What roof pitch do I need for a roof window?

Pitch is simply the angle the roof surface makes with the horizontal, measured in degrees. A typical house roof in south Essex sits somewhere between 30 and 45 degrees. A shallow rear extension might be 10 or 15. A steep Victorian front gable can run past 50. The window itself is happy across almost all of that range, but two things vary with the angle: the flashing that seals it into the roof, and the way the glass behaves once the sun is on it.

Most manufacturers quote a working range for each window, and for the common centre-pivot and top-hung units that range is roughly 15 to 90 degrees. The 90 figure is not a typo. These windows can go into a vertical or near-vertical roof, such as the steep face of a mansard, provided the right flashing is used. The number that matters more in practice is the lower limit, because that is where most of the trouble sits.

There is also a comfort point in the middle. A window fitted between about 20 and 40 degrees is easy to reach, easy to open and closes under its own weight without a fight. Very shallow and very steep both make the day-to-day use of an opening window slightly more awkward, which is worth knowing before you commit to a size and an operation type.

Under 15 degrees, a standard roof window and its standard flashing are outside their designed range, and water is the reason. On a steep roof, rain runs off fast and the flashing only has to shed a thin sheet of water past the frame. On a shallow roof the water moves slowly, sits longer against the top edge of the window, and finds any weakness in the seal. Wind-driven rain off the estuary makes that worse, because it pushes water uphill against the flow.

You have two honest options at a low pitch. The first is a window specifically rated for low pitch, which is fitted on a timber or insulated kerb that lifts the glass to a steeper effective angle and gives the flashing something to work against. The kerb does the job the roof slope would otherwise do. The second is to treat the roof as flat and fit a flat-roof rooflight built for the purpose, which is a different product with a different upstand and a different seal. We cover that route in full on our flat roof skylight installation pages, and it is often the better answer on a modern single-storey extension where the roof was only ever laid at a couple of degrees for drainage.

The line between the two is not a hard rule, but as a working guide anything below about 10 degrees belongs to the flat-roof family, and 10 to 15 is the awkward middle where a kerb-mounted unit usually makes more sense than a standard one. Get this wrong and the roof looks finished, passes a quick glance, and then weeps at the head of the window the first time the wind swings round to the south east.

Pitch does not just decide whether the window fits. It decides how much summer sun the glass collects, and that is the question this business is built around. The flatter the roof, the more it points at the open sky rather than at a compass direction, and the more of the high summer sun it catches almost square on.

A steep south-facing roof at 45 degrees takes the July sun at a glancing angle for much of the day, so a good part of the energy reflects off the glass rather than passing through it. Drop that same roof to 15 degrees and the sun strikes far closer to head-on, so more of it comes straight into the room. This is why a shallow south-facing extension roof can overheat even with a modest area of glazing, while a steeper pitch on the same elevation is more forgiving.

Southend faces south across the Thames Estuary, with an open horizon and light bouncing off the water, so a south-facing roof here collects sun from mid-morning until it swings west with very little shading to interrupt it. On a shallow pitch that adds up quickly. The fix is not a smaller window, it is the right glass: solar-control glazing with a G-value around 0.3 or below on a south-facing roof, against the 0.5 to 0.6 of clear double glazing that is perfectly sensible facing north. If you want the longer version of that argument, our solar-control glazing pages set out the numbers by orientation. The short version: work out your pitch and your orientation together, because a shallow pitch and a southern aspect is the combination that turns a bright room into a hot one.

The flashing kit is matched to the roof covering as well as the pitch, and the two decisions are linked. Slate and plain tile sit relatively flat, so their flashing can hug the roof closely. Deep-profile interlocking tiles and pantiles stand proud, so they take a different flashing that steps over the contours to keep the seal tight. When you order a window you are really ordering three things: the unit, the flashing for your covering, and any insulation collar that goes around the frame.

On a pitched roof the window normally beds into the plane of the roof, so it does not need the raised kerb that a flat-roof unit relies on. That keeps the profile low and the sightline clean from the garden. Where a window has to go into a very shallow pitch on a kerb, the kerb needs insulating properly, because a bare timber upstand is a cold bridge that will mark the ceiling around it in a hard winter. None of this is exotic, but it is the sort of detail that separates an installation that stays dry for its full life from one that looks right on the day and disappoints later.

Whatever the pitch, a new or enlarged roof window is notifiable under Building Regulations. Part L covers the thermal performance of the glass and Part K covers safety glazing where the window sits within reach. That applies to a swap as much as a new opening, so if a tired unit is coming out, the replacement is the moment to bring the glass specification up to current standards rather than matching what was there.

You can measure a roof pitch from inside the loft without going anywhere near the roof. Hold a spirit level horizontally against a rafter, mark a set distance along it, say 300mm, then measure straight down from the end of that mark to the rafter. A rise of about 173mm over 300mm is 30 degrees; about 250mm is 40 degrees; about 300mm is 45 degrees. A phone with an angle app held flat against the underside of a rafter gives you the same answer in seconds. Either way you want a number, not a guess, because 12 degrees and 16 degrees send you to two different products.

Once you have the pitch, note which way the roof faces and whether anything shades it, then read those two facts together. A steep north-facing roof is the easy case and takes standard clear glazing. A shallow south-facing roof is the one to think hardest about, both for the flashing and for the glass. If you are on a genuinely pitched roof and want the fitting done properly, our pitched roof skylight installation service is built for exactly that, and you can see typical figures on our skylight installation costs page.

If you are not sure which side of the low-pitch line your roof sits on, that is the useful thing to ask us first. Tell us the pitch you measured, the roof covering and the orientation, and we will tell you whether it is a standard window, a kerb-mounted low-pitch unit or a flat-roof rooflight, and what glass it should carry. Ring 01702 898232 or request a quote, and we will look at the whole roof rather than just the hole you want to put in it.