Where two roof slopes face each other across a narrow gap, or a slope runs up to a taller wall, there is nowhere for a normal gutter to hang. The solution that builders have used for centuries is the box gutter: a rectangular channel sunk into the roof, lined in metal, and fed from both sides. In older towns it is common between the pitched roofs of terraced houses, behind the front wall of a shop, and in the valleys of double-pitched buildings where a central trough replaces the usual V. A box gutter does a lot of work in a small space, and it has little margin for error. If its fall is wrong, if an outlet blocks, or if the lead splits, the water has only one place to go, and that is into the building. A leak usually traces back to how the gutter was built, how its falls and drips were set, or one of a handful of known faults.
Where you find a box gutter
The name describes the shape. A box gutter has a flat floor, called the sole, with a vertical side rising on each edge. The sides may be the roof slopes themselves, formed in timber, or a mix of roof and masonry. Because the channel sits below the line of the roof, its sides must be high enough to keep the water in when rain is heavy.
You find the form in four places. The first is between two parallel pitched roofs, where a pair of terraced houses or a double-fronted building meets in the middle. The second is where a pitched roof meets the side of a taller building and the gutter follows the base of the wall. The third is behind a chimney stack, where a small gutter, often called a back gutter, carries water round it. The fourth is at the foot of a parapet, which makes the channel a close relative of the parapet gutter. The difference is mainly where it sits and what holds up its edges. A parapet gutter lies behind a front wall. A box gutter may lie anywhere two roofs meet.
Box gutters are often invisible from the street and the garden, so they get little attention. A house owner may live in a building for a long time without being aware of one. What alerts people is a stain on a ceiling under a party wall, or a damp patch in a first-floor room that is nowhere near an outside wall. Because the channel carries water from two roofs, any fault in it can let in a surprising amount of water in a short time.
The box gutter is also not the same as a valley. A valley is the V-shaped meeting of two slopes, where the lead lies on the slope itself. A box gutter has its own level floor and its own sides. Our separate guide to lead valleys deals with the V-shaped form.
How a box gutter is built
Underneath the lead is a carpentry job. The floor of the gutter is a deck of boards or plywood carried on joists or bearers that run across the channel. The sides, the cheeks, are formed by the roof structure or by firring pieces nailed to it. Everything is shaped so that the finished surface slopes towards an outlet at one end, or towards a sump in the middle.
The lead then goes down as one continuous sheet across the floor and up both sides. This is the point at which a box gutter differs from most other leadwork: the metal is wide enough to run from the top of one cheek to the top of the other, with no joint across the floor. Joints lie across the fall, at the drips, and on the sides they are formed by tucking the lead under the slates or tiles on one edge and dressing it up the wall or the lap on the other.
The width of the sheet determines the code it needs. Supplier data for cast lead lists code 5 and 6 for flashings and gutters of moderate size, and codes 7 and 8 where the load is heavier. We choose the code for each gutter from its width, length and the area of roof that drains into it, and we name it in the quote. A heavier lead is not an upgrade for its own sake. It is the correct specification where the sheet is wide and long.
Beneath the lead sits an underlay, a fleece or building paper that keeps the metal from rubbing on the boards. It matters because lead moves with temperature, and rubbing against rough timber wears it thin. Our page on underlay for lead goes into the choices.
The edges are the last detail. The lead is turned up under the roof covering on one side by a margin that depends on the pitch and the exposure, and it is fastened at the top with clips, never nailed through the flat of the sheet, because a nail through the sheet is a hole. Clips hold the edge down while letting the metal move.
Falls, drips and outlets
Three features decide whether a box gutter drains: the fall, the drips and the outlet. They need to work together.
The fall
Water only leaves a gutter if the floor slopes. For flat surfaces, BS 6229:2018, as quoted by manufacturers, recommends designing to 1:40 so that the finished fall comes out at about 1:80 after allowing for the deck and for deflection. Many old box gutters were built shallower than that, or have sagged. A fall that is too slight leaves a film of water standing in the sole after every shower. Standing water finds any weak point, and it freezes in winter.
The drips
Each drip is a small drop built into the boards, repeated at intervals so the floor steps down along its length. The lead goes over the step, so that the upper sheet laps down over the lower one and water flows across the joint, not through it. Drips do two jobs. They break a long run of lead into lengths short enough to expand and contract without tearing, and they let the gutter reach a useful depth without needing a huge slope. A long box gutter with no drips is a gutter waiting to split. How far apart drips should be depends on the code of lead and on the roof, which is why we set them out on site.
