Sun tunnels on a pitched and on a flat roof

Pitched roofs

The collector is flashed into the covering exactly like a small roof window: apron, soakers, head flashing and an underlay collar.

The kit has to match the covering profile. A flat kit on a profiled tile leaves a gap under every roll, the same defect that affects roof windows.

The run is usually longer, because the tunnel drops from the roof slope through a loft to the ceiling below. Bends are often needed to route past structure.

Flat roofs

The collector sits on a small kerb, and the covering is dressed up it and terminated under the unit, exactly like a flat roof rooflight.

Same rules: adequate kerb height, square, positioned where water reaches it moving rather than ponding against it, insulated on the outer face in a warm roof.

Why flat often performs better

Because the ceiling is usually directly below the roof, with only a shallow void between. The run is short and straight, which is exactly what a tunnel wants.

A 250mm tunnel with a 400mm straight drop delivers considerably more light than the same tunnel on a two metre pitched run with two elbows.

Collector shape

Usually domed on a flat roof, which gathers light from a wider angle including low morning and evening sun. On a flat roof that matters more than on a pitch, because a flat collector facing straight up gathers less at the ends of the day.

Condensation

Less of a risk on a flat roof, because there is less tube in cold space. The ceiling collar is still sealed.

Same tube, different roof end

The tube, the diffuser and everything inside a sun tunnel are the same whichever kind of roof it comes through. What changes is the roof component and how it is weathered, and manufacturers make distinct kits for each.

Fitting a pitched roof kit to a flat roof, or the reverse, is not a matter of improvisation. The flashing arrangements are entirely different and neither works in the other situation.

The pitched roof kit

A dome or a low profile glazed top mounted on a flashing that interleaves with the roof covering, exactly as a roof window’s flashing does.

The profile is matched to the covering and they are not interchangeable: flat for slate, deep for profiled interlocking tiles, specific patterns for pantiles. A flat flashing on a profiled tile leaves open voids under the tile channels where it crosses them, water gets under within a season, and because it enters below the flashing it appears inside as a leak that seems to come from nowhere.

That is the most common installation fault on self fitted tunnels, and it is exactly where a cheap quotation economises, because the flashing is a separate order line and nobody can see which one was used.

The underlay matters just as much. Water reaching the roof membrane has to be carried around the opening rather than into it, and cutting the underlay without dressing and lapping it correctly turns the membrane into a funnel.

The flat roof kit

A dome or flat glazed top mounted on a small upstand rather than a flashing, and the principles are those of any flat roof rooflight.

The upstand lifts the unit clear of standing water. The roof covering is dressed up the full height of it and terminated properly. The unit sits on top with its own flange overlapping that termination, so water landing on the dome runs onto membrane rather than into a joint.

Getting the order right matters: the covering goes up the upstand before the unit sits on it, not after.

Upstand height, which is the detail that decides it

Every flat roof holds some water. A small dome bedded straight onto the deck sits in it after every shower, and a seal designed to shed water is not designed to be submerged.

The same 150mm working minimum applies as on a flat roof rooflight, with the deck falling away from it and an outlet that actually runs.

On a green roof the upstand has to clear the growing medium as well, so on a sedum roof with 80mm of substrate you are looking at 230mm or more, plus a vegetation free margin of gravel or paving around it so roots and growing medium stay off the upstand.

Insulation on the outer face of the upstand applies here too, on the same principle as a rooflight kerb, though the perimeter is much shorter so the absolute loss is smaller.

Why flat roof tunnels frequently perform better

Not because of the roof detail, but because of what is usually below it.

Flat roofs are most often over single storey extensions, so the tunnel drops directly into the ceiling of the room below. That is a short straight run, frequently under a metre, and light loss compounds with length.

A one metre straight run at 350mm delivers considerably more than a four metre run with two bends at the same diameter. So a flat roof tunnel over an extension is frequently the best performing installation of the lot.

The same applies to bungalows with pitched roofs, for the same reason: it is the run length that matters rather than the roof type.

