Sun tunnels on flat and pitched roofs

Every sun tunnel has three parts. At the top is a clear dome, usually acrylic or polycarbonate, that catches daylight. Below it is a tube with a reflective inner surface that carries the light down. At the bottom is a diffuser, a frosted or prismatic panel set in the ceiling that spreads the light into the room.

Those three parts do the same job on any roof, and the diffuser looks identical whether the dome is above slate or above felt. What differs is everything that joins the parts to the house: the flashing or base around the dome, the angle and length of the tube, and what the tube passes through on its way down. A sun tunnel is a small opening, typically a few hundred millimetres across, so it asks far less of the roof structure than a full-size rooflight. It still has to be watertight, insulated and finished, and the way you get there depends on the roof.

A pitched roof is one that slopes: tiled or slated semis, terraces, cottages and most Devon houses built before the 1960s. The dome sits on the slope, and rain runs down past it, so the flashing has to guide water around the opening and back onto the roof covering.

For this, makers supply flashing kits matched to the covering. Profiled or interlocking tiles take a flashing moulded to the tile profile. Plain tiles and slate take a flat, flexible flashing that is dressed under the courses above and over the courses below. Thatch and unusual coverings, such as stone slate or old pantiles, are a specialist question and are treated one house at a time. Our page on fitting a rooflight in a thatched roof sets out why.

The dome should be fixed to the rafters or to a timber frame between them, not to the tiles. A small opening often fits between two rafters, which means no structural trimming at all, one reason sun tunnels are quicker than larger rooflights. If the opening lands on a rafter, a short timber is fitted on each side to carry the load.

The slope also changes what the dome looks like from outside. On a steep roof the dome sits at an angle to the sky, so the light it catches changes through the day as the sun moves. Many tube kits include a dome that rises slightly proud of the roof so that it still catches light low in the sky, and that matters most in winter.

On a flat roof the dome stands on a raised base, often called an upstand or kerb, fixed to the deck. The base lifts the dome clear of standing water and gives the roof membrane something to turn up against. That is the flat-roof equivalent of the flashing kit on a slope.

The membrane matters. Flat roofs in Exeter and around it are covered in several ways: GRP fibreglass, single-ply membranes such as EPDM, torch-on or built-up felt, and in older extensions, bitumen felt that may be near the end of its life. The base has to be weathered into whichever one you have. On GRP, the base can be laminated in. On a single-ply roof, the membrane is dressed up the outside of the base and fixed. On felt, the layers are lapped up and over. If the existing membrane is old and brittle, that is the moment to talk about renewing that area of it, so that the new work is not weathered into material that will fail first. We cover the same point for larger units on flat roof rooflights.

Flat roofs also have a build-up. A warm roof has insulation above the deck. A cold roof has insulation between the joists with a ventilated void above. The tube has to pass through whichever you have without breaking the insulation or letting damp air into the void. In a cold roof, the tube gets its own insulation wrap and the void stays clear around it, so the ventilation path isn't blocked.

A flat roof is also walked on by roofers, gutter cleaners and the occasional homeowner. A dome on a raised base is more exposed to a dropped tool or a ladder foot than one sitting in a slope. Tell us how the roof is reached, and we will choose the dome and base with that in mind.

The tube is where pitched and flat roofs part company most. Under a pitched roof, the ceiling often sits well below the roof slope, with a cold loft between them. A tube dropping from the dome to the ceiling may need to run a metre or more, and if the dome and the chosen ceiling position are not aligned, the tube has to bend.

Rigid tubes are straight lengths of reflective metal, joined with adjustable elbows to turn the angle. Flexible tubes are a ribbed, reflective tube that can be bent by hand to follow a route around a rafter or a pipe. Each has a place, and we set out the trade-off on rigid or flexible sun tunnels. As a rule, the more bends and the longer the run, the less light arrives, so a straight, short route is the better route wherever the house allows it.

Under a flat roof, the story is simpler. The ceiling is only the depth of the joists and the plasterboard below the roof deck, so the tube may be a few hundred millimetres long, enough to bridge the roof build-up and no more. A short tube loses little light. That makes flat roofs one of the most effective places for a sun tunnel, particularly over a bathroom or a corridor in a single-storey extension.

A sun tunnel is a metal or plastic tube linking a cold outside with a warm room. The dome is a single or double skin of plastic, and the tube is thin. Warm, damp air from a bathroom or kitchen rises to the diffuser, and if the surface above it is cold, the moisture condenses on the underside.

