The short answer
A lead box gutter is a channel lined with lead that collects rain where two roofs meet or where a roof stops at a parapet wall, then carries it to an outlet. Royston's brochure lists Codes 5 to 8 for parapet and box gutters, because the lead carries more water and takes more wear than a flashing does. Falls and drips decide how long one lasts.
What a box gutter does
Some buildings have roofs that stop short of a wall, either at the front behind a parapet or between neighbours. A trough runs along the foot of that wall, and the rain from the roof slopes drains into it. That trough is the box gutter. It is lined with lead, sits on a timber or board base and is bordered by the roof on one side and the parapet or party wall on the other.
A box gutter differs from the cast iron or plastic gutters hanging off an eaves. It is part of the roof itself, so when it fails the water goes into the building, not onto the ground. It also takes the run-off from a whole roof slope, which is why it needs a thicker sheet than a flashing and a sound outlet at the end.
Where a pitched roof meets another at an angle, a similar trough forms in the valley between them. That is a pitched valley gutter, and it appears in Royston's brochure beside the box gutter. Our page on lead valley versus dry valley compares the valley options in more detail.
Because the gutter is hidden, the first sign of trouble is often on a ceiling.
Lead codes for parapet and box gutters
Royston Lead's cast-lead brochure marks use D, parapet or box gutters and tapered valley gutters, against Codes 5, 6, 7 and 8. Those are 2.24, 2.65, 3.15 and 3.55 mm thick, and they weigh 25.40, 30.05, 35.72 and 40.26 kg per square metre.
Code 4 is not on that list. A gutter takes the water from a large area, is walked on when someone clears it and is the part of the roof most likely to hold standing water, so the heavier codes are the ones named for it.
Which of the four suits a given gutter depends on its length, its width and the roof area draining into it. A short gutter between two small roofs asks less of the lead than a long one behind a tall parapet. We measure the run, and the quote names the code.
Lead sheet is tied to BS 6915 through Approved Document C. BS 6229, the flat roof standard, covers flexible membranes only, not lead, so its falls do not apply here. Our page on how thick roofing lead is gives the whole table.
Falls, drips and outlets
A lead gutter works only if the water runs out of it. That depends on the fall, the gentle slope along the gutter bed towards the outlet. If the bed is flat or has sagged, water stands in the lead, and standing water will exploit any weakness in a joint.
Lead is also soft and moves as it heats and cools, so a long sheet laid along a gutter would creep and eventually split. To prevent that, the gutter is laid in lengths that step down by a drip, a small change in level at each joint, with the upper sheet overlapping the lower. Water runs over the step, and each sheet can expand and shrink without dragging on the next.
The outlet matters as much as the lead. A blocked or undersized outlet backs the water up over the sides of the gutter and over the lead upstands, and it can spill into the building. Leaves and moss collect there, so clearing the outlet is the one piece of maintenance a box gutter needs.
If the base has to be re-laid to restore the fall, the work is larger than relaying lead, and we show you the photographs before quoting for it.
Signs of failure and what we do
These signs point to a box gutter in trouble:
- a damp patch on an upper ceiling or the party wall inside, below the gutter run;
- water marks on the face of the parapet or the brickwork beneath it;
- standing water or a build-up of moss and leaves in the gutter after rain;
- cracks or splits in the lead, most often at the drips and the corners;
- lifted or patched sheets from earlier repairs.
If you can see the gutter from a window, a photograph on a dry day is useful. Otherwise, we survey it for you, by ladder or camera, and show you what is there.
A small split near a drip may be patched. A gutter with several faults, a sagged bed or lead that has been repaired many times is usually better relined, and we tell you which applies, with photographs to show why. Our page on why a lead roof leaks goes through the causes.
You receive one written price, and the lead roofing hub shows how the rest of our leadwork fits together.
