Do sun tunnels leak?

The flashing

The collector is flashed into the roof exactly like a small rooflight: an apron below, soakers up the sides interleaved with the covering, a head flashing above, and an underlay collar connecting the membrane to the unit.

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

Because the collector is small, there is less to go wrong than on a full rooflight.

The usual cause of water

Condensation inside the tube. A cold tube passing through a cold loft, with warm humid room air at the bottom, will condense moisture on its inner surface. That runs down and collects on the inside of the diffuser.

It looks exactly like a leak and it is not.

Telling them apart

The same test as always: does it happen when it has not rained? If it appears on a cold dry morning, it is condensation. If it follows rain, particularly driving rain from a particular direction, it is the flashing.

Preventing the condensation

Insulate the tube through the cold space, and seal the ceiling collar so warm moist air cannot enter the loft around the tunnel. Both are part of a proper installation, and the collar is the more important of the two.

In a bathroom, where the air is wettest, both matter most.

The diffuser seal

The diffuser is sealed to the ceiling. That seal is what keeps room air out of the tube, and it is a component that can be renewed if it ever degrades.

Properly fitted ones do not

A sun tunnel has fewer ways to leak than a rooflight: a small fixed dome, no opening sash, no gasket under daily thermal stress. When water does appear at the diffuser it is usually not rain at all.

Condensation first, because it is the common answer

A metal tube crossing a cold loft is a cold surface. Warm moist air from the room below meeting it condenses, and the water runs down the tube and arrives at the diffuser looking exactly like a leak.

The tell is timing. Condensation appears in cold weather, is worst after the room has been used, and bears no relation to whether it rained. If water appears after a bath on a January evening but not during a September downpour, that is what it is.

The cure is inside the loft rather than on the roof: insulate the tube where it crosses unheated space, and seal the ceiling ring airtight so room air cannot get into the tube at all.

The three genuine leak causes

The wrong flashing profile. Kits come matched to the covering, and a flat flashing on a profiled interlocking tile leaves voids under the tile channels. Water gets under within a season and appears inside as a leak that seems to come from nowhere.

The underlay cut without being dressed. Water reaching the roof membrane has to be carried around the opening rather than into it. Cutting the underlay without lapping it correctly turns the membrane into a funnel, and this is the fault we find most often on self installed tunnels.

Joints in the tube. Sections should be taped and sealed at each joint. An open joint in a loft admits nothing on its own, and it lets condensation escape into the loft rather than staying in the tube where it can be diagnosed.

Insects in the diffuser

A seal to check rather than an insect problem. A tunnel is sealed at the roof and at the ceiling, so anything inside got in through a gap, and the same gap will let moisture in.

What to send us

A photograph of the diffuser and a note of whether the water follows rain or cold evenings. Between those we can usually tell you whether it is a roofing job or a loft job, and the loft job is much cheaper.

Where the water actually gets in when it does

Almost never through the dome, and almost never through the tube. Three places account for nearly all of it.

The flashing at the roof. Wrong profile for the covering, or dressed over the tiles rather than interleaved with them. On a deep profile tile a flat flashing bridges the rolls and leaves open channels underneath.

The underlay collar. Where the membrane has been cut and not dressed around the opening, water reaching the underlay in driven rain arrives at the tunnel and has nowhere to go but in.

The joints in the tube. Rigid tunnels join in sections and those joints need taping. An untaped joint in a loft void does not leak rain, but it lets moist air into the tube, and the condensation that produces looks exactly like a leak at the ceiling diffuser.

Telling a leak from condensation

Timing. Water appearing during or shortly after rain is a leak. Water appearing in cold weather without rain, particularly on a still frosty morning, is condensation inside the tube.

Condensation is fixed by insulating the tube through the cold loft space and sealing the ceiling diffuser so warm room air cannot get into the tunnel. A leak is fixed on the roof.

The maintenance side of the question

Most tunnel problems that appear years after installation are not installation faults, and two checks catch nearly all of them.

The flashing and the covering around the dome. Tiles slip, mortar weathers, and a flashing that was correct can be disturbed by later roof work. A look from the ground with binoculars once a year is enough to spot lifted or displaced material.

The dome itself. Acrylic domes can craze or crack with age and UV exposure, and a hairline crack lets water straight into the tube. Glass domes do not, which is one argument for specifying one.

Debris around the dome

A dome low on a slope collects leaves and moss against its upslope side, and that dam holds water against the flashing. Under a tree it is worth clearing annually.

Where a leak follows other work

A tunnel that starts leaking after a roof repair, a solar panel installation or a chimney job almost always has a disturbed flashing rather than a failed one.

Worth mentioning to anybody who works on that roof afterwards, because a tunnel dome is easy to stand on and easy to disturb.