The short answer
A lead parapet gutter or box gutter is a flat-bottomed channel lined with lead, set behind a parapet wall or between two roof slopes. It collects rainwater and carries it to an outlet. The lead is laid in lengths with drips, small steps that let one length run over the next, so water keeps moving and the metal can move too.
On many terraces and larger buildings, the roof drains into a hidden channel instead of an open gutter. When that channel fails, the damp shows inside.
Where these gutters are found
A parapet is a short wall carried up past the roof edge. Where the roof surface runs back to that wall there is a channel, and that channel is the parapet gutter. A box gutter is similar but sits between two slopes, often where two roofs meet, or along the party wall of a terrace.
Why not use an ordinary gutter?
Because there is no eaves overhang to hang one from. On a Victorian terrace, a shop or a church, the roof falls to an internal channel instead. The lead lining makes that channel watertight and is shaped to the timber or concrete base beneath.
Because they are hidden from the ground, these gutters are easy to forget. Leaves, moss and silt collect, outlets block, and water backs up. The earliest warning is often a stain on an upper-floor wall or ceiling below the line of the gutter, not anything visible outside.
In Taunton and the surrounding towns, older terraces, commercial premises and larger public buildings are the likeliest to carry this kind of gutter. If you are buying such a building, make sure the survey includes the gutter outlets, and ask to see photographs of them.
Lead codes for gutters
Lead for gutters is heavier than lead for ordinary flashings, because the gutter carries water continuously and takes foot traffic during maintenance. The Royston Lead brochure lists parapet, box and tapered valley gutters under codes 5, 6, 7 and 8. Code 5 is 2.24 mm, code 6 is 2.65 mm, code 7 is 3.15 mm and code 8 is 3.55 mm.
Which code do we choose?
That depends on the span, the position and the exposure. A short, sheltered gutter might take a lighter weight than a long run on an exposed gable. We decide after the survey and name the code in the written price, so you know what is being laid.
Thicker lead resists tearing and wear better, but size still matters. A long sheet creeps and cracks however heavy it is, which is why gutters are laid in lengths. Our answer on why lead has size limits explains the movement.
Our answer on lead codes gives the wider picture, and the colour marking that goes with each code is the same for every use of the metal.
Drips, falls and outlets
A drip is a step in the gutter floor, usually formed in the timber beneath the lead. One length of lead finishes at the top of the step, and the next starts below it. The upper sheet laps over the lower, so water passes across the joint and cannot back up.
Why does the gutter need a fall?
A fall is a slope. Without one, water stands in the channel, and standing water finds pinholes and joints. The gutter slopes towards an outlet, a hole or pipe that discharges the water, often into a hopper and downpipe. BS 6229:2025 sets no fall for lead, because it covers flexible membrane flat roofs only. The fall for a lead gutter is set out by the roofer to suit the timber layout and the building.
The outlet is the weak point. If it blocks, water rises over the lead's upstand, the turned-up edge at the side, and runs into the wall. A cage or grating at the outlet keeps leaves out, but it needs clearing.
Check the outlet from a safe window, or with a roofer's help, twice a year. A clear outlet after autumn leaf fall is the simplest protection a hidden gutter has, and it costs nothing.
Why they leak and what repair involves
The common faults are blocked outlets, cracked laps, lead that has crept at the drips and failed linings under old repairs. Bitumen painted over a split often hides the real problem for a season. Our answer on sealant on lead flashing explains why that approach fails.
Can a lead gutter be patched?
A small split in sound lead can be repaired. If the lead has thinned, the drips have failed or the base timber is rotten, a patch only delays the work. A rebuild means stripping the lining, repairing the boarding, and laying new lead in correct lengths with new drips.
Reaching the gutter makes up a large share of the effort. One behind a high parapet may need scaffold or a tower, and scaffold on a public pavement brings in a Highways Act 1980 s169 licence. The scaffolder and we arrange that between us. After the survey you receive a single written figure, built on photographs of your own gutter.
Our lead work page sets out the rest of the lead details on a roof, from flashings to valleys, for anyone wanting the wider picture around a gutter.
More on box gutters
Does my house have a hidden gutter behind the parapet?
Look for a roof that falls inwards behind a parapet wall, or two slopes meeting in a flat channel. There is often no external gutter on that side of the house. Send us a photo of the roofline and we will tell you what you have.
Can the gutter outlet be cleaned without a roofer?
Only from a safe, stable position, and many outlets are not reachable that way. Clearing silt and leaves helps, but the roof edge is a fall risk. If you cannot see the outlet from a window, leave it to a roofer and ask for a check.
The full picture: Lead work in Taunton ยท More short answers