The answer
Often, yes. Every roof window has a minimum roof pitch set by its maker, and a shallow roof above that line takes one with the standard flashing, sometimes with a low-pitch kit that gives water more help leaving the frame. Below the line, a roof window is the wrong product, and a flat roof rooflight set on an upstand does the job instead. The pitch is measured at the survey, not guessed from the ground.
Why pitch matters to a window
A roof window is built to shed water down a slope. On a steep roof, rain runs off the glass and round the frame fast. On a shallow roof it moves slowly, collects at the sill, and sits against the joint for longer.
That is the whole reason for a minimum pitch. Below it, water lingers and is more likely to find a way under the flashing, especially in the wet spells of a Nottingham winter, when leaves and moss add to the problem. Manufacturers test their windows across a range of pitches and publish the lowest angle they stand behind.
Pitch is the roof's angle from horizontal, in degrees. Low pitch is common on porches, rear additions, single-storey extensions, garages and some post-war houses, where a shallow roof covered in large-format tiles or sheet material is normal. Many Victorian and Edwardian terraces have a steeper main roof but a shallow lean-to at the back, and it is often the lean-to that needs light.
If you want the wider picture of how a roof window is chosen and fitted, our roof window installation in Nottingham page starts there.
How we find out what your roof allows
We measure. A pitch you estimate from the ground is often wrong by several degrees, and several degrees can decide between a standard window and a different product.
At the survey we check the angle with a level on the rafters, then check what the roof is made of, how it is covered and how it drains. A tile or slate roof needs one type of flashing, a profiled sheet roof another, and a flat membrane roof needs a different solution altogether. We also look in the loft or the ceiling space for the rafter layout, because a low roof often has less space to frame the opening.
The result is a clear answer: this window, with this flashing, at this pitch, or not this window. The written quote that follows is fixed, so there is no surprise on the day.
It is better to ask this question before choosing the window than afterwards, because the choice of window and the choice of flashing depend on it. A window you fell for in a brochure might have a minimum pitch above your roof.
Low-pitch kits and the right flashing
Several manufacturers offer kits that let a roof window sit on a shallower roof than the standard version. They usually raise the flashing around the frame and change how it laps over the covering, so water runs away from the joint rather than pooling.
A kit is not a loophole. It extends the usable range down to a stated angle and no further, and it depends on being fitted exactly to the maker's method. That is part of why a window on a low-pitch roof is a job for an installer who knows the detail, not something to put in as a quick afternoon job.
The flashing matters just as much as the glass. It has to match the roof covering: a flashing made for plain tiles is not the same as one for slate or for a profiled sheet. Upstands and side flashings must be wide enough to shed a long wet spell, and the underlay and any insulation around the frame are fitted so the whole joint drains outward.
A low roof also tends to hold leaf litter and standing water longer. So we check what is above the window. A gutter or a tree that drops debris on the glass can cause trouble in autumn, and the position can sometimes shift a little up or down the slope to keep it clear.
When it is too shallow: flat roof rooflights
If the pitch falls under the window's minimum, there is still a good way to bring light in. A flat roof rooflight sits on a raised kerb or upstand fixed to the roof structure, and the kerb lifts the glass clear of standing water. Rooflights of this type are designed for roofs at or near level, and they suit lean-tos, extension roofs and shallow dormers.
They come in fixed or opening versions, in single units or in longer runs, and they can be much larger than a roof window. A wide, flat rooflight over the deepest part of a room is good at lifting light under an overcast sky, because it faces the brightest part of the sky. Our page on flat roof rooflights describes the options.
The trade-off is the build. A kerb has to be framed, insulated and waterproofed, which is more work than a roof window going between rafters. The view differs too: a roof window gives you an outlook, while a flat rooflight mostly gives sky.
For a sloping glass roof over an extension, a roof lantern is another option. See roof lanterns if the room is large.
Rules that still apply on a shallow roof
A low pitch does not change the regulations. A new opening in the roof is normally notifiable to Building Control, since it changes the structure and its heat loss, and replacing a window counts as controlled work as well. We handle the Building Control notification where the work is notifiable.
On energy, Part L allows a roof window in an existing home no more than 1.4 W/m²K of heat loss (the alternative is a band B rating), and gives a flat rooflight a looser limit of 2.2 W/m²K. Pick the glazing that meets the limit for the product, and check the whole unit, not only the glass.
Planning is usually simple. Most house rooflights are permitted development so long as they stand no more than 15 cm off the slope and below the ridge line. That does not extend to flats and maisonettes, and listed buildings and homes under an Article 4 direction need a separate check. The Planning Portal has the national guidance.
The survey is free and the quote that follows is fixed and in writing. Installation follows current Building Regulations. You also get the 10-year workmanship guarantee, put in writing.