In brief
Yes. A sun tunnel can go into a flat roof, provided it is fitted with a flat roof flashing or upstand kit made for the tunnel and dressed into the roof covering. A pitched-roof flashing is shaped for tiles and does not suit a flat deck. The covering, the thickness of the roof build-up and the roofer who lays it decide the details, so we agree them in writing first.
- A flat roof uses a different flashing kit from a tiled roof, made for the tunnel
- The kit has an upstand that is dressed into the roof covering by the roofer
- Roof build-up depth affects the tube length and the way the ceiling is finished
- Fitting while a new roof is laid keeps the number of cuts through the covering to one
- The covering, whether felt, single-ply, GRP or asphalt, changes how the upstand is finished
Why a flat roof needs its own kit
On a pitched roof, water runs off the tiles and around the flashing under gravity. A flat roof has little fall, so water can sit against anything that rises from the surface. A sun tunnel flashing made for tiles has a skirt that is tucked under the courses above and lapped over those below, and none of that works on a continuous deck.
The makers therefore supply a flat roof version of the kit. The tube still carries light down to the ceiling, but the base is an upstand, a raised collar that stands above the finished roof surface so the dome sits clear of any standing water. Its height follows the maker's instructions for the model, much as a flat rooflight kerb is raised clear of the roof by the maker's stated height.
How the upstand meets the roof covering
The watertight line on a flat roof is the covering itself. The roofer dresses the membrane up against the upstand and finishes it according to the system in use, so the covering and the tunnel base become one continuous layer. The method differs by covering:
- Single-ply membrane: the sheet is welded or bonded up the upstand and sealed at the top edge, with a manufacturer's flashing detail
- Felt: layers are lapped up the upstand and the top edge is protected with a fixing and a cap
- GRP: the laminate is taken up the upstand while it is being laid, which makes it a strong time to fit the kit
- Asphalt: the roofer forms the upstand detail as part of the asphalt work
Whoever lays the covering should be told the make and model of the tunnel before work begins, so the right upstand is on site and the detail is built into the roof, not added afterwards.
Fit it while the roof is being laid
The cleanest result comes from fitting the tunnel into a roof that is under construction or being re-covered. The hole is cut in the deck, the upstand is fixed, and the roofer takes the new covering up to it in the normal run of work. There is one watertight junction, made by the person responsible for the whole roof.
Where a sun tunnel is part of a flat-roofed extension, we send the opening size and upstand position to the builder or roofer in writing before the deck goes on.the unit is ordered against the build programme, so the unit is on site when the roofer needs it. The sequence is then straightforward: roofer's deck and membrane, our fitting, the electrician if a light kit is wanted, and the plasterer for the ceiling.
Cutting into an existing flat roof
A sun tunnel can also be added to a finished flat roof. That means opening the covering, cutting the deck, fixing the upstand and then making good the covering around it, which usually calls for a roofer who works in that system. The age and condition of the existing roof matter: a covering that is near the end of its life may be better replaced than cut.
If the roof is insulated above the deck, the build-up is thicker, so the upstand sits higher and the tube may be shorter than it appears from below. We measure the depth through the roof, not just the loft or ceiling void, during the survey. On a cold roof or a warm roof the position of the insulation also affects how the tube is sleeved and sealed.
Inside the room
Flat roofs often have a shallow void between the deck and the ceiling, so the tube is short. A short run loses little light, which is a good outcome, and how much light a sun tunnel gives explains the factors. A rigid tube is the usual choice for a short, straight run, and rigid or flexible sun tunnel covers the options.
Joists run across a flat roof, so a tube that falls between two of them needs no trimming, while a tube positioned over a joist needs the position changed or the joists trimmed. We mark the tube position against the joists during the survey. The diffuser is then fixed to the ceiling, and the plasterer finishes around it.
What goes in the quote
A flat roof sun tunnel quote lists the model and tube diameter, the flat roof kit, the type of tube and length, the diffuser, any light kit, the maker's lead time, and what the roofer or builder must provide. It also says who makes good the covering. Prices follow those items, as set out in what a sun tunnel costs.
Every rooflight is fitted to current Building Regulations. A flat roof sun tunnel is also a roof penetration, so it is worth telling your insurer or warranty provider if the roof carries one. For the wider range, see our sun tunnels page.
Related questions
- Can a sun tunnel go in a green roof or a roof terrace?
- It depends on the build-up and the maker's kit. We look at the layers above the deck on the survey and tell you what the roof can take.
