What are lead parapet and box gutters?

A parapet is a low wall that rises above the edge of a roof. Where a sloping roof stops short of it, the two meet in a trough, and that trough is lined with lead to catch the water running off the slope. This is a parapet gutter. A box gutter follows the same logic somewhere else: a flat-bottomed channel formed between two roofs, or tucked behind the roof edge, with sides and a base.

Both use lead sheet in bays with drips, and both have an outlet at one end or in the middle to drain the water into a downpipe or a hopper head. They are common on terraces, town houses, chapels and mill buildings, where the roof sits behind a wall and a conventional gutter could not be hung.

They differ from a sloping valley, where two slopes meet at an angle. The sloping version is covered in our guide to lead valleys in Stroud: how they fail and are renewed. A box gutter is level or nearly so, which brings its own problems.

These gutters carry a large catchment of roof in a narrow channel, and in heavy rain they run full. Lead has to survive standing water, foot traffic during maintenance, and repeated cycles of warming and cooling across a long run.

Leadworx and Royston Lead place the heaviest codes in this kind of use. Their tables list parapet, box and tapered valley gutters under Codes 5, 6, 7 and 8, with Code 6 at 2.65 mm for large roofs. Thicker lead is slower to fatigue, and it takes the folds at the sides better. The sheet is limited in length, and the gutter is divided with drips so each length can move. MyJobQuote refers to flashing lengths of 1.5 metres, and gutter lining is kept to workable lengths for the same reason.

BS 6229 sets the ideal fall for flat roofs at 1:40 on the design, with a finished minimum of 1:80, as IKO and Rockwool both note. A good lined gutter is built with a fall to the outlet.

The common faults follow from the design. A blocked outlet fills the channel until water spills over the lowest point, which is usually into the building. Leaves, moss and bird nests are the usual cause, and a hidden gutter behind a wall may go unchecked for years.

Standing water is the second problem. A gutter laid without enough fall holds a pond, and the lead beneath it wears faster and lets grit collect. Third, lead cracks at folds and drips where it has been overstressed, and the cracks open slowly. Fourth, the timber boards beneath the lining rot when moisture finds its way in, and a soft deck gives no support to the lead above.

The first warning is often indoors: a damp patch on a ceiling below the parapet, or staining along an upper wall. Outside, look for water marks on the parapet face, overflow stains and plants growing in the gutter. A zoom lens or binoculars trained on the parapet from across the street often shows whether the channel is clear and which stretch is holding water. Water pouring over the lip of a rainwater head during a downpour is another giveaway. Our guide to old roof lead covers what age does to the metal.

Small defects can be mended. A split fold or pinhole may be patched or welded, and a blocked outlet needs only clearing. Our answer on lead slate and pipe flashings covers the outlet junctions where lead meets a pipe.

Where the lining is worn across its length, or the deck below is rotten, renewal is the sound choice. The work involves lifting the old lead, replacing soft boards, rebuilding the fall, laying new lead in bays with drips, and dressing it up the parapet face and under the roof covering at the sides. In a building with a parapet, the work is often done from a scaffold, and MyBuilder's 2026 guide gives scaffold of about £1,125 for a typical re-roof, which gives a sense of how access shapes the bill.

Lead flat roofs are quoted by MyBuilder at £150 to £200 or more per m², though a gutter lining is a smaller and more complicated job. We price it from your own roof after the survey, with the scaffold, strip, boards, lead and making good as separate lines.

Many parapet gutters sit on listed or conservation area buildings. Stroud District Council's guidance treats the whole of a listed building as protected, so repairs that change its character can need consent. Renewing in lead again is usually the gentlest route, though the conservation officer should hear about it first. Stolen or failed lead is not a reason to change metal without asking.

When we survey, we photograph the gutter, the outlet and the deck, and the written quote follows. A written 10-year guarantee covers our workmanship, and the debris leaves with us. Our other leadwork is described on the lead roofing page. For a survey, ring 01453 933613 or send photographs of the parapet through WhatsApp or the form.