Very rarely. The roof unit is a small component flashed into the covering in exactly the way a roof window is, and the detail is short, simple and well proven. When water does appear at a diffuser it is far more often condensation forming on the tube in a cold loft than rain finding its way past the flashing.
How a pitched roof unit is weathered
The same principle as a roof window.
A flashing kit matched to the covering, with the underlay collar lapped so anything reaching the membrane is carried out over it.
The mechanism is on will a roof window leak.
Why it is simpler than a rooflight
Because it is small. A short perimeter means fewer courses disturbed, a smaller flashing and much less covering to reinstate, so there is less to get wrong.
The flashing kit
Chosen for the covering, not the tunnel.
A kit for profiled tiles differs from one for slate, exactly as with a roof window, and using the wrong family is the mistake to avoid.
What a kit does is on what is a flashing kit.
The underlay collar
The detail that decides it. The membrane is cut and dressed around the opening so water reaching it runs out over the flashing rather than in behind it, and the lap direction is everything.
Debris above the unit
The commonest cause of an actual leak.
Moss and leaf litter collecting on the covering above the head dams water at the flashing, which is where it then finds a way past.
A slipped tile above it
Frequently the real fault. Water entering through a covering fault higher up the slope runs down the underlay and appears at the first interruption, which is the tunnel.
On a flat roof
A small kerb, weathered as any upstand.
The covering is dressed up the upstand and terminated under the roof unit, and the same rules about height and lap apply as to a rooflight.
That is on can a sun tunnel be fitted to a flat roof.
Why water in the tube is usually not a leak
Because a tube in a cold loft is a cold surface.
Moist air in a roof space condenses on any cold object, and a metal tube crossing a loft is exactly that, so water forms on the outside of it and runs.
The mechanism is on do sun tunnels cause condensation.
How to tell them apart
Weather, as always. Water appearing in cold weather with no rain is condensation. Water appearing during or after rain, particularly driving rain, is a leak.
The full method is on is it condensation or a leak.
Water inside the tube
A different and rarer fault. Water actually within the tube points at the roof unit seal or at a joint in the tube sections, and it is diagnosed from the loft rather than from the room.
Tube joints
Taped and sealed on assembly. Sections are joined and taped so the tube is closed along its length, which keeps loft dust and any moisture out of the light path.
Why a closed tube matters
For appearance as much as anything. Dust entering a tube settles on the inside of the diffuser, and a tunnel that has gone dim after a few years is often simply dirty inside.
The dome seal
Where the roof unit itself is sealed.
The dome or glazed head is sealed to its base as a manufactured component, and that joint very rarely fails within the life of the covering.
An old unit
Where a polymer dome ages. A polycarbonate dome embrittles and crazes over decades exactly as a dome rooflight does, and an aged one can crack.
The parallel is on can a dome rooflight be replaced with glass.
Diagnosing from the loft
The advantage a tunnel has. Unlike a rooflight, the whole route is inspectable from inside the loft, so a torch and a look at the tube in wet weather usually answers the question.
What we look for first
The covering above the unit. Debris, a slipped tile and the head flashing are checked before anything is attributed to the tunnel itself.
What we would not do
Seal round the outside of the flashing.
A bead round a flashing traps water that the detail was designed to shed, and it makes the next diagnosis considerably harder.
Why is on can a leaking rooflight be resealed.
Wind driven rain on the coast
The condition that finds a marginal detail.
Rain arriving nearly horizontally off the sea is pushed at a flashing rather than falling on it, which is when an inadequate lap shows up.
What maintenance prevents
Almost all of it.
Clearing debris from above the unit once a year, when the gutters are done, prevents the commonest cause of a leak at a sun tunnel.
Where a tunnel was fitted badly
The pattern we see most. A unit fitted quickly with the flashing dressed over rather than under the covering above it, and no underlay collar formed, will pass water in a driving south westerly.
It is a workmanship fault rather than a product one, and it is corrected rather than sealed.
Guarantees
Two documents, as always. The manufacturer stands behind the unit and the installer behind the flashing and the collar, and water almost always arrives through the second of those.
What we look for on a sun tunnel
We check the tube and the loft before we go on the roof, because the great majority of water reported at a diffuser turns out to be a loft problem rather than a roof one.
The whole picture is on the sun tunnel installation page.
Thinking about this job? We survey across Worthing, West Sussex and the South Downs and give you a fixed price before any work starts, with a 10-year workmanship guarantee.