Fitting a sun tunnel through a tiled or slate roof, with the flashing matched to the covering

A sun tunnel needs a pitched roof that drains freely past it, a flashing that matches the covering, and a clear route down through the loft. Everything else follows from those three.

The dome sits on a short base, often called a curb or collar, that stands a little proud of the tiles. Around that base sits a flashing skirt. On the upper side of the dome the skirt slides under the tiles above. On the lower side it lies over the tiles below. At the two flanks it is dressed so that water is steered round the dome and not trapped against it. Get that sequence right and the dome is simply an obstacle in a stream, and the stream goes round it.

Because water moves by gravity, the pitch matters. A steeper slope sheds water quickly and leaves little chance for wind to push it uphill under a lap. A shallower slope sheds slowly and lets driven rain travel further under each tile. The flashing has to suit the slope it is on, so the pitch is the first thing we measure. Flashings are made for a stated range of pitch, and the measurement is taken, never assumed.

For a wider view of the product itself, the sun tunnel installation hub sets out how the dome, tube and diffuser work together. This page stays on the roof.

Tiled and slated roofs are different materials with different habits, and the flashing follows the material. Identifying the covering correctly is what decides the detail.

The table gives the outline. The sections below give the reasoning, because two roofs with the same covering can still need different handling depending on age, exposure and what lies underneath.

Older roofs deserve a slower look. A slate roof laid in the 1930s and a slate roof laid last year both look like slate from the garden, but the nails, the battens and the underlay beneath them may be quite different. The survey looks at the underside from the loft as well as the top from a ladder, because the loft shows what the tiles are really sitting on.

We walk the route before we quote it. Four things are measured: the roof pitch, the rafter spacing, the batten gauge and the path the tube will take from dome to ceiling.

The pitch is taken with a digital level on the rafter. The rafter spacing is measured from centre to centre in the loft. Domestic rafters are typically set roughly 400 to 600 millimetres apart, and a sun tunnel is designed around that gap. A standard 250 or 300 millimetre tube fits between two rafters with room to spare. A larger diameter may land close to a rafter edge, and the position of the dome can then be shifted along the slope to keep clear. The guide to choosing a sun tunnel diameter explains how size relates to the room; here the point is that the diameter also has to suit the structure.

The batten gauge is the vertical distance between battens, which is set by the tile or slate being used. It fixes where the flashing skirt can be supported and which courses will need cutting. A position that puts the dome neatly in the middle of a course saves cutting. Moving it a hand-span up or down the slope can turn a fiddly detail into a tidy one, so we look at the tile pattern from the ground before settling on the spot.

The route is the line from the roof opening down to the ceiling. It should avoid the water tank, electrical cables, the ceiling joists and any ducting. A straight run gives the most light, and the difference between rigid and flexible tubes matters if a bend is unavoidable. Only when the route is clear do we fix the roof position.

Concrete and clay tiles with a wave or a roll are the easiest coverings to recognise and the most forgiving to work with, provided the flashing follows the profile exactly.

Profiled tiles interlock at their sides and lap at head and tail. Because they are large, a single tile covers a good deal of roof, and removing a few opens a generous working space. A flashing made for profiled tiles is formed with a skirt that can be pressed down into the shape of the tile, so that it sits close to the surface all the way across. Aluminium-backed pliable flashing can be moulded by hand, while pressed metal skirts arrive already shaped for common profiles. Either way, the aim is the same: the skirt should follow the tile, so there are no gaps underneath for wind-driven rain to use.

The sequence is straightforward. The tiles in the working area are lifted and stacked. The battens are checked and, where the opening crosses them, cut back and supported. The base of the dome is positioned on its frame. The flashing skirt is laid so that its upper edge disappears beneath the tile course above and its lower edge rests on the course below. Then the tiles are replaced around it, cut where they meet the skirt.

The side troughs need particular care. On a profiled tile the trough between two ribs carries the water. Where a trough meets the flashing it has to be directed onto the skirt, not past its edge. Good flashing kits supply side wings for this. Where a wing does not quite reach, the tile is cut to sit neatly over the edge of the skirt, and any small gap is filled with a tile-matched closure. The finished result should look as if the dome had always been part of the roof pattern.

Tile cuts are made with a wet cutter or an angle grinder fitted with the right blade, in a spot where the dust can be managed. A clean cut matters for looks and for seating. A chipped edge sits badly and draws the eye.

