Last updated 22 October 2026
Start with the but
Textbook roofs are rare. A Leicestershire roof might have a shallow back addition, a hip at each end, trusses where the plans showed rafters, or a flat extension that ponds after every shower. Each of those is a reason someone has told a homeowner a skylight won't work.
We treat the obstacle as the first thing to survey. What follows is each one in turn: what makes it awkward, how a rooflight or roof window gets past it, and when the right answer is to leave that roof alone. For a homeowner's walk through placement, the separate guide on where a skylight can go on an awkward roof goes into more detail on placement.
Low pitch
The obstacle. Every roof window is tested down to a minimum pitch, stated in its fitting instructions. Below that angle, rain moves too slowly off the glass and can be driven back up under the flashing by wind.
The way round. Check the real pitch with a gauge, not by eye, then match it. Some makers produce roof windows and flashing kits rated for lower slopes. Below those, a flat rooflight on a built-up kerb lets the roof be treated as a flat-roof detail, with the glass set at its own fall. On a shallow back addition behind a terrace, that can be the neat answer. The choice is set out in our page on a skylight on a shallow-pitched roof.
When it is no. If the slope is below every rated product and a raised kerb would project further than the planning limits or the house allow, we say so, and look at a sun tunnel or a different slope.
Purlins and trusses
The obstacle. A purlin is a heavy horizontal beam that holds the rafters up partway along the slope, and it tends to sit right where a loft wants its light. Trussed rafters are factory-made triangles of timber, each one designed as a whole, so a cut in one weakens it.
The way round. With cut rafters and a purlin, the opening moves up or down the slope to clear the beam, and the rafters around the hole are trimmed and doubled. The purlin stays whole. With trusses, the first move is a light sized to fit between two of them. Two narrow roof windows side by side can do the work of one wide one. Where a wider opening is wanted, the trimming has to be designed for that roof before anything is cut.
When it is no. Cutting a purlin to suit a window, or cutting a truss without a design, is off the table.
Flat roofs with no fall
The obstacle. A flat roof is meant to fall slightly towards its outlet. Some never did, and some have sagged, so water sits in a shallow pond. A rooflight in the middle of that pond is relying on its seals more than any seal should.
The way round. We find where the water collects and keep the light out of it, then build the kerb high enough that the glass and its seals sit well clear of anything standing. The rooflight itself is tilted a little, which lets rain run off. On a roof that needs new covering anyway, the fall can be put back at the same time.
When it is no. If the deck under the covering is soft or rotten, a rooflight should not go into it until the roof itself is sound.
Hips and valleys
The obstacle. On a hipped roof the slopes meet at angled corners, and the rafters get shorter as they near each hip, so the clear run for a light shrinks. A valley is the inside corner where two slopes meet, and it carries water from both at once.
The way round. The opening goes where the rafters run full length, set back from the hip line so its ridge tiles and the flashing do not fight each other. Near a valley, the light stands far enough away that the water running down the valley never meets the flashing collar.
When it is no. On a small hipped slope with no full-length rafters left in the right place, there may be nowhere a light can sit and drain properly.
Dormers
The obstacle. A dormer takes a bite out of the main slope, and its own roof is short, often flat or very shallow. Every junction between dormer and roof is another joint to weather.
The way round. Either the main slope beside the dormer takes a roof window, where there is a clear run of rafters, or the dormer's flat roof takes a small kerbed rooflight. The survey measures the run first, because a short slope limits the size more than anything else.
When it is no. If the remaining slope is only a narrow strip between dormers, forcing a light into it would leave flashings overlapping. A sun tunnel through the dormer roof may reach the room instead.
Chimney stacks
The obstacle. A stack has its own flashings and, on the uphill side, a small gutter behind it to turn the water. A roof window crowding it disturbs both.
The way round. Keep a sensible gap so each has its own flashing with room to lap the tiles properly. Where the stack sits mid-slope, the light goes beside it rather than below, out of the water it sheds.
When it is no. On a narrow terrace slope where the stack takes up the only clear bay, we look at the other slope or another product.
Solar panels
The obstacle. Panels tend to be on the sunniest slope, which is often the one you wanted the light on.
The way round. The rear or side slope may suit better anyway, since a south-facing loft can overheat under too much glass. If the array has a gap in the right place, a light can sit in it with clearance for both flashings. Moving panels is a job for whoever looks after the system, and we plan around their visit.
When it is no. If the array covers the only usable slope, the honest answer is a different slope, a sun tunnel, or no light on that roof.
Conservation and listed limits
The obstacle. Rooflights on a house are usually permitted development, within limits on how far they stand proud of the slope and where they sit. Those rights can be taken away. A listed building needs listed building consent, a flat or maisonette has no permitted development rights of its own, and an Article 4 direction can remove the rights for a whole area.
The way round. We check the planning position for your address before ordering. On a period roof, a conservation rooflight sits low with the slates, with a slim dark frame, and is made for exactly that setting. The full rules are in our planning permission and Building Regulations guide.
When it is no. If consent is refused, the light does not go in. That decision belongs to the council, and we work to it.
Your roof, looked at properly
All of this is settled on a ladder and in the loft, not from a photo. The survey is free, the quote that follows is fixed and in writing, and it says what we found and how we would deal with it, or that we would not. Send a photo of your awkward bit on WhatsApp, or ask for a survey.