A flat roof sun tunnel, dressed into felt, EPDM or GRP

The dome of a sun tunnel is a round cap on a short tube. The tube carries light down through the roof build-up and the ceiling. The cap is only as dry as the collar it stands on.

On a slate or tile roof, the flashing around a tunnel works with gravity. Water meets the collar and is steered round it to the next course. A flat roof gives the water no such help. It is laid to a slight fall, and a tunnel sitting on that surface is a small obstruction in the water's path. Rain stops, collects and slowly drains past. In winter it may freeze there and thaw again. Any weakness in the joint between covering and collar is tested far more often than it would be on a pitch.

So a flat roof light tube is not the pitched roof product with a different flashing. It is a tunnel set on its own upstand, which is a short vertical rise in the roof surface, and the upstand is where the work is done. The dome and tube are the familiar part. The upstand, the collar and the way the covering is dressed up them decide whether the ceiling below stays clean for the life of the unit.

If you are still deciding whether a tunnel suits the roof at all, the sun tunnel installation hub sets out the options. The rest of this page assumes the roof is flat, the room below is dark and the question is how to fit one properly.

A flat roof tunnel stands on a raised ring, and the height of that ring sets how well the covering can be sealed to it. As a working rule, the waterproof covering should rise at least 150 millimetres above the finished roof surface before it is tucked under the dome's skirt.

That figure is the usual minimum for any upstand on a flat roof, whether for a rooflight kerb, a door threshold or a wall abutment. We state it here from general practice rather than from a particular product sheet, and the manufacturer's own instruction for the chosen tunnel is what we follow on the day. It exists for a simple reason. Standing water, splashing rain and drifted leaves all reach a few tens of millimetres up a vertical surface. Below that height a joint is wet often. Above it, a joint is wet rarely.

Most flat roof tunnels are supplied with a moulded collar, a short round or square base that provides this upstand. Some are built from a timber kerb made on site and clad in the roof covering. The moulded type is quicker and suits most domestic roofs. The timber kerb earns its place where the tunnel diameter is large, where the roof is thick with insulation, or where the tube must be offset to miss a joist.

The upstand also does two quieter jobs:

• It lifts the dome clear of settled water, so the seals at the dome are not sitting in a puddle.
• It gives the covering a firm vertical face to bond to, instead of a soft rim of insulation.

Before the covering is touched, the opening has to be made, framed and lined. The order matters, because each layer is sealed to the one beneath it.

A typical domestic flat roof has a timber deck over joists, a vapour control layer, rigid insulation and a weatherproof covering on top. That is a warm roof. Some older roofs are cold roofs, with insulation between the joists and a ventilated void above the ceiling. The tunnel treats them differently, and we establish which you have at the survey by looking from the loft edge, or by lifting a corner of the covering where a safe place exists.

1. Mark the centre of the opening from the ceiling below, then transfer it up through the deck with a plumb line, so the tube lands where the diffuser is wanted.
2. Cut the opening to the tube diameter plus the collar seat. Where a joist is in the way, trim the opening between the joists with short timbers fixed across, so the load is carried round the hole.
3. Fix the upstand collar to the deck with its flange lying flat on the surface and its wall standing upright.
4. Turn the vapour control layer up to meet the tube or collar and seal it, so the warm damp air of the house cannot drift into the insulation.
5. Dress the covering over the flange and up the collar, then fit the dome and its ring over the top.

Two points deserve a closer look. The first is the joist spacing. Flat roof joists commonly sit at 400 or 600 millimetres apart, and the gap between them limits the tube diameter that fits without trimming. A 350 millimetre tube sits between joists at 400 millimetre centres with only a little to spare, so a small sideways error is a real problem. Our page on choosing a sun tunnel diameter explains how the tube width is chosen. The second is the direction of the joists, which decides which way the trimming runs.

A flat roof is also usually thin from top to bottom. The tube may be only a few hundred millimetres long, so the light path is short and the tunnel is bright. A short tube is one reason flat roofs often suit this product well. The page on tunnel length, bends and how much light arrives covers what a longer run costs in light.

Bitumen felt is the commonest older flat roof covering, laid in two or three layers, and it is usually fused to the deck with heat or cold-bonded with adhesive. It forms a good detail around a collar because the layers are made to be turned up a vertical face.

The sequence is straightforward in principle. A first layer, often called a base or underlay, is cut to the collar and turned up the upstand. The flange of the collar is then bedded on it. A second, cap layer is dressed over the flange and taken up the vertical face. Each layer is lapped so that the joint faces down the fall, in the same way as tiles, never up it. The corners of a square collar need folded or cut-and-lapped pieces, which is why a round collar is tidier in felt. The curve takes a continuous strip and removes the corners where water likes to gather.

