How a flat-roof sun tunnel is built
On a pitched roof, the dome sits in a flashing that tucks under tiles or slates. A flat roof has no tiles to tuck under, so the dome is raised on a base, often called a kerb or upstand, that stands up from the surface. Water runs across a flat roof slowly, and it can pool, so the dome has to sit above it. The roof covering, which may be a membrane, felt or fibreglass, is then turned up the outside of the base and finished so water can only run away from the joint.
Below the base, the tube drops through the roof build-up. A flat roof is a stack of layers: the covering, insulation, a deck, joists and a ceiling. The tube has to pass through all of them without leaving a gap where warm air from the room can reach cold timber. At the ceiling, the diffuser finishes the opening and spreads the light into the room.
It is the same product in a different setting, and the quote names the base, the covering it joins and the tube, so nothing is left to be settled on the roof. The dome catches daylight, the tube carries it, the diffuser delivers it. What changes is the weathering at the top and the care taken with the layers below.
What the survey checks
Flat roofs differ more than pitched ones, so the survey asks more questions.
How deep is the roof? The distance from the top surface to the ceiling is the length of the tube. A flat roof is often shallower than a pitched roof space, which can help: a short run delivers more of the daylight it receives. The page on light output explains why length matters.
Where are the joists? The tube sits between joists. If the spacing is tight, or a joist is in the way of the position that would light the room best, the opening moves a little, or the tube is trimmed in with doubled timbers. That is ordinary carpentry, and it is planned before anything is cut.
How is the roof built up? A warm roof has insulation above the deck. A cold roof has it between the joists, with a ventilated void. The two need different detailing at the tube, and the survey finds out which you have. A sealed, insulated tube section through the build-up keeps warm room air away from cold surfaces and the ceiling dry.
What is the covering, and how old is it? The base has to be joined to the existing covering, so its type decides the method. An ageing covering is worth knowing about before the hole is cut, so the two jobs can be planned together. We say so at the survey.
Falls, pooling and the dome
A flat roof is not truly flat. It has a gentle fall so that water drains towards a gutter or outlet. A dome base should sit where water will run round it, not behind it. Where the fall is slight, a small tapered piece or a back gutter above the base can steer water around the sides. On a roof with almost no fall, the details matter even more. We have a longer piece on rooflights on a flat roof with no fall, and the same logic applies here.
The same goes for the height of the base, which is chosen with the roof covering in mind. A taller base keeps the dome clear of standing water and leaves, and the covering turns up it further, which makes a safer joint. Our page on rooflight kerbs and upstands covers the base in detail, because a sun tunnel base follows the same ideas as a flat rooflight kerb.
When a flat rooflight wins
A flat roof offers a choice a pitched roof does not. Because the room often sits directly under the roof, with only a shallow build-up above it, a flat rooflight can be an option instead of a tube. A flat rooflight gives more light and a view of the sky. A sun tunnel gives a smaller pool of light and takes less room in the roof.
Where the room is small, the route is awkward or the ceiling is the only place light can come in, the tunnel is often the right call. Where the room is larger and the build-up is shallow, a rooflight may serve it better. The page on flat roof rooflights covers that product. If you cannot decide, sun tunnel or rooflight compares them room by room.
Rules, guarantee and the next step
A sun tunnel in a flat roof is a small opening, and planning permission is rarely the sticking point. If your home is a flat, the usual householder rights do not apply, so ask the council first. Everything else is in the planning guide. We deal with Building Control when it is needed.
Our sun tunnels in Leicester page covers the product from dome to diffuser. To find out whether yours will work, book the free survey or send a photograph of the roof and the ceiling below on WhatsApp. We measure the build-up, look at the covering and give you a fixed written quote. The fitting comes with the 10-year workmanship guarantee.
Some people call a sun tunnel a skylight. In the trade it is a tubular rooflight. On a flat roof, it is a neat way of lighting a room that is a long way from a window.