Condensation, and where the difference lies

On a pitched roof with a cold loft, the tube crosses unheated space and must be insulated along that stretch, with the ceiling ring sealed airtight so warm room air cannot get into the tube at all.

Without both, moisture condenses inside the tube and runs down to the diffuser looking exactly like a leak. It is the commonest complaint about sun tunnels and it is entirely preventable.

On a warm flat roof, where the insulation sits above the deck, the tube passes through insulated construction rather than cold space, which reduces the condensation risk considerably.

What it introduces instead is a vapour control layer question. That layer sits below the insulation and has to be carried up and sealed to the tunnel, so moist air cannot pass into the roof build up where it would condense out of sight. Getting that wrong on a warm roof produces wet insulation, which is expensive and slow to discover.

On a cold flat roof, where the insulation is between the joists, the situation is closer to a pitched cold roof and the tube needs insulating where it crosses the ventilated void.

Dome or flat glazing at the roof end

A domed top collects light from lower sun angles, which helps at the beginning and end of the day and in winter. It also sheds water and debris better than a flat top.

A flat glazed top is more discreet, which matters where the roof is overlooked from an upper window, and it is what a conservation officer will want to see on a designated property.

Glass rather than polycarbonate is worth the difference on any south facing position. Polycarbonate yellows and hazes under ultraviolet over fifteen to twenty years, at which point output drops noticeably. It is a replaceable part rather than a reason to change the installation, and glass does not degrade at all.

Planning and consent

On both roof types, a sun tunnel is generally permitted development on an unlisted house outside any Article 4 direction, because the roof component is small and low and comfortably within the 150mm projection limit.

On a listed building, consent is required regardless. A sun tunnel is not exempt for being small, and that catches people out because a 350mm dome on a rear slope feels too minor to need permission.

Where a full rooflight would not be consented, the tunnel is frequently the option that will be, because the intervention is one opening between rafters rather than a trimmed structural opening and it is largely reversible. That is a genuine argument to put to a conservation officer.

What to send us

A photograph of the roof, a photograph of the space between the roof and the ceiling position, and a note of which room you want lit.

On a flat roof, add a photograph showing how the roof meets the house and, if you can see it safely, where water sits after rain. On a pitched roof, tell us what the covering is, because the flashing profile follows from it.

The tube and diffuser are the same, the top is not

Manufacturers make distinct pitched and flat roof versions, and the difference is entirely at the roof end.

On a pitched roof, the dome sits on a flashing that interleaves with the covering, exactly as a roof window does. The flashing profile has to match the covering: flat for slate and plain tile, deep for pantile and profiled interlocking tile.

Below it, an underlay collar is dressed and lapped so water reaching the membrane passes around the opening.

On a flat roof, the dome sits on a kerb. The waterproofing is dressed up that kerb and terminated into the unit, and the unit sits on top. It is a flat roof detail, not a lapped covering detail.

Why the flat roof version frequently performs better

The run.

On a flat roof over a single storey extension, the ceiling void is shallow and the tunnel travels a short straight distance with no bends at all. That is the ideal case, and light loss is minimal.

On a pitched roof the tunnel has to cross a loft, which frequently means length and at least one bend to get from a workable roof position to the right ceiling position. Bends cost more light than length does, so the pitched installation is frequently the weaker performer despite being the more common.

What each needs detailing correctly

On a pitched roof: the right flashing profile, the underlay collar, and the covering reinstated over the flashing rather than under it.

On a flat roof: a kerb high enough to clear any water the roof can hold, insulated on its outer face continuous with the roof insulation, and falls that carry water past the kerb rather than into a dead corner beside it.

Both need the tube insulated through unheated space and the diffuser sealed to the ceiling, or the tunnel runs with condensation in winter.

The dome shape

On a pitched roof, the slope sheds water and debris and a dome stays reasonably clear.

On a flat roof there is no slope to help, so a domed top matters more than it does on a pitch: a flat top holds water, dulls, and collects debris around its edge.

Consent

A pitched roof dome is visible from the ground and subject to the usual questions on a designated building, though its small size makes it a far easier proposition than a rooflight.

A flat roof dome on a rear extension is frequently not visible at all, which removes the question.