Where the felt meets the dome's skirt, the top edge of the upturn is tucked up under the lip of the skirt, so that water running down the dome meets the covering from above. A felt upturn left with its raw top edge showing would let water behind it. A neat finish takes a metal or plastic trim that holds the top edge firm.

The working risk with felt is heat and age. Torch-applied felt near a timber collar and a plastic dome needs care, and on a roof with insulation below, cold-bonded methods are often the safer choice. Older felt that has gone brittle will not turn a tight angle without cracking. In that case the sensible route is to renew the felt in a wide circle around the tunnel, or to recover a small roof in full when the covering is near the end of its life. A tunnel fitted into felt that has a short life left will outlast the felt.

EPDM is a synthetic rubber membrane, supplied as a single large sheet and usually bonded to the deck with adhesive. On an EPDM roof the detail is cleaner than felt, because the sheet can be cut to the roof in one piece with no laps across the main area.

Around a tunnel collar the installer forms the sheet up the vertical face and bonds it there. The corners of a round collar are smooth, and the rubber stretches to follow the curve without creasing. A square collar needs a formed corner, which is a small piece of pre-shaped rubber or a folded corner closed with special tape, and this is the place where a rushed job goes wrong.

Some EPDM systems come with a matching flashing for pipe penetrations, a moulded rubber boot that slips over a round tube and is bonded to the sheet. Where the tunnel maker supplies a round upstand, this is the neat answer. Where the collar is a timber box, the membrane is wrapped round it and finished with a termination bar and a flexible sealant along the top edge.

The rule for EPDM is that the adhesives and tapes belong to the same system as the sheet. A sheet from one maker joined with a primer from another is a weak joint from day one. We use the membrane manufacturer's own accessories wherever they exist, and we follow their cure times, because EPDM bonds take time to develop and stay sensitive to damp and cold while they do.

GRP, or glass-reinforced plastic, is laid as a liquid resin over glass matting and cures into a single hard shell. It is the covering most often found on newer flat roofs and extensions, and it makes a very sound seal around a collar because the roof and the upstand become one piece.

The tunnel collar is fixed to the deck, then the GRP is laid over the roof and taken up the upstand and onto the flange, all as continuous laminate. There is no joint between roof and collar to fail, because there is no separate joint. The collar is usually built from a material the resin will bond to, such as a GRP moulding or a timber kerb primed for the purpose. A plastic collar that resin will not grip would defeat the whole idea, so the collar is chosen for the covering, not the other way round.

Two practical points apply. A GRP roof is laid in dry weather over a dry deck, so the tunnel is best installed at the same time as the roof, or before a new top coat is laid. A roof that is already finished can still take a tunnel, but the new laminate has to key into the old, so the old surface is ground back to give a clean, rough edge. And GRP is rigid. A large sheet across a roof that moves slightly with temperature will show any stress at the collar, which is why the corners of a GRP upstand are rounded, never sharp.

The resin cures hard, and the finish around the collar, with its top coat and a neat termination, is durable and looks tidy from the garden and from any upper window.

The table sets the coverings against the points that matter most for a tunnel. It is a guide to what is typical, since every roof is different and the survey confirms the detail.

The fourth group, liquid-applied coatings, is found on some older roofs that have been recoated. They are dressed around the collar with a reinforcing fleece and several coats. The same rules apply: a round collar, a compatible primer and a clean vertical face.

A tunnel collar is a small dam on the roof, so its position matters as much as its detail. Flat roofs are laid to a fall, commonly around 1:40 as designed, so that after settling the finished roof still drains at 1:80 or better.

Put the tunnel where the water has a clear way past it. A collar placed just above an outlet can starve the outlet or leave a pool in the dead corner between collar and upstand wall. A collar placed hard against a parapet leaves a narrow trough behind it where leaves collect. We look for a clear margin of deck on every side, and we set the tunnel slightly to the side of the main water path when the room allows. Where a tunnel must sit on the high side, a small timber cricket, which is a wedge shaped so that water divides around the collar, directs the flow away and stops a pool forming on the uphill face.

Position also controls where the light lands. The tube comes straight down from the dome, so the diffuser in the ceiling sits directly below the roof opening. If the ideal spot for light is in a different part of the room from the ideal spot on the roof, a rigid or flexible bend moves it, at some cost in brightness. Our page on rigid or flexible sun tunnels sets out the trade.

A flat roof light tube is frequently put in over a bathroom, hallway or landing, where the room below is a box with no window. In those rooms the tunnel is often the only daylight source, so the diffuser position is chosen first and the roof detail is built to suit it.

