The short answer
A box gutter is a lined channel set between two slopes or along a parapet. When the lining splits or the outlet blocks, water backs up and enters the walls and ceilings. A failed gutter is usually relined in place with EPDM, GRP or lead, after the deck, falls and outlets have been put right, so the new lining sits on a sound base.
What a box gutter is and why it fails first
On many industrial units, mills and older shops, two pitched roofs meet in a valley that is too shallow or too wide to be an ordinary gutter. Instead, a rectangular channel is formed between them, lined, and drained through outlets. A parapet gutter is the same idea, running along the foot of a low wall around the roof edge. Both carry the run-off of a large area into a narrow channel, so each metre of gutter carries more water than any other part of the roof.
That is why they fail early. Linings were often felt, asphalt or lead, and they age in the sun and in the damp. The channel holds leaves, moss and silt, which keep the lining wet and block the outlets. Footfall damages it as well, since the channel is where people walk to reach plant. Water in a box gutter has nowhere else to go, so a lining that has split lets it straight into the wall or the room below. Stains on the ceiling beneath a valley, or damp patches at the foot of a wall, are often the first sign.
Why patching a failing box gutter rarely lasts
A split can be patched, and as a stopgap that is sensible. But a lining that has cracked in one place is usually weak along its length, and the next failure appears a few metres further on. Patches also add thickness and edges where silt gathers. If the channel is also ponding, because its fall has been lost or the timbers below have sagged, the repair is working against the geometry.
Before deciding between patching and relining, we look at four things. The lining: is it brittle, split or lifting at the upstands? The deck beneath: is the boarding sound or soft from long damp? The fall: does water drain to the outlet or sit in the middle? And the outlets: are they large enough, clear and free to carry the flow? Manufacturers citing BS 6229 aim for a finished fall of 1 in 80 on flat roofs, and a box gutter should shed water towards its outlet in the same spirit. Our survey photographs show what we found, so you can see the case for relining before you pay for it.
Relining options: EPDM, GRP and lead
There are three common ways to reline a box gutter. EPDM is a rubber membrane cut to the length of the channel, which suits long runs because there are few joints; it is glued to the deck, turned up the sides and finished under the roof coverings. GRP, glass-reinforced plastic, is brushed on in liquid form and sets hard as one lining, which helps in awkward shapes and short, difficult channels. Lead is the traditional metal: the gutter is lined in lead sheet, with drips cut in to take the fall, and it can last for many decades.
MyBuilder's 2026 flat roof guide gives rates per m² of £90 to £130 for EPDM, £95 to £150 for GRP and £150 to £200 or more for lead. These are for roof areas, not for gutters, so they give scale rather than a price. Lead sheet is graded by code: Royston Lead lists codes 5 to 8 as suitable for parapet and box gutters, so the heavier codes are the usual choice for a gutter that must bear foot traffic.
On a listed mill or a stone-built building, lead may be the sensible match; elsewhere, EPDM or GRP is often more practical. Relining choices are discussed alongside the wider roof in our guide to the best roof systems for Stroud commercial buildings.
What goes into a relining job
Work starts by clearing the channel and stripping the old lining back to the deck. The boards are checked and soft or rotten timber is cut out and new boarding fitted. Falls are restored with tapered boards or firring pieces so that water runs to the outlet. Outlets are cleared, enlarged or replaced if the channel is carrying more than they can pass, and an overflow is added where it is missing, since a blocked outlet is what turns a heavy shower into a flood inside.
The new lining goes in with upstands turned up the walls and under the roof coverings, and the edges are dressed and sealed. Where the wall behind is leaking as well, the flashing or coping above is dealt with at the same time. On a lined parapet, the lining is tucked behind a new flashing to stop water running down the back of it.
Finally the channel is tested with water before we leave, and the site is cleared. The quote covers the strip, deck repairs, falls, lining, outlets, access and waste, and our 10-year workmanship guarantee comes in writing.
Timing, upkeep and keeping a lined gutter working
Box gutters can be relined in most seasons, but adhesives, sealants and cold-applied products all have limits on temperature and damp, and an open channel must not be left in rain. Our page on commercial roof replacement in winter covers which systems cope with cold, and a dry window is worth waiting for when the situation allows.
Once relined, a box gutter needs very little, but that little matters. Leaves, silt and moss should be cleared twice a year, more often under trees, and the outlets checked after storms. Walking in the channel should be kept to a minimum, and a duckboard fitted if staff must pass along it. This is the kind of job a planned maintenance visit picks up.
If the roof as a whole is tired, a liner on the gutter alone will not stop leaks elsewhere. Our guide to liquid roof coatings explains one way of extending the life of the sheets either side. The commercial roofing page covers the wider service.