The short answer
On a cottage, a conservation rooflight should be small: narrow enough to sit between two rafters, short enough to stay well below the ridge and clearly smaller than the windows in the walls beneath it. Think of a pane roughly the size of a bedside cupboard door rather than a patio door. A cottage roof is a modest surface, and a rooflight that is too large will dominate it from the lane and from the garden.
Start with the rafters
The rafters set the first limit. In an older cottage they are rarely evenly spaced, and in cob and stone houses they can wander by an inch or two over the length of the roof. A rooflight that fits between two neighbouring rafters needs no structural change to the roof, which keeps the work lighter and keeps the old timbers whole. One that is wider has to have a rafter cut and a new trimmer fitted, which is a structural alteration and has to be done with proper support, and it is Building Control work.
So the first job is to measure the clear gap between rafters, from inside the roof space. Typical spacing is in the range of 400 to 600mm between centres, but a survey is the only way to know what yours are. Many small conservation rooflights are made to suit that gap, and some makers will build a size to order where the standard ones do not fit.
Scale against the building
The better test is by eye. A cottage usually has small windows with a vertical shape, and a rooflight looks best when it echoes them. A common rule among conservation officers is that roof openings should be clearly smaller than the windows below and should not compete with them. Line the rooflight up with a window or a door below if you can, so it reads as part of the pattern of the elevation and not as something dropped on the roof.
Height matters as much as width. Keep the rooflight in the lower half of the slope where possible, and well away from the ridge. Under a slate roof, set it so the frame lands neatly on the courses, which saves cutting slates awkwardly around it. Our guide to conservation rooflights on slate roofs explains how the flashing and the slate courses interact.
Sizes that tend to work
Conservation rooflights are made in a spread of sizes. As a general guide, the range runs from roughly 550mm wide by 800mm long up to around 1,000mm wide by 1,500mm long, though makers differ and bespoke sizes are available. On a cottage, the lower half of that range is almost always the right place to start.
- Small, around 550 by 800mm. Suits a narrow gap between rafters, a landing, a bathroom or a stair with a low ridge.
- Medium, around 700 by 1,000mm. Suits a bedroom or a small kitchen slope where the roof is a generous size.
- Larger, 800mm and up. Rarely suits a cottage. It can work on a long rear slope that is not seen from outside, or on an extension.
Two small rooflights, spaced evenly, often look better than a single large one, and they spread daylight along a room more fairly. That is worth asking about if the room is long and narrow.
Thatch, cob and low ceilings
Cottages bring their own limits. On a thatched roof, a rooflight is dressed in by a thatcher, and the frame has to sit within the thatch line rather than pushing through it, so only small openings are practical. Read can a rooflight go in a thatched roof before you set your heart on a size.
Inside, the ceiling often follows the roof slope and is low, with the rafters close above. A larger rooflight in a low ceiling looks overpowering, while a small one reads as a neat, deliberate opening. Cob and stone cottages also tend to have lime plaster, so the lining is best formed in lime to match, which is an extra reason to keep the opening modest: less plaster to match, fewer joins to blend in.
How much daylight a small opening gives
A small rooflight can do more than its size suggests. Overhead glass sees the open sky, and a window in a thick cottage wall sees only a slice of it, so a pane the width of a tea tray in a sloping ceiling often brightens a room more than people expect. The room still matters. A box room under the eaves needs very little, while a deep kitchen at the back of a cottage may want two openings.
Splaying the lining helps a small pane go further. If the roof is deep enough, widening the opening towards the ceiling spreads what the glass collects, and it makes a modest rooflight look intended. On a shallow slope the lining is short and a straight one is neater. Our page on what a splayed rooflight lining is sets out when each makes sense.
Opening or fixed is another choice. A fixed unit is the simplest and the slimmest. An opening one adds ventilation, which helps in a bathroom or a loft bedroom, but the frame is slightly deeper and the choice of size is narrower.
Planning and consent for a cottage
If your cottage is listed, any rooflight needs listed building consent, and size and position will be discussed with the conservation officer before a pane is ordered. If the cottage is in a conservation area, or in a national park, permitted development usually still allows a rooflight that projects no more than 150mm, but an Article 4 direction can take that right away, which is worth confirming with the council's planning team. See our answer on rooflights in Dartmoor National Park if yours is on the moor.
Getting the size right on your roof
You do not need to settle the size yourself. Send photos of the roof and the room through the quote form, and we will survey: measuring the rafter gaps, checking the ceiling below and looking at the roof from the lane. The size, the lining and the finish then go into your written quote, so you can see what the room will look like before the roof is touched.
If you would like to compare options first, the answer on glazing bars in conservation rooflights covers the one detail that changes how a small pane looks. The full picture is on our conservation rooflights page, and when you are ready, ask for a quote.