The outlet
At the low end, the water enters a rainwater pipe, a hopper head or a sump. The outlet has to be large enough for the heaviest rain, and an overflow set a little above it gives a safe route if it blocks. A box gutter with a small outlet and no overflow will fill during a storm and spill over the lowest edge, which is often an inside wall. Our page on blocked hopper heads covers one common outlet fault.
Why box gutters fail
Lead has a long life when it is properly installed. The cost guides we rely on put a figure of over 80 years on flashings and 50 to 100 years on lead flat roofs, as typical ranges that depend on workmanship and exposure. When a box gutter fails early, the cause is usually one of the following.
- Ponding. A shallow or sagging floor holds water, which accelerates wear and adds weight to timber that is already struggling.
- Splits from thermal movement. Lengths that are too long, drips too far apart, or sheets fixed rigidly leave the lead nowhere to go as it warms and cools. It cracks, usually along a fold, and the crack is often a fine line.
- Blocked outlets. Leaves, moss and silt gather at the low end. Once the outlet is blocked, the water rises up the cheeks and into whatever is behind them.
- Undersized channels. Roofs have sometimes been extended, or an extra dormer added, so the gutter takes more water than it was built for.
- Rotten timber below. Slow leaks feed wet rot in the boards and bearers. Once the structure moves, the lead above it tears.
- Patching with the wrong materials. Bitumen, sealant and tape can hold for a season, then peel. They also complicate any proper repair afterwards, since the lead must be scraped clean before it can be joined.
It is worth separating a fault in the lead from a fault where it meets other parts. Water often comes in at the junction of the gutter and a wall, because a flashing has lifted, a joint has opened, or pointing has failed. The stain on the ceiling looks the same.
Our guide to repairing split lead explains the methods available for a local split. Where the split is the sign of a wider problem, the right answer is to deal with the cause, and a patch over a sagging floor does not do that.
Repair, relining or a different material
The decision comes down to how far the problem extends. A single split at a drip, a flashing that has lifted, or an outlet full of silt are local faults, and a repair puts them right. Repair is our first choice where it will hold, and the survey pictures let you judge the fault for yourself.
Relining is the answer when the lead is cracked in several places, the fall is wrong, the boards are soft, or earlier patches have failed. It means stripping the gutter, correcting the carpentry, and laying new lead with drips and outlets set out to suit. Because it is a larger job and involves access, it costs more, and there is no published national figure for it. Our page on box gutter relining cost explains what moves the price, with the fixed figure written up after the survey.
Some owners ask about a different material. A fibreglass lining, GRP, is sometimes used in box gutters on modern buildings, and a rubber membrane can be a cheaper route on a plain, accessible channel. Both have a place, and both have limits. On an older building, particularly a listed one, lead is the matching material, and changing it may need consent. See our page on a GRP box gutter for the argument in detail. GRP, like EPDM, is a flat roof system whose typical life is shorter than that quoted for lead, and the figures vary by source.
Approval under the Building Regulations has size thresholds, set out by the Planning Portal, and a lone box gutter usually falls beneath them. If the house is listed, B&NES Council may still require consent, so ask it first.
Keeping a box gutter working
Most of what keeps a box gutter in good order is simple. It needs regular clearing, an occasional look, and attention to small faults before they grow.
The outlet is the part to watch. Clear it at least twice a year, and again after a storm that has stripped leaves from nearby trees. If your gutter is reached from a window or a safe access hatch, a hand and a trowel are enough. If it is at height, or on a roof you cannot reach safely, ask for it to be done by someone with the right access. Our guide to parapet gutters describes the same routine for the front-wall version.
Look at the lead when you can. Fine cracks, white powdery patches, lifted edges and signs of repair with sealant all deserve a closer look. A white coating on lead is normal. It is the surface weathering to a protective patina, not a sign of decay.
Keep the gutter free of load. People store tiles, slates and planks in box gutters, or walk through them in work boots, and lead is soft. If a gutter is a route for access to a chimney or a skylight, a duckboard protects it.
Finally, ask for the gutter to be included whenever a roof survey is done. A survey of the slopes alone misses the part that carries their water. On our visits we check the floor, falls, drips and outlet, take pictures of what is there, and say whether it needs a repair, a relining or nothing at all. The lead roofing service page describes the wider work, and if you can reach a window that overlooks the gutter, a photograph taken after heavy rain showing any standing water is a useful thing to send ahead.