Short answer
It depends on the model, and the manufacturer's data sheet sets the limit. Many roof windows are made for pitches starting at around 15 degrees, with some low-pitch flashing options at the bottom of the range, and they work up to a vertical wall. Below the minimum, a roof window is the wrong product. A flat rooflight on a kerb is the usual answer for a very shallow roof.
Roof window minimum pitch: what the manufacturer sets
A roof window is built to shed water down a slope, so the pitch of the roof is part of its specification. Every manufacturer publishes a range for each model, with a lowest and a highest pitch, and the fitting only carries the guarantee when it is installed inside that range. The roof window minimum pitch therefore is not one number for the whole market. It is whatever the data sheet states for the particular unit going in.
As a general guide, centre-pivot roof windows from the main makers commonly start at around 15 degrees. Top-hung and other opening types have ranges of their own, which are usually narrower at the shallow end, so switching the opening style can change whether a unit is allowed on a given roof.
At the steep end the limit is rarely a worry. Most roof windows will go on to a near-vertical slope. The lower end is where a roof tends to run out of options.
Low-pitch flashing and the options at the bottom of the range
Towards the bottom of a unit's range, the standard flashing is not always enough. Water runs slowly down a shallow slope and can be pushed back by wind, so the flashing needs a deeper apron and a more generous overlap on the tiles or slate. Manufacturers supply low-pitch flashing kits and sometimes an adapted underlay detail for this reason.
The roof covering sets its own limit too. A clay tile, a concrete interlocking tile and a natural slate each have a minimum pitch at which they will lap and shed water correctly. If the covering is already below its own minimum, the window is not the issue, and the survey notes it. Our guide to VELUX flashing kits shows what each kit includes and how it is chosen for the covering.
We never fit a unit outside its range, and we do not stretch a flashing kit to cover a pitch it was not made for. If the numbers do not line up, the job plan says so before an order is placed.
- Check the model's pitch range on the data sheet
- Check the roof covering's own minimum pitch
- Choose a flashing kit made for that pitch and covering
- Where the pitch is below range, look at a kerb-mounted rooflight instead
Why very shallow pitches suit a flat rooflight on a kerb
When a roof is nearly flat, such as a lean-to, a porch roof or a low-pitched extension covered in a membrane, a roof window will not sit right. The better product is a flat rooflight, fixed to a raised kerb and weathered with the roof membrane. The kerb lifts the glass clear of standing water and gives the membrane a firm edge to turn up against. A typical upstand for this is around 150mm.
A flat rooflight is also built to deal with the kind of roof a shallow pitch resembles. It can be glazed with a fixed pane, or fitted with an opening mechanism, and is available in larger sizes than most pitched units. Our flat roof rooflight installation service covers that route, and the minimum pitch for a flat rooflight answers the question from the other side.
There is a grey area between the two, where a pitch is low but not flat. In that band the surveyor compares the pitch against the unit ranges, and the choice between a low-pitch roof window and a kerb-mounted rooflight is made on the numbers rather than by habit.
How pitch is measured on the survey
We measure the pitch directly. A digital level or inclinometer held on a rafter gives the angle in degrees, and we take it at more than one point, because an old roof can have a slight sag or a change of slope near the eaves. The reading nearest to where the window will sit is the one that counts.
From inside the loft we can also read the pitch off the rafter itself. From outside, where the roof is safe to reach, we confirm the covering, its lap and its condition. Both readings go into the written survey notes together with photographs of the slope.
A pitch reading also depends on what the roof is built from. A slope that looks even from the garden can differ by a few degrees between the eaves and the ridge, and the reading is repeated at the height where the head of the window will sit. If two readings straddle the manufacturer's minimum, the unit is chosen for the lower figure, and the plan records both readings so the choice can be checked later.
| Measurement | Why it matters |
|---|---|
| Pitch in degrees | Checked against the unit's range |
| Roof covering | Has its own minimum pitch and lap |
| Position on the slope | Pitch can change near the eaves |
| Rafter spacing | Decides the size of unit that fits |
Where the pitch goes in the job plan
With the numbers in, the written job plan gives the model in its unit row and the kit chosen for the pitch and covering in its flashing row. If the roof falls below the range for a roof window, the plan says that a kerb-mounted rooflight is the proposed unit and explains why. You read this before you commit to anything.
The same facts feed the Building Control paperwork where the job is notifiable. Cutting a roof open for a new window is building work, and the pitch, the unit and its thermal figure all belong in that file.
For the full picture of how a pitched unit is chosen and fitted, read about roof window installation, or the roof window cost answer if you want to know what moves the quote. A skylight on a low pitch roof is not a lost cause. It is a question that the survey settles with a tape and a level, and the plan records the answer.