The short answer
A sun tunnel stays watertight because of its flashing. This is a shaped sheet that sits round the base of the dome, is formed to the roof covering and is lapped under the tiles or slates above it, so water runs over it and off. A careful installer checks the laps, the fixings and the battens before any tile goes back.
Water only goes where gravity lets it
A roof sheds water by overlap, not by sealing. Every tile sits over the one below it, and the water runs down that staircase and away. A hole in that pattern, even a small one, needs to be rebuilt as part of the pattern.
A sun tunnel makes such a hole. The tube is perhaps a foot or so across, and the dome that sits on top of it is a little wider. The roof covering has to be lifted round that area and put back in a way that still tells water where to go. The part that does the telling is the flashing.
Think of the flashing as a short piece of roof with its own overlaps. Its upper edge tucks under the course of tiles above the tube. Its sides run beneath the tiles to left and right, and its lower edge lies over the tiles below. Water coming down the slope meets the dome, splits round it, runs along the flashing and leaves onto the covering beneath. Nothing has to be stopped or held, since all of it is directed.
The same principle applies to every roof penetration, including the smaller jobs such as a soil vent. The difference with a sun tunnel is that the dome is a large, smooth obstacle in the water's path, so the flashing has to cope with more flow round it.
Matching the flashing to the roof covering
A flashing made for one covering will not work on another. The shape has to follow the profile of what is under it.
| Covering | Flashing shape | What to watch |
|---|---|---|
| Plain clay or concrete tile | Flat, with a stepped skirt | Laps over two courses |
| Interlocking tile | Profiled to the tile ribs | Gaps at the side channels |
| Natural or fibre slate | Flat, lapped slate by slate | Fixings near slate edges |
| Pantile | Formed to the wave | Crests and troughs |
Most sun tunnel manufacturers supply a flashing for each of these, and some supply a soft, malleable one that can be moulded by hand to an unusual profile. Lead is another option where the detail needs to be worked closely to an old covering. The choice is made at the survey, once the roof has been looked at from inside and out. Our page on fitting a sun tunnel through a tiled or slate roof walks through the roofing side of the job in more detail.
Crawley's housing spans a wide range, from the New Town neighbourhoods built from 1949 onwards to the Victorian terraces and villas that survive in northern Southgate. Roofs of different ages carry different coverings, and the covering is confirmed on site. A flashing ordered without looking at the roof is a flashing that might not fit.
What a careful installer checks before the tiles go back
The flashing is fitted in a fixed order, and each stage is checked before the next one hides it.
- The opening. The roof is opened to a size that suits the flashing, and the battens are trimmed and supported round the gap. Loose battens mean a loose flashing.
- The underlay. The membrane is cut and dressed up to the base of the tube so any moisture that gets past the covering is carried down and out. It is lapped, not butted, and not left torn.
- The base. The tube base is fixed square and level to the roof structure. A base that rocks or twists will move the flashing with it.
- The laps. The flashing upper edge goes under the course above and the sides go beneath the tiles beside it. We look at how each lap sits, because a lap set the wrong way round is the commonest way for water to find its way in.
- The fixings. Clips and nails are placed where the covering will hide them. No fixing is left open to rain.
- The cover. Tiles are cut to fit round the flashing, not forced over it, and relaid in the original bond.
Sealant has a place, but as a finish, not as the structure. The water path should work with the sealant left out. That is our rule for the job, and it is why the detail is checked before the covering goes back.
The dome, the upstand and the warm roof space
The flashing is half the story. The dome and the tube make up the rest.
The dome is fixed to a collar that stands above the flashing. That collar raises the joint between dome and base above the level where water flows, so the water never has to cross it. On a flat roof the same idea is delivered by an upstand, and the roof membrane is dressed up it. Our page on sun tunnels on flat roofs covers that version.
Inside, the tube is sealed along its length and lagged, so moisture in the warm air from the house does not meet the cold tube and condense into drips. Condensation is a separate matter from rain, although people sometimes mistake one for the other. Our guide to preventing condensation in a sun tunnel explains the difference.
The work is installed to current Building Regulations and carries a 10-year workmanship guarantee, which you can read about on our guarantee page. Where the work is notifiable, we handle the Building Control notification.
Flashing questions people ask
Is lead better than an aluminium flashing?
Neither is better in every case. Lead moulds to an irregular covering and lasts a long time. A factory flashing is quick to fit on a modern tile that it was made for. The roof decides.
Can a flashing be added to an old sun tunnel?
If the existing flashing has failed, the sound answer is to replace the unit with one that has the right flashing for the covering. See our answer on replacing an old sun tunnel with a newer one.
Does a steeper roof make it easier to keep dry?
A steeper pitch sheds water faster, so the flashing has an easier task. A low pitch needs more lap and more care, since water moves slowly and can be pushed sideways by wind.
Does rain on the dome make noise?
It can. The dome is a thin curve of plastic or glass, and a downpour can drum on it. Our answer on keeping a sun tunnel quiet in the rain deals with that side.
To have the flashing matched to your roof covering, get a quote and we will look at the roof at the survey. The full service is on our sun tunnel installation page.