Box Gutter Lining: Repair or Replace?

A box gutter is a channel set between two roof slopes or behind a parapet wall, built into the structure of the building rather than hung on the outside. A valley gutter is the same idea where two pitches meet. Because the channel sits inside the roof, a leak does not drip harmlessly onto the pavement. It runs straight into the ceiling below, the stock or the wall cavity.

Most failures come from the same few causes. The old lining, whether lead, bitumen felt or asphalt, cracks where it has been stretched by heat and cold. Leaves, grit and moss build up and hold water against the joints. A blocked outlet lets the channel fill until water finds the lowest weak point. Foot traffic from other trades crushes the surface. Once water stands in the channel, every cold spell works on it.

The first job on any box gutter is to find out how far the damage has gone. Fixing a split in the lining is a small repair. Water that has been sitting under the lining for a season is a different matter, because the timber boards or metal deck beneath may be wet. Our survey includes a look from inside the building where we can get at it.

Lining suits a gutter whose base is dry, firm and level enough to take a new layer. The usual systems are a liquid-applied coating reinforced with fabric, a sheet of single-ply membrane such as EPDM glued or welded into the channel, or glass-reinforced plastic laid in one piece. Lead can be fitted again too, and Royston Lead's sheet guide lists Code 5 and heavier for parapet and box gutters. Each has its own limits, so the choice turns on the shape of the channel, the number of laps and upstands, and who needs to walk there.

Lining does not suit a gutter whose timber is soft, whose metal deck is corroded through, or whose structure moves. A new layer on a wet deck traps the moisture, and blisters follow. Where the channel has no fall at all, a lining leaves a permanent pond.

Where we find that, we show you the photos and price replacement as a total, with new boards, falls and outlets. Many gutters sit between lining and full renewal, with a section of deck cut out and the rest lined. We explain which parts fall where.

However good the lining, it is only as sound as the drainage feeding it. For flat and low-pitched roofs, BS 6229:2025 keeps a minimum 1:80 finished fall for general areas and internal gutters. Design guidance from IKO, quoting the earlier 2018 edition, suggests aiming for 1:40 so that the roof, once built, still meets the minimum after settlement and tolerances. A gutter laid flat will hold water however good the lining is.

Outlet size matters just as much. BS EN 12056-3 is the standard for working out roof drainage, and the sums use the roof area feeding the gutter and the rainfall the designer allows for. A gutter that served a small roof when it was built may now take water from an extended slope or a new plant deck. If the outlets cannot clear a heavy downpour, the channel fills and spills over the lining's upstand.

That is why an overflow, set slightly higher than the main outlet, belongs in the specification. It tells the occupier something is blocked before water reaches the ceiling. Our wider notes on commercial roof drainage problems cover ponding and larger roofs.

Lining is often chosen because the business can carry on trading. Most of the work happens in the gutter itself, with limited disruption inside, but the channel must be dry on the day of application, so weather decides the programme. We agree with you how staff and customers will be kept clear of any access points, and the site is cleared and made safe when each shift ends.

Price follows gutter length and width, the count of outlets, upstands and corners, how sound the deck is, and the access needed. A gutter on the first floor of a terrace can mean a tower or scaffold, and the licence for any that sits on the public highway comes from the highway authority, so it is worth asking who arranges it. After the survey you receive one total in writing. When you collect other quotes, ask each to confirm what the figure covers, VAT and scaffolding included.

If you are weighing up a wider job, our guide to commercial roof condition surveys shows how gutters fit into the full picture, and our commercial roofing page lists the other work we take on.