The short answer
A leak at a union usually comes from one of three faults: the union does not sit tightly on the tiles around it, the roll beneath it has lifted or been cut short, or the fixings are loose and let the piece move. Cheap kits make all three more likely. A roofer lifts the nearby tiles, finds which fault it is, and replaces or refits the parts.
The union is the small piece where ridge meets hip. It carries water from three directions, so any weakness there shows quickly as a stain.
What a union is and what it has to do
On a hipped roof, the ridge tiles and the two hip lines come together at a point. A union is the moulded piece, usually plastic, that sits at that point. It joins the end of the ridge to the top of each hip, holds the tiles in position and closes the gap beneath them. Because three runs meet in one spot, this joint handles more run-off than anywhere else along the apex.
How should a union fit?
It should sit flat on the roll and on the tiles beside it, with its arms in line with the ridge and the hips. The tiles either side bed down on it with an even gap, and the fixings go through the points the maker marks. Nothing should rock when pressed. Where the union stands proud, tilts, or leaves a visible gap against a tile, wind-driven rain gets past it. The fit depends on the tile too, because a union cut for one profile will not close properly against another.
Replacing a leaking union on its own often fails if the surrounding parts are still wrong, so the whole junction gets a look.
The roll beneath the union
Under the union runs the ventilated roll, the strip that seals the ridge line. At a union the roll has to stop and start in the right places. If it runs short of the union, there is an open stretch of apex where only the plastic stands between the weather and the roof space. If it overlaps awkwardly, it can lift the union up off the tile.
Why does the roll position matter so much?
Because the roll does the sealing and the union only covers it. A roll that is out of position under a union makes the best-fitted piece leak. When one side of the roll peels away, rain runs beneath the plastic, and the stain turns up on the ceiling below, often a distance from where the water got in. Our page on the roll in a dry ridge kit explains how it is dressed.
Birds and insects use the same gaps. A lifted roll at a union is an open door to both, and a nest in that spot can force the parts further out of place.
Failure modes of cheap dry systems
Low-cost kits show a pattern. The plastic of the union is thin, so it flexes in wind and cracks at the screw holes. The mouldings are generic rather than shaped to the tile, which leaves gaps that rain finds. The screws supplied are short, or too few, so the piece works loose. Colour fades or becomes brittle in the sun on some parts.
Is a leaking union a sign the whole ridge is faulty?
Not always, but it is worth checking. A single cracked union on a sound ridge is a local repair. If several unions are leaking, the cause is more likely the kit, the fitting method or the timber. BS 8612 is the specification dry-fix systems are tested against, and a kit with no test behind it has less to show for itself. A kit that is tested can still leak if the fitter skips the maker's steps, which is why we ask to see the fitting instructions for whatever system is already on the roof. Union leaks that appear within the first winter after a new ridge can point to the fitting rather than to age.
For the full list of weak points, see our page on dry ridge failure. For hip-specific faults, the page on hip tile repair may help.
Finding the cause and fixing it
Finding the cause means taking the union apart. A roofer lifts the adjacent tiles, checks the roll, looks at the fixings and tests whether the timber below is sound. A hosepipe test is sometimes used to trace where water gets in, though an experienced eye usually spots the fault first. If the stain on your ceiling is directly below a union, the fault is likely close by. If not, the water has travelled.
Can a leaking union be repaired without redoing the ridge?
Often yes. A cracked union comes off and a part made for your tile goes on, the roll is re-laid and re-stuck at that junction, and the tiles refitted. If the fixings have pulled out of soft timber, they go into sound wood or the batten is renewed. If the kit is the wrong one for the tile, a repair only delays the next leak, and a full refit is the better buy.
You see photos of the junction before we price anything, and the written price names the parts. More on the system is in our guide to dry ridge and verge. A photo of the stained ceiling helps us place the problem.
More on leaking unions
Can I seal a leaking union with silicone?
Sealant can hide a gap for a short time, but it does not fix the cause and it can stop the ridge ventilating. Silicone also traps water in places the roll was meant to shed it. A roofer finds the fault and puts the parts right instead of coating over them.
Is a leaking union an emergency?
Where water is dripping through the ceiling, treat it as urgent: a leaking roof gets made safe first and mended properly after. A slow stain that appears only in heavy rain is less urgent, but it still wets the timber, so book a survey rather than waiting for the next storm.
The full picture: Dry ridge and verge repairs in Taunton ยท More short answers