A tube is a hole through the insulation, and a bare metal tube carries the winter cold straight into the room. Part L of the Building Regulations is concerned with the thermal performance of roofs, and a tunnel through a roof must not undo it.

The thermal detail on a warm roof has three parts. The insulation is cut tight to the collar, with any gap filled, so there is no path for air round the tube. The vapour control layer is sealed to the collar, so that damp air from the room does not reach cold surfaces. And the tube itself has an insulated sleeve through the depth of the roof, or a collar with insulation built in. Without those steps the upper part of the tube can get cold enough for warm bathroom air to condense on it and run back down. Our page on preventing condensation in a sun tunnel goes through the causes, and tunnels, loft insulation and heat loss deals with the heat that goes the other way.

On a cold roof the detail is different. The tube crosses a ventilated void and needs an insulated sleeve through it. The ventilation gap around the tube has to stay open, so the sleeve is sized to leave the airflow clear.

Rain falls straight onto a flat roof and hits the dome from directly above, with nothing to shelter it. A dome is a thin shell, and thin shells carry sound. The patter of a shower on a dome can reach the room below through the tube, especially where the tube is short and the diffuser is a thin plastic plate.

We treat this as part of the specification, not as an afterthought. The choices are the thickness and build of the dome, a second inner dome that gives an air gap, the way the tube is fixed so that it does not ring, and a diffuser that closes the tube firmly. Our guide to keeping a sun tunnel quiet in the rain covers the dome itself. On a flat roof near the Gatwick approach, where aircraft are a daily sound, the same build helps against the sky as well as the shower.

What we write in the quiet spec for a flat roof tunnel is the dome build, the tube fixing and the diffuser. It is a short note, and it comes with every quote, so you can see the choices before any work is booked. More on that is in The Quiet Spec.

Fitting a sun tunnel to a flat roof is work on the roof structure and its covering, so it touches the Building Regulations. The parts that matter are Part L for thermal performance and Part A where joists are cut or trimmed. Whether a notification is needed depends on the work, and where it is, we handle the Building Control notification. The page on whether a sun tunnel needs Building Regulations approval answers that in full.

A flat roof also raises a question that a pitch does not: how a person reaches it. The roof may be reached from a ladder against the extension wall, from a loft hatch or from a first-floor window. We plan access at the survey. A low-load roof is worked on from boards that spread the weight. Once the tunnel is in, the dome needs only occasional cleaning, since a raised dome sheds dust in the rain.

Where a flat roof is also being recovered, the best moment for the tunnel is at the same time. The covering is laid around the collar as one job, instead of being cut into later. If the roof is already sound and recent, the tunnel goes in as a separate day's work, with the covering opened in a small circle and dressed back. Our wider page on flat roof rooflight installation covers the larger openings that suit a flat roof, for when a tunnel turns out to be too small for the room.

For a rough idea of what the work involves, our rooflight and skylight costs page sets out the drivers. The cost of a flat roof fit depends on covering, roof depth and access more than on the tunnel itself.

A flat roof tunnel is decided mostly before the roof is opened. The survey checks a short list of facts, and each one changes the detail.

• The covering and its condition, because it sets how the collar is dressed.
• The roof build-up, warm or cold, and its depth, because it sets the tube length and the sleeve.
• The joist direction and spacing, because they set the largest tube that fits without trimming.
• The fall and the nearest outlet, because they set where the collar can stand.
• The room below, and what the diffuser will look like against the ceiling.

With those answered, the job is predictable. The opening is cut, the collar is fixed, the covering is dressed and the dome goes on. Inside, the diffuser is fitted and the ceiling made good.

How tall should the upstand be on a flat roof sun tunnel?

The covering should rise at least 150 millimetres above the finished roof surface, and the tunnel maker's instruction overrides that where it asks for more. A taller upstand helps where snow or deep insulation is expected.

Can a flat roof light tube go in an existing felt, EPDM or GRP roof?

Yes. The covering is opened in a small circle, the collar is fixed to the deck and the covering is dressed up it. The existing roof should be in good condition, so that the new detail joins sound material. A covering near the end of its life is better renewed in the same visit.

Does a round or square collar suit a flat roof better?

Round. It has no corners for water to pool in, and each covering can be formed round it in a continuous sweep. Square collars work, but each corner needs a formed piece.

Will the tunnel make the flat roof colder?

Not when the insulation, vapour layer and tube sleeve are all dealt with. The roof loses only what the glazed dome and tube allow, and an insulated sleeve keeps the tube wall close to room temperature. Get a quote and the survey will confirm the detail for your roof.