A valley is the internal angle where two roof slopes meet and a box gutter is a lined channel between a roof and a wall or parapet. Both carry far more water than an eaves gutter and both sit inside the building envelope rather than outside it, which is why a failure here wets the structure instead of running down an outside wall.
Lead valley gutter detail: how it is supposed to work
The lining sits on boards in the angle, dressed up under the tiles or slates on both sides, in lengths short enough that thermal movement does not tear it, with laps arranged so water always runs onto the piece below. The two failures are lengths that are too long, which split at the fixings from repeated expansion, and laps that are too short or the wrong way round. Neither is visible once the tiles are back, which is why the detail matters more than the material.
The conservatory box gutter problem
Where a conservatory or a lean to roof meets the house wall, a box gutter runs along the junction and takes the water from both. It is shallow, often has very little fall, and is frequently the lowest quality part of the whole structure. Debris collects in it, the outlet is small, and when it backs up the water goes over the back edge and into the wall or the roof void rather than outside. Most conservatory leaks reported as roof leaks are this junction.
Access for maintenance is part of the design
A valley or box gutter that cannot be reached to be cleared will eventually block, and blocking is what causes the failure. A sound design gives a way to inspect and clear it, keeps the outlet generous, and provides an overflow so that a blockage shows itself outside the building rather than inside it. Where there is no overflow, the first sign is a stain on a ceiling.
Garden room guttering, and where the water goes
A garden room is a small building and its rainwater still has to go somewhere that is not the base of its own walls or the neighbour's boundary. That usually means a gutter and downpipe to a soakaway sized for the roof area, not a pipe discharging onto the lawn beside the timber frame. Approved Document H deals with rainwater disposal and it applies to the water even where the building itself is exempt.
Questions people ask about valley gutter repair
Why does my conservatory leak where it joins the house?
Usually the box gutter at that junction is blocked, has too little fall, or its flashing into the wall has failed. Clearing it is the first step and the fall and the outlet are what to look at if it recurs.
How long does a lead valley last?
Properly sized and laid lead is a very long life material, commonly outlasting the covering around it. Lead that splits early has almost always been laid in lengths that are too long or fixed so it cannot move.
Can a valley be repaired without stripping tiles?
Rarely and not well. The lining runs under the tiles on both sides, so a genuine repair means taking them back. Sealant applied over the top from above is a short term measure that traps water underneath.