In brief
Sometimes. Every roof window and its flashing comes with a minimum roof pitch set by the maker, and the window can be fitted only at or above the pitch stated for that exact combination. Below that limit, the usual answer is a flat roof window or a rooflight on a kerb, which is made for shallow slopes. We measure the pitch on the survey and match the product to it.
- The maker sets a minimum pitch for each window and flashing pairing
- Pitch is measured on your roof, not read from the drawings
- The covering, such as slate or interlocking tile, changes what is allowed
- Below the limit, a flat roof window or kerb-mounted rooflight takes over
- The product and flashing named in the quote must suit the pitch
Why pitch decides the product
A roof window is built to shed water down a slope. The frame, the gutter around it and the flashing that joins it to the roof covering all rely on gravity to carry rain past the joints. On a steep roof, water runs away quickly. On a shallow one it moves slowly, can pool at the upper edge of the window, and is more easily pushed sideways by wind.
Makers test their windows and flashings against a range of pitches and publish the lowest pitch at which each one can be used. That figure is not a general rule of thumb. It belongs to a particular window, a particular flashing and often a particular roof covering. Using a window outside its range is a departure from the instructions, and it is the kind of detail that causes trouble years later.
What the maker's limit depends on
Three things move the limit. The first is the window itself: some models are designed for lower slopes than others. The second is the flashing kit. The same window can have a different minimum pitch when paired with a flashing for plain tiles than with one for slate, and a low-pitch roof often needs a flashing made for shallow slopes. Our guide to flashing kits for tiles and slate shows how the covering changes the kit.
The third is the covering and the underlay below it. Some coverings, such as slates, need a steeper roof to shed water properly even without a window, and the manufacturer of the covering sets its own limit. Where the roof is already close to the lowest pitch its covering allows, there is little room to add a window and stay within both sets of instructions.
Because the figures differ between models and change as makers update their ranges, we read them from the current literature for the product you are buying, and name them in the quote.
Measuring the pitch properly
Pitch is the angle of the roof slope, normally given in degrees. Drawings can show it, but drawings are sometimes drawn to a different angle than the one built. Roofs also vary: an old extension, a dormer cheek or a lean-to can sit at a lower angle than the main roof, even on the same house.
On the survey we measure the slope where the window is going to sit, not a different part of the roof. We also note what the roof is covered with, how it is battened, and whether the existing roof has a felt or breathable underlay. If the roof is part of a new build, we ask the builder for the pitch the roofer has set out and check it against the rafters once they are up.
You can get a rough idea yourself from the roof drawing, but the measured figure is the one the quote relies on.
Options when the roof is too shallow
If the pitch is below the limit for a standard roof window, there are still ways to bring daylight in.
- A flat roof window. A product made for shallow slopes, fitted on a low upstand or frame suited to the roof and weathered to the maker's instructions.
- A rooflight on a kerb. A frame built up above the roof surface so water drains clear of the joint. A kerb commonly rises at least 150mm over the finished surface of the roof, as the maker directs.
- A different roof window range. Some makers offer windows with their own low-pitch flashing, and the choice moves with the covering.
- A sun tunnel. Where the room needs light rather than a view, a small dome on the roof and a tube to the ceiling asks little of the structure.
The right choice depends on the room, the covering and what the builder is willing to alter. A flat rooflight on a kerb also gives a different look from a roof window, which sits flush with the slope, so we talk through the appearance before anything is ordered.
Planning it with your builder
On an extension or a new roof, the pitch is a decision the builder can still change. A small adjustment to the roof angle can bring a window within the maker's range and keep the design you wanted. That conversation is far easier before the rafters are up. We send the maker's minimum for the chosen window to the builder or architect in writing, so the roof is set out to suit it.
On an existing roof the angle is fixed, so the choice is the product. Have a look at the range on our roof windows page, or see what a fitted window costs in how much a roof window costs to fit. The quote that follows the survey names the unit, its flashing and the delivery date confirmed when we order.
Related questions
- Can I use a roof window on a very shallow pitch if I add more sealant?
- No. Sealant is not a substitute for the maker's pitch limit. The joints are designed to drain by gravity, and added sealant on a roof that is too shallow tends to trap water. A product made for the pitch is the sound choice.
- Does a low pitch affect which glazing I can have?
- The pitch rule applies to the window and flashing, not the glass. Glazing options are chosen for the room, such as heat, noise or privacy, and then confirmed against the maker's range for the pitch.