On a pitched roof, the tube passes through the loft, which is cold. The tube should be wrapped in insulation along the loft section, a standard accessory with most kits, and the joint between the tube and the ceiling should be sealed so room air can't climb the outside of the tube into the loft. Without that seal, damp air gets into the loft and can show up as mould on timbers or wet insulation. Our answer on whether sun tunnels cause condensation goes into the causes.

On a flat roof the tube is short, but the dome and base are in the same position as a pitched roof dome, and the cold bridge is smaller rather than absent. Insulation at the base matters more than on a slope, because the base is a solid ring through the roof build-up. A well-made base has an insulated core. A site-built timber base needs insulation fixed inside it before it is clad.

In either case, a bathroom is the hardest test. Many homeowners want a sun tunnel there, since a small bathroom with no window is a classic reason to fit one. We discuss the room side in sun tunnels for bathrooms, landings and hallways.

Most homes are one or the other, but several Devon houses are a mix, and the survey has to look at the exact roof over the exact room.

Lean-to and kitchen extensions

These are often low-pitched or flat roofs hung off the back of a tiled house. A lean-to may look flat from inside yet have a slight fall of a few degrees, which counts as pitched for some kits. The rule is to match the kit to the actual pitch and covering.

Dormer flats

The flat roof over a loft dormer is often a small, awkward area, with lead or felt at the edges. A sun tunnel there can serve a stair or landing below. The depth is limited and the structure is tight, so it is worth knowing what is under that flat before choosing.

Cob and stone cottages

Thick walls and irregular roof builds across the Culm and Exe valleys make for uneven rafters and varying depth, so the tube route is worked out on site rather than drawn on paper.

Rafter spacing, a chimney breast, water tank pipework or a wiring run can all force a bend, and a bend costs light. We check the route from above and below before quoting. It is also why the finish schedule records the roof build and the room.

Makers publish a range of roof pitches that each flashing kit suits. A kit made for a steeper roof will not sit right on a shallow one, and a flat-roof base is not meant for a steep slope. If your roof falls near the edge of a kit's range, say so at survey and we will match the product to the roof. We note the range used in the written quote.

Which way the roof faces matters less than people expect, because a dome collects light from a wide patch of sky and the tube carries it on. A north-facing slope still delivers, though a south-facing slope delivers more on a bright day. What matters more is the tube: a short, straight, clean one beats a long, bent one every time. On a flat roof the dome points straight up, so aspect hardly matters at all.

Light output also depends on the diameter. A wider tube carries more light, but needs a bigger opening, a bigger flashing or base and a bigger hole in the ceiling. The sums are laid out on how much light a sun tunnel gives.

A sun tunnel dome on the roof of a house usually falls under the same permitted development rights as a rooflight, which are set out in Class C of Part 1 of Schedule 2 to the GPDO. The dome should not project more than 150mm beyond the plane of the roof, and should not sit higher than the highest part of the roof. A low-profile dome is generally comfortable within that, but a raised flat-roof base and dome together may need checking against the limit. Flats and maisonettes do not have householder permitted development, and listed buildings need consent. Conservation areas and national parks can restrict what you can do, so check with the local planning authority where it varies.

On Building Regulations, cutting into the roof structure is building work, and Building Control may expect the unit to meet the thermal requirements of Part L, as it would for a rooflight. A sun tunnel passing through a loft or roof build-up must be insulated and sealed to meet them, and where the work is notifiable we handle the Building Control notification. The wider rules are on our page covering rooflight planning and Building Regulations in Devon.

Once the roof end is sound, the work moves down into the room, and it looks the same on either roof. The opening in the ceiling is cut cleanly, the tube is fixed to a collar or ring, and the diffuser is fitted to finish the opening. The edge where the diffuser meets the ceiling is the line your eye lands on from across the room or from the stairs.

In a plasterboard ceiling, the ring is set flush and the plaster or filler is brought up to it, then the paint line is cut in clean. In an older house with a lath and plaster ceiling, the cut has to be supported and scored so the plaster around it doesn't crack. Our answer on fitting a skylight in a lath and plaster ceiling explains the method, and a sun tunnel opening, though small, follows the same rules.

All of this is recorded in the finish schedule we write with every quote: the room, the ceiling and roof build, how the opening is formed, the finish, and whether it is left ready to paint or painted. For a sun tunnel it includes the roof covering, which tells you whether pitched or flat details apply. The rest of the service is described in the sun tunnel installation overview, and when you are ready, ask for a quote and a finish schedule for your roof and room.