What Is the Minimum Pitch for a Roof Window?

Why a roof window has a lowest angle

A roof window sits in a hole cut through the slope, with a metal flashing kit (the pressed sheet that overlaps the surrounding tiles or slates) sealing the gap. The whole arrangement depends on gravity. Water arrives on the glass, runs down to the bottom edge of the frame, passes over the flashing apron and carries on over the roof covering. On a steep slope that happens quickly and cleanly. On a shallow one the water moves slowly, lingers on the glass and sits in the joints, and a strong gust can drive it uphill under the lap of the flashing.

Dirt makes it worse. On a gentle slope, leaves and grit settle along the lower edge of the frame and hold a line of damp against the seal. That is why makers publish a minimum angle for each window type and why the figure is not something to bend. Going below it can void the product's weather performance, and a leak at a roof window is awkward to trace because the water often travels along a rafter before it shows on the ceiling.

What to do if your roof is shallower

Many extensions and porches have a pitch lower than a main roof, and so do some dormer cheeks and garden rooms. If the slope is under the window's limit, you have three real choices. The first is a low-pitch version of the same window, which some manufacturers supply with a deeper flashing and extra sealing. The second is a flat rooflight on a raised kerb, which is made for almost level roofs and sheds water off its own upstand. The third is to build the roof up, using a tapered timber frame to lift the pitch where the window goes, though that changes the look of the roof and may bring in planning or building control questions.

For nearly level roofs, our page on flat roof rooflights covers kerbs, glazing and how they are weatherproofed. The page on roof window types compares the main opening styles.

How the pitch is measured

Guessing the angle from the ground is unreliable, because eaves overhangs and perspective fool the eye. The accurate way is to place a digital level or an angle finder along a rafter or a straight batten lying on the roof covering. In the loft, you can lay the same level against the side of a rafter and read the angle directly. A tape and spirit level also work: hold a one-metre level horizontal against the slope, measure the drop at the far end, and the ratio gives the angle.

Check the pitch at the exact spot the window will go. A roof that looks uniform can include a kick at the eaves, where the lower few courses are laid at a flatter angle, and a window placed in that kick may sit below its limit even though the main slope is fine. Any change in the slope near the planned position is worth noting on a sketch before you order a window.

Other checks that sit alongside pitch

Pitch is one of several things that decide whether a window works in a given spot. The rafters must be able to take the opening, which usually means trimming members and adding a header, and the roof covering must be compatible with the flashing kit. Heavy profiled tiles, thick slates and thatch each call for a different kit. If the window is meant to serve a loft conversion, the room beneath has its own rules on headroom, fire safety and insulation. Our guide to roof window building regulations sets out what building control looks at.

A worked example on an extension roof

Picture a rear kitchen extension with a mono-pitch roof that runs down from the house wall to a gutter at the garden edge. The designer drew it at a gentle angle to keep the eaves low, and the owner now wants a roof window to light the dining end. The drawn pitch looks about right, but once the covering is added the finished slope can be a degree or two flatter than the plan showed, because of the thickness of insulation and the fall built into the structure. That small difference can move a window from comfortably inside its limit to just outside it. The sensible order is to settle the window model first, then fix the pitch, rather than the other way round. Where there is any doubt, a low-pitch kit or a flat rooflight removes the risk entirely.

We start every window job with a free survey, measure the slope and rafters on site, and take pictures of the slope and the framing, which you get to keep. Our quote is fixed and in writing, and if your pitch rules out the window you had in mind, we say so and show you what will work. You can read more on the roof windows in Basingstoke page.