The short answer
Roof windows suit most pitched roofs, but limits apply. The pitch must be within the window's rated range, the covering must have a matching flashing kit, and the rafters must be in a condition to be cut and trimmed. Very shallow pitches, flat roofs, some thatch and fragile roof coverings need a different product or a different approach, and a survey settles which.
Pitch is the first limit
Every roof window is rated for a range of roof pitches, and the figures vary by model. Typical centre-pivot windows are designed for pitches somewhere in the middle range of a normal house roof, while windows for shallower or steeper slopes are separate products or need extra kits. Check the manufacturer's range rather than assuming.
The pitch of the roof covering is a separate limit. Marley's guidance says Canterbury clay plain tiles need 40 degrees, Ashdowne 35 and Hawkins 30, while interlocking concrete tiles vary from 12.5 to 25 degrees depending on the profile. A natural slate roof, under BS 5534, needs at least 20 degrees, subject to headlap. If the existing roof is near the limit of its covering, adding a window does not change that, but the flashing detail gets more demanding.
For flat or almost flat roofs, the right product is a flat rooflight on an upstand, not a roof window. See our flat roof skylights guide.
The covering decides the flashing
The flashing kit is the set of formed aluminium or lead pieces that closes the gap where the frame meets the tiles or slates. It has to match the covering. Plain tiles, interlocking tiles, pantiles, slate and stone slate each use a different kit, and some windows come with several options.
Profiled tiles are the most forgiving, because the kits are made for them. Plain tiles and slate need a flatter, stepped flashing. Natural stone slate, as found on Cotswold-edge cottages, is heavy and hand-sorted, so the detail is often bespoke lead work rather than an off-the-shelf kit. Thatch, asbestos cement sheets and some old sheeting are another category, where the answer may be no, or a very different method.
If your covering is unusual, say so in your enquiry. Our page on roof window flashing kits explains the options in detail.
Rafters, structure and loft space
Fitting a window usually means cutting through at least one rafter. The structure around the opening then has to be rebuilt with trimmers, short timbers that carry the load round the hole. Rafters have to be in a condition to take that, and the spacing between them decides how wide a window fits without any cutting.
Older roofs can have rotten, bowed or undersized timbers, and modern roofs can use trussed rafters that must not be cut without an engineer's design. Where a window is wider than the gap, trimming is a structural alteration and needs Building Regulations approval. Our page on trimming rafters for a roof window goes through how it is done.
Two rules of thumb help before anyone climbs a ladder. First, look in the loft and see how the rafters run and how far apart they are, because a window that fits between two of them saves structural work. Second, note any insulation, wiring or pipework in the likely spot, as those items can move the position. Photographs of both help us advise you before we visit.
It also matters what is inside. Water tanks, flues, ductwork or a chimney breast can limit where an opening can go.
Building age and special status
Listed buildings need listed building consent for roof windows, and conservation areas may have an Article 4 direction that removes permitted development rights. Where permitted development applies, the window must stay within 0.15 metres of the slope's surface and below the original ridge, and windows in side slopes must be obscure-glazed.
In the North Wessex Downs and other protected landscapes, roof windows can still be permitted development, but dormers cannot. Ask the planners at Swindon Borough Council if you are in the borough, or at the neighbouring council if you are not.
Period roofs often suit low-profile conservation rooflights. Our page on roof windows and skylights sets out the products.
When the answer is no, or not yet
A pitched roof in sound condition is rarely a barrier by itself. The usual obstacles are specific: a covering that is too fragile to lift, a pitch outside the product's range, or a structure that needs strengthening first. Each has an answer, but the answer may change the cost.
Sometimes the roof needs work before a window can go in. A tired covering makes little sense to cut into, because the surrounding slates or tiles will not survive being lifted. Broken glass, damaged frames or earlier botched work around an existing window may also need sorting first; see broken roof window glass for that scenario.
If the roof is due for replacement soon, fitting the window during the re-roof is cheaper and tidier than doing it first. If the roof is sound, the window can go in on its own.
How we check a roof for a window
Our surveyor checks the roof both on the outside and from within the loft, identifies the covering, measures the pitch, checks the rafters and any services in the way, and photographs it all. You then get an explanation of what will fit and what will not, with the pictures and a written quote.
If a different position or a smaller model avoids cutting any rafters, we will tell you, because the smallest job that meets the need is the one we put forward. Our skylights and roof windows page lists the work we do, and you can also ask us on 01793 626096 or WhatsApp.