The short answer
A standard roof window is made for slopes from about 15 degrees of pitch upwards, and the exact range is printed in the maker's data for each model. Below the minimum, rainwater leaves a flashing too slowly to rely on it, so a shallow roof usually takes a rooflight on a kerb. We measure your pitch at the survey and check it against the range for the window you have in mind before anything is ordered.
- Free survey, fixed written quote
- 10-year workmanship guarantee
How pitch is measured
We measure on the rafter, since a sagging tile covering can mislead. Slopes on one house can differ: a rear extension, a hipped end and the main roof may each have their own pitch, and a lean-to over a kitchen can be the shallowest of the set. Each slope that is going to take glass is measured on its own, and the readings go on the survey sheet beside the rafter spacing and the type of covering. A reading on a hip or a valley rafter gives a different angle from the main slope, so we take it on a common rafter.
A 15 degree slope rises about 270 mm over each metre of run
What goes wrong on a shallow slope
A roof window depends on gravity. Its flashing carries water down and round the frame, and the steeper the slope, the faster that water clears. As the slope flattens, rain driven by wind can be pushed up under the edges, and water lingers at the joints where it can find a way in. Snow also sits longer on a low roof and presses against the frame while it melts. Leaves and moss gather on a shallow slope above the frame, and the window ends up wetter for longer than it would on a steeper roof.
The covering has a minimum pitch of its own. Every make of tile and slate states the lowest slope it is meant to be laid on, and a window cannot improve on that. If the covering sits near its own limit, both limits are checked together, and the specification of the flashing kit goes onto the quote.
Roofs that fall below the range
Shallow roofs are not out of reach of daylight. A garage conversion, a lean-to or a porch with a gently sloping roof can take a flat-roof rooflight on a kerb, where the kerb gives the upstand that the low fall lacks. Some makers also build roof windows for shallow slopes in separate ranges, and we look at what exists for your model. A garden room with a mono-pitch roof is another case, where the slope can be as gentle as a lean-to and a kerb-mounted rooflight is again the likelier fit.
Over a wide extension, a roof lantern puts a ridge and glazed sides onto a low roof. Where a hall or bathroom lies under an awkward stretch of roof, a sun tunnel asks for a much smaller opening. Our guide to flat roof rooflights covers the kerb-mounted options.
A kerb-mounted unit is a rooflight, so it is held to 2.2 W/m²K against 1.4 W/m²K (or Window Energy Rating band B) for a roof window, and with its kerb it can stand more than 150 mm proud. That is worth knowing early, since it can change the planning position, and the page on permission for a roof window explains how it is judged.
| Slope | Glass that suits it | What we check |
|---|---|---|
| Flat to very shallow | Rooflight or lantern on a kerb | Fall of the roof, kerb height, structure |
| Shallow but pitched | Roof window from a low-pitch range, or a rooflight | The maker's published minimum for the model |
| Within the standard range | Roof window in the slope | Rafter spacing, covering, flashing kit |
Checking your own roof before the visit
From the loft you can lay a phone with a level app along a rafter, and the angle shows on screen. Write down the reading for each slope you hope to glaze, and note which room sits below it and roughly how high the ceiling is. Stand a ruler against the rafter to note its depth and the gap to the next one. Photographs of the roof from the garden, and of the rafters from inside, go with it.
Send both to WhatsApp 07880 284133 or info@orchardskylightsmaidstone.co.uk and we will say whether the pitch looks workable. For a firm answer, book the free survey. We measure with our own level, set the pitch against the window's range, and the flashings guide for Kent roofs covers what goes round the frame once it is chosen.
