Why a flat rooflight needs any pitch at all
A rooflight called flat is not meant to lie dead level. Glass that sits at zero degrees collects rain on its upper face. The puddle does nothing alarming, but it stays there. When it dries it leaves behind whatever the rain was carrying, and a pane that looks clean on the day of fitting is streaked a season later. Roof-Maker, a UK rooflight maker, says plainly that a flat rooflight fitted completely level will soon have water pooling on it, and that a pitch of five degrees is enough to stop that.
A second reason sits underneath. A flat roof is normally given a fall, partly because pooled water that freezes can damage the roof membrane. The same logic applies to the rooflight and the roof around it. Water should leave, not wait.
For a skylight that opens, there is a practical extra. A pane holding a shallow puddle will tip that water towards the room the moment it is lifted. That is a point in favour of a proper fall on any opening unit, whatever the minimum on the data sheet says.
The figures makers actually give
People expect a regulation to settle this. It does not. The pitch of a flat rooflight is a manufacturer's installation requirement, set for each product, and the maker is the one who decides what counts as fitting its unit correctly. We opened two makers' pages for this answer, and they do not quite agree.
- Roof-Maker, fixed flat unit: 5 degrees minimum, 15 maximum.
- Roof-Maker, wall abutment unit: 10 degrees maximum.
- Sunsquare, conventional fixed unit: 3 degrees minimum.
Roof-Maker states that its fixed flat rooflights should go in at a minimum of five degrees, and that fixed units can be installed up to fifteen degrees. Its wall abutment rooflights, the type that butts up against a house wall, stop at ten. Sunsquare, writing in a trade product article, says a conventional fixed skylight should be fitted at a minimum of three degrees in a flat roof, and notes that where more drainage is wanted the figure can go up to five degrees.
The gap between three and five looks small. In practice it decides how much timber goes into the upstand, so it is worth reading the right data sheet. Other makers publish their own numbers, and we quote only what we have read on a maker's page. Whatever unit goes on your roof, its own sheet is the authority.
What the angle looks like on the roof
Degrees are hard to picture on a roof. Simple trigonometry turns them into a rise you can measure with a tape. Five degrees is a rise of about 87 millimetres over each metre of run, so a rooflight a metre and a half deep is lifted by around 130 millimetres at its high end compared with its low end. Three degrees is about 52 millimetres per metre. These are not large steps, and in a finished roof they are not obvious, but a level pane and a five degree one behave very differently in rain.
It also helps to separate three different slopes that are often muddled together:
- The fall of the roof. The slope of the whole deck towards a gutter or outlet. It is often gentle, and it can be lost altogether on an older roof.
- The pitch of the glass. The angle the rooflight sits at. This is the figure the maker sets.
- The slope of the upstand top. The raised frame under the glass can be cut or built so that its top edge carries the pitch even when the roof below does not.
The second is what this page is about. The first has its own guide, fitting a rooflight to a flat roof with no fall, and the third is the way the second gets achieved.
When the roof is flatter than the unit wants
Many flat roofs have a gentler slope than the minimum a rooflight asks for. That is normal, and it is not a reason to give up on the skylight. Roof-Maker's guidance says that where the roof is flatter than the required angle, a larger angle can be created with a small timber frame. In the trade that frame is the upstand or kerb, and it is built so the glass leans the right way while the roof around it keeps its own fall.
The same maker points out that its roof lanterns do not need the five degrees, because the pitched sides of a lantern shed water themselves, so a lantern kerb is built level. That is a real difference between the two products, and it is one of the reasons the choice between a flat unit and a lantern is not only about looks. Our guide to roof lantern or flat rooflight weighs the others.
In practice the work runs in this order. The survey measures the roof's own fall in the spot where the opening will go. The upstand height is set so the top edge reaches the maker's pitch. Then the roof covering is dressed up and over that raised frame, which is the final stage of weatherproofing. The upstand has more to do than carry a slope, and rooflight kerbs and upstands goes through the rest.
What to do before you order anything
Do not buy a rooflight on size alone. Three things are worth settling first, in this order.
- Choose the unit. Its data sheet carries the minimum pitch, and sometimes a maximum too. A figure from a different brand, or from a web page that is about something else, does not transfer.
- Find the roof's own slope. A straight edge and a level across the area tell you what you have. A photo of any standing water a day after rain helps us before we arrive.
- Decide what has to change. If the roof gives the unit its pitch already, the fitting is standard. If it does not, the upstand is built to make up the difference.
The rules around the opening itself, such as the planning limits for a rooflight and the energy standards, are covered in skylight planning permission and building regulations. They do not set a pitch. The pitch is the maker's.
The wider picture of glass, upstands and the roof sits on our flat roof rooflights page. If you want your roof looked at, ask for the free survey. We measure the fall, check the figure for the unit against it, and the fixed written quote that follows says what is being built to get the glass to the right angle.