Question answered
What is the minimum pitch for a roof window?
The short answer
There is no one minimum pitch for a roof window: every model has its own lowest angle on the maker's data sheet, and the tiles or slates around it carry a minimum of their own. Pitch means how steeply the roof rises, given as an angle. If your roof is below the limit for a standard roof window, the usual choices are a different product, a rooflight raised on a kerb, or a different layout. A survey measures the real angle first.
No single angle suits every roof window. Each model has a lowest pitch printed on its data sheet, and the tiles or slates around it have a limit of their own. Both have to be met.
What pitch is and how it is measured properly
Pitch is how steeply a slope climbs, expressed in degrees above level. A steep gable might be well over 40 degrees, a lean-to over a kitchen or a porch can be close to the minimum its covering allows, and a flat roof is deliberately near zero but still has a fall so water runs off.
Eyeballing it from the garden is unreliable. Roofs look steeper from below and shallower from above, and an extension slope can differ from the main roof beside it. We measure along the rafter inside the loft with a digital angle finder, or along the slope with a level and rule, and note the result for each slope that a window is planned for.
Measure the same slope twice. A figure that changes by a degree or two between readings usually means the rafter is not straight, and that is worth knowing before a frame is fixed to it.
Two limits: the window and the covering
A roof window is only as low-pitch as the weakest part of the system. The first limit belongs to the window and its flashing kit, the metal surround that weatherproofs the frame. The data sheet gives the range of pitches it is designed for, and that sheet is the final word, not a general rule of thumb or a salesman's memory.
The second limit belongs to the covering. Marley's published minimums start at 12.5 degrees for its shallowest interlocking concrete tile, reach 35 degrees on its concrete plain tile and 40 degrees on the Canterbury clay plain tile. For natural slate, BS 5534 indicates a minimum of 20 degrees at moderate exposure with a 115 mm overlap, as quoted by Cupa Pizarras. If your slope is only just above the covering's limit, adding a window cut into it leaves less margin than the covering was laid with.
Both limits must be met together. A window that is happy at 15 degrees does not make a 35 degree tile suitable at 15.
Why shallow slopes cause trouble
On a shallow slope, rain runs off more slowly and spends longer sitting at every overlap, which is where a window is most exposed. The frame interrupts the flow, and water has to be carried round it by the flashing, past the head and down both sides. If the angle is too low, wind can push that water uphill under the upper edge of the kit.
That risk is higher on an exposed coast, where gales and driving rain test every detail, and a lean-to with no shelter from the weather shows weaknesses sooner than a tall roof tucked between neighbours. A roof with no margin in its pitch has less to spare when the weather turns.
The same logic is behind our advice to follow the data sheet exactly. The kit was tested within a range of pitches, and outside that range nobody can say how it will behave.
When the roof is too shallow
There are real options. Some makers produce windows or flashing sets for lower pitches, and a different product from the same range may bring the job back inside its limits. We will tell you if the model you had in mind is not the right one.
A rooflight is another route. In Approved Document L terms, a rooflight stands proud of the roof on a raised kerb, whereas a roof window lies flush within the slope. Lifting the glass on a kerb lets water drain off it on every side, which suits very shallow slopes and flat roofs. See our answer on flat rooflights for how that works.
Altering a roof to make it steeper is a larger piece of work with planning and Building Regulations questions of its own. Ask the Adur & Worthing planning team and your building control body before you consider it.
What we check before we quote
We measure the angle, read the model's data sheet and identify the covering with its own minimum. Photographs of the slope itself accompany the written quote, which carries a single total figure after the survey.
The same visit tells us whether the opening can be worked from inside the house and how much time it needs; both are covered in our answers on fitting a roof window from inside and fitting time. For the width and height of the window itself, see roof window sizes. The full range of this work is on our skylights and roof windows page.
Low-pitch roof windows: two more questions
Is a lean-to roof too shallow for a roof window?
Sometimes, not always. Many lean-tos are steep enough, but some sit close to the covering's own limit. We measure the angle, check it against the window's data sheet and the tile or slate minimum, and tell you whether a roof window or a kerbed rooflight suits it.
Does the pitch change how a roof window is flashed?
Yes. Each flashing kit is made to work across a band of angles, and the kit chosen has to suit both the covering and the slope. A kit stretched beyond its band is a familiar reason for leaks round a new window.
Tell us about your roof
Describe what is happening and we will call you back to arrange a visit.
- Photographs of what we find
- A written quote with one total price
- Or call 01903 299063
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