Can a roof window be fitted in a low-pitch roof?

Why makers set a minimum pitch

A roof window sits in the slope of the roof and sheds rain down and around itself through the flashing. On a steep roof the water runs off quickly. On a shallow roof it runs slowly, can pool against the upstand, and is more likely to be driven back under the flashing by wind. For that reason each maker states the range of pitches a window may be used on, and installing outside it can void the maker's warranty on the unit.

The limit belongs to the window, the flashing kit and the roof covering together. A plain tile or slate covering usually needs a steeper slope than a sheet metal or membrane roof, so the same window can have a different permitted pitch on two roofs. The only reliable source is the maker's current instructions for that exact model.

Our answer on which roofs can take a skylight covers the wider question, and the main roof windows page sets out the types of window.

Where low pitches turn up

The main roofs of many houses sit well inside the normal range, so the question mostly arises on extensions and additions. A single-storey rear extension with a lean-to roof often has a shallow slope, as does a dormer, whose roof is usually flatter than the main roof. A porch, a garage conversion and a link between two buildings are other common cases.

Pitch is measured as an angle, and the difference between a roof that qualifies and one that does not can be small. We measure it at the survey with a level, not by eye, and write the angle on the quote.

• Pitch measured at the survey
• Maker's limit checked for the model
• Roof covering and flashing kit matched

The options below the limit, side by side

When the pitch is below the limit, there are three routes. Each has a different cost, appearance and amount of building work, and none is automatically the best.

A flat rooflight on a shallow slope is the choice that most often works, since it is made for roofs that do not shed water quickly. Our guide to flat roof rooflights compares the types, and our answer on the minimum pitch for a flat rooflight explains why even those have a lower limit.

What changes if you use a rooflight instead

A flat rooflight is built for a kerb and sits above the roof surface, so the look changes from a window flush in the slope to a glass panel on a low frame. Some people prefer the cleaner look of a frameless or low-profile rooflight, which our guide to frameless flat rooflights covers.

The opening type changes too. A fixed rooflight lets light in, while an opening one gives ventilation, by hand or by electric motor. Roof windows give an easy reach to the handle from inside the room, whereas a rooflight on a shallow roof is often out of reach and may suit an electric opener, which is explained in our guide to electric opening rooflights.

Planning is the same question in both cases. Permitted development can cover a rooflight or roof window going into a roof, with the conditions explained in our rooflight planning guide, and the address is something we check for you. The council decides.

Getting a firm answer for your roof

The survey settles the question in one visit. We measure the pitch, identify the covering, find the make and model limits in the current catalogue, and tell you which route fits. Cutting into the roof for a new window is building work, and we lodge the Building Control notification whenever the work is notifiable.

After the survey the written quote lays out the options that suit your roof, taken from several makers when the roof permits, and gives the glass, the frame, the opening and the lead time for each of them. Book a free survey and we will measure your roof and say which route fits.