Plain clay tiles are laid in a double lap, so there are two layers of tile over the bulk of the roof. That gives excellent weather protection and a demanding detail, because every cut course has to be rebuilt the same way.

Plain tiles are small, commonly about 265 by 165 millimetres, and each tile is nibbed on to a batten. Around a sun tunnel the tiles in the working area are lifted course by course and set aside in order. The flashing for plain tiles is typically a flat skirt, often with lead at the edges, because lead can be dressed over the small step of each course and will hold its shape. The upper edge of the flashing goes under the courses above. The sides are dressed up and over the tile edge. The lower edge lies over the course below.

Where the tiles return around the dome, the courses are cut and staggered so that the joints do not line up in a straight seam. A straight seam gives water a direct path. A staggered joint breaks that path, which is the whole point of a double lap. Replacing the tiles in their original order also keeps the roof looking continuous. Hand-made tiles vary in colour and texture, and putting them back in their own sequence hides the work.

Older plain-tile roofs carry a patina that new tiles do not share. If a tile breaks while it is lifted, we match it against spare tiles kept from the original roof, or taken from a hidden slope, so that a mismatched colour does not sit next to the new dome. On a house in a conservation area, the appearance of the dome matters as much as its detail. A low-profile dome in a dark finish suits such a roof.

Slate is thin, hard and brittle. It lasts for generations, and it needs a careful touch from the first foot on the roof to the last nail.

Slates are laid in a double lap with a head lap that depends on the pitch: a shallower roof needs a bigger overlap. Each slate is held by nails near its head, and the slate above covers those nail holes. That means the fixings are hidden, and it also means any slate you lift must be handled so that it is not cracked or its nail holes enlarged. Crawl boards or a roof ladder spread the load, because walking directly on slate can snap it.

The standard flashing for a sun tunnel through slate is a lead apron with a slate-covered upper side. Lead, worked to shape with a dressing tool, forms a tight fold against the base of the dome and lies flat against the slate face below. At the sides, the lead is dressed up against the collar and covered, with the slate courses cut to close over its edge. Lead is chosen because it is soft enough to be dressed into the exact line of the slate courses and stays put once it is in place. Lead sheet is supplied in codes by weight, and flashings of this kind are normally made from the lighter codes, as a rule of thumb around code 4.

The slates beside the dome are trimmed so that the gauge carries on unbroken and the pattern of joints is kept. Slate cutters or a slate axe are used, and edges are cleaned with a rasp. Where original slates cannot be reused, matching Welsh or imported slates are sourced to suit the colour and thickness of the roof.

Fibre-cement slate behaves differently. It is lighter and somewhat more forgiving to cut, but its edges chip if they are handled roughly, and its fixings are usually copper nails or rivets. The flashing approach is similar, using lead or a pliable skirt, and the same staggering of cut courses applies.

Once the covering is lifted, the structure is exposed. The opening for a sun tunnel is cut in the underlay and, where needed, in the battens, with the rafters left intact.

A standard sun tunnel is sized to sit between two rafters. The batten or battens in the path of the dome are cut back and supported on short noggings, which are offcuts of timber fixed between the rafters. Those noggings carry the cut ends of the battens and give the dome base something solid to rest on. Because the opening is a hole of only a few hundred millimetres, the rafters themselves are not cut. A larger opening, or a position that crosses a rafter, is a different job that calls for trimming and sometimes a structural check. That is nearer to fitting a roof window, and the roof window installation is a separate job.

To mark the opening, a pilot hole is drilled upwards from the loft, so the exact centre can be found from the roof side. A circle is then drawn around it with a trammel or by following the base template. The hole is cut from the loft or from above depending on access, with the dust kept under control and the loft insulation moved aside rather than crushed.

The underlay is handled with care. It is the second line of defence, and it carries any water that gets past the tiles down to the gutter. Around the dome base, it is cut and lapped so that it runs onto the collar, not past it. Taped joints hold it in place, and the loose edges are dressed so that they cannot flap in the wind. The underlay detail is out of sight once the tiles are back, but it is part of why the roof stays weathertight over the long term.

With the structure ready, the flashing is fitted. Its job is to lap correctly, stay in position and let water run over it without finding an edge to creep under.

The base of the dome is fixed to the roof structure through the collar, not through the flashing alone. That way the dome is held by the timber, and the flashing is left free to do its own work of shedding water. The skirt is then fitted around the base and set in the lap sequence described above: upper edge under the covering, lower edge over it, sides dressed to channel water away.

