Box gutters
Why does a conservatory box gutter leak?
A box gutter hides in the roof edge, so its leaks show elsewhere. This covers how it fails, how we trace the fault and which falls and lead codes apply.
The short answer
Because the lining or one of its joints has failed, or because the gutter cannot drain. A box gutter is built into the roof edge, and a split, a lap that has opened, a blocked outlet or standing water will each send a drip into the room, usually some distance from the fault. Tracing it means testing the channel, not guessing from the stain. Where the lining is sound and the outlet was blocked, clearing it ends the leak; where the lining has gone, a new lining is the repair.
4 min read ยท Part of Conservatory roofs
What a box gutter does on a conservatory
A box gutter is a timber-framed channel that collects rain from a roof slope and carries it to an outlet. On a conservatory it sits in the roof edge or in the valley where one roof meets another, so it takes the run-off from every tile or sheet above it and has no room for error.
The channel has four parts:
- the boards or deck under it, which give it its shape and its fall
- the lining, which is the waterproof skin and is usually lead or a flat-roof membrane
- the outlet, where the water leaves the channel through a pipe or into a hopper
- the edges, where the lining turns up under the roof covering on one side and the wall or fascia on the other
Because the channel is hidden under the roof covering, nobody sees it fail. The first sign is a stain, a drip at a window head or damp lining paper under a ceiling, and by then the water has often travelled along a timber before it reached the room.
Where the leak starts
A box gutter fails in a small number of ways, and each one leaves its own clues. We go up and look, then run water into the channel to see where it goes. A wet patch on the ceiling lining is a clue, not a diagnosis.
| Symptom | Cause to test for | Likely verdict |
|---|---|---|
| Drip only in heavy rain | Blocked or undersized outlet, gutter filling and spilling | Repair |
| Standing water after rain stops | Channel without enough fall, so it ponds | Check |
| Drip along one seam | Split lining or a joint that has opened | Repair or replace |
| Stain beside the wall | Edge detail lifted, or the lead at the wall | Repair |
Leaves, moss and a dead bird can all block an outlet, and a gutter that is blocked will spill over its edges even if the lining is perfect. We clear the outlet first and test again, so you never pay for a new lining that a handful of debris was hiding. If the water then drains away cleanly and the leak stops, the gutter was never the failed part.
Falls, lead codes and the lining
Water has to travel to the outlet, so the channel needs a slope. For membrane-lined roofs, the NFRC says BS 6229:2025 sets a minimum 1:80 finished fall for general roof areas and internal gutters. IKO puts it this way: "To achieve a 1:80 fall, the code of practice says that the roof should be designed for a fall of 1:40." BS 6229 covers membranes only, so it does not govern a lead lining.
For lead, Royston Lead's cast-lead brochure lists Codes 5 to 8 for gutters of the box type, alongside parapet and tapered valley gutters. Code 4 is listed for flashings and soakers but not for box gutters. The code is the thickness of the sheet, and the specification fixes the code for each detail, so a lining of the wrong thickness is a fault of the original build as well as of age.
Standing water after the rain has stopped points to a fall that is wrong. Correcting it means opening up the boards below, so a ponding channel is a rebuild and never a patch.
Repair, relining or a new roof
The verdict follows what the test shows. A blocked outlet is cleared and the gutter tested again. A split in an otherwise sound lining can be repaired where the lining is accessible and the fall is right. A lining that is worn along its whole length, or a channel that ponds, calls for a new lining on corrected boards.
Sometimes a failing box gutter belongs to a roof that is otherwise nearing the end of its working life, and then a patch buys little for the money. Then it makes sense to renew the gutter with the roof, so that the new covering is laid to a gutter that will outlast it. Our guide to conservatory roof leaks covers the other places water gets in, and the page on how a conservatory roof meets the house wall deals with the edge beside the house. The conservatory roofs page shows how the checks fit together. We quote a repair or a rebuild as a single written figure, and only once the gutter has been opened up and looked at properly.
More questions
Can a blocked box gutter overflow into the conservatory?
Yes. When the outlet clogs, the channel fills and spills over whichever edge is lowest, and on a conservatory that can be the side next to the room. Clearing the outlet and testing the gutter is the first step, because it separates a blockage from a lining that has failed. Where it keeps blocking, we look at the outlet size.
Can a leaking box gutter be patched rather than replaced?
Where the fault is a split in a lining that is otherwise sound and the gutter drains properly, a repair is reasonable and we will say so. A lining worn along its length, or a channel that ponds because the fall is wrong, needs a new lining on corrected boards. You hear which of the two it is before any figure is written.
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