stays warm as well as bright.
03Finding yours
How to find out what pitch your roof is
You can get a good estimate from inside the loft in a few minutes, and a survey gives you the exact figure.
The simplest check is the roof plan or the loft conversion drawings, if you have them. They usually state the pitch. If not, you can estimate it from the loft side with a spirit level and a tape, or with a free angle-measuring app on a phone held along a rafter (the sloping timber that carries the roof covering). Treat the app as a guide only.
- Go into the loft and find a clear stretch of rafter.
- Hold a level flat against the underside, or lay the phone along it.
- Read the angle, and take it twice in different spots to check.
- Note it down with the roof covering, such as plain tiles, concrete tiles or slate.
- Send it to us, or leave it for the free survey, where we measure properly.
04Your options
If the roof is below the minimum
A low slope does not rule out daylight. It changes which product suits the roof.
There are two sensible routes. The first is a roof window with a low-pitch kit, where the maker offers one for that model and angle. The second is to treat the roof as flat or nearly flat and fit a rooflight on a kerb, which is the raised collar that lifts the glass above the roof surface. Both are installations we carry out, and the right one depends on the slope and the roof covering.
Pitched roof window with kit
Sits in the plane of the slope, looks tidy from outside, and suits a pitch inside the maker's published range for that flashing.
Rooflight on a kerb
Sits on a raised upstand, copes with very shallow or flat roofs, and the kerb can be insulated for a warm finish.
If you are choosing between the two, our page on what pitch a flat rooflight needs covers the other end of the scale.
05Quick guide
A rough guide by roof type
These are broad guides to help you picture your own roof, not a substitute for the maker's data.
| Roof type | Typical pitch | Likely route |
|---|---|---|
| Steep loft room roof | 35 to 50 degrees | Standard roof window |
| 1930s semi | 30 to 40 degrees | Standard roof window |
| Low-pitched bungalow | 15 to 25 degrees | Check the model limit |
| Shallow extension roof | Under 15 degrees | Low-pitch kit or kerbed rooflight |
Those pitches vary from house to house, and plotland and chalet bungalows in Laindon and Wickford can sit at the lower end. Measure before you choose. When you are ready to size the opening, roof window sizes and placement is the next step, and the full picture is on our roof windows page.
Low roofs, flashing kits and the angle you can get away with
Can I fit a roof window on a 10 degree roof?
Not with a standard model. At that angle you would need a model and kit the maker approves for it, or a rooflight on a kerb. We check which applies when we survey.
Is there a maximum pitch as well?
Yes. Most roof windows are made for slopes up to 90 degrees, but some models stop lower. The maker's data sheet states the range for each one.
Does the roof covering change the answer?
It can. Slate, plain tiles and profiled tiles each use a different flashing, and the allowed pitch is tied to the flashing. Tell us the covering when you ask for a quote.
Do I need Building Control for a new roof window?
Cutting a new opening can be notifiable, and replacing a whole rooflight is notifiable under Part L. We handle the Building Control notification where the work needs it. See do rooflights need Building Regulations.