Fixing positions are chosen so that any nail or screw is covered by the layer above it. A fixing in the open face of a flashing is a place where water could enter, so any that cannot be hidden is sealed with a compatible sealant and placed high in the profile. The answer on how a sun tunnel is kept watertight goes through the layers in a shorter form.

Colour is a final consideration. Flashing and dome bases come in grey, black and terracotta finishes among others, and a sensible match lets the detail sit quietly in the roof. Lead weathers to a soft grey, which suits slate and dark tile well. A bright-finished skirt on a weathered roof will draw the eye for a season or two and then settle.

The dome is the noisiest part of a sun tunnel, because it is a thin curved sheet standing in the weather. The roof covering and the fitting details decide how much of that sound reaches the room.

Rain drumming on a dome can carry into the tube, and from the tube into the ceiling below. On a tiled or slated roof the effect is softer than on a flat roof, because the pitch sheds water quickly and the dome presents a small surface. Even so, the choice of dome build, the seating on the collar and the way the tube is fixed and supported all affect what is heard. Quiet-minded choices include a double-skinned dome, a firm seat on a compressible seal, and tube fixings that do not pass vibration into the ceiling. The guide to keeping a sun tunnel quiet in the rain goes into these in detail.

For a loft bedroom under the Gatwick routes, or a hallway beside a busy road, the quiet spec that comes with every quote records what we heard at the survey and which build we propose. You can read how that document is put together on The Quiet Spec page. It is written for the room, not for the product.

The tube runs through the loft, and the loft is where heat, cold and moisture meet. Treating the tube as part of the thermal envelope keeps the room below comfortable.

Where the tube passes through loft insulation, the insulation is fitted snugly around it, and the tube itself is chosen or wrapped to limit heat loss. In a cold loft, an uninsulated tube can be a cold surface, and warm air from the room can condense on it. A well-fitted tube with a sealed ceiling connection keeps room air from reaching the cold surfaces. Both subjects have their own pages: sun tunnels, loft insulation and heat loss, and preventing condensation in a sun tunnel.

A sun tunnel through a tiled or slate roof is a single-visit job for a straightforward route. The work is planned so that the roof is open for as short a time as possible.

The day follows a steady order. Dust sheets go down in the loft and the room. The position is confirmed. The roof opening is prepared, the dome base and flashing are fitted, and the roof is made weathertight first. Only then is the tube run down through the loft and the ceiling opening cut, so that the inside work happens under a closed roof. The diffuser is fitted, the ceiling is made good and everything is cleared away. How long each step takes depends on the covering and the route, and the page on how long a sun tunnel takes to fit gives realistic expectations.

The weather is watched. A tiled roof is opened only when the forecast allows it to be closed the same day, and a temporary cover is ready in case rain arrives sooner than expected.

A sun tunnel is a small alteration, but it still touches the roof, the ceiling and the loft, so the rules apply. We install to current Building Regulations, and we handle the Building Control notification where the work is notifiable.

Whether a particular job is notifiable depends on what is involved. The page on whether a sun tunnel needs Building Regulations approval sets out the position in plain terms. The work is covered by our 10-year workmanship guarantee, which is a workmanship guarantee and is not insurance-backed.

These are the questions that come up when a homeowner looks at the roof and wonders how a round dome will sit on it.

Will the tiles be cut?

Some will. A few tiles around the dome are cut to fit neatly against the flashing, and the rest are lifted and relaid in their original order. With profiled tiles, the cuts follow the natural line of the profile.

Can a sun tunnel go on a slate roof without cracking the slates?

Yes, with care. Slates are lifted, not levered, and their fixings are released gently. A crawl board spreads the load while working on the slope. Any slate that cannot be reused is replaced with one that matches the roof in colour and thickness.

Does the flashing colour need to match the roof?

It helps. A finish close to the tile or slate makes the dome sit quietly on the roof. Lead weathers to grey, and suits slate particularly well.

Will it look out of place on the front slope?

A dome is small and low. The rear slope gives the tidiest look, and the position can be shifted along the roof to sit centrally in a course. Where the light route from the front slope is better, a low-profile dome in a matching finish keeps the look calm. Which slope is best for light is covered on the page about fitting on a north-facing roof.

To talk through your own roof, send a photograph and the room you want to brighten through the quote form, and we will arrange a survey.