Commercial Box Gutter Leaks: Repair or Line?

A box gutter is a lined, square-sided channel that collects water from two slopes meeting in a valley, or from a roof that runs up against a parapet wall. Shops, warehouses, terraced industrial units and older commercial parades often have them, and the lining may be lead, felt, asphalt, metal or a single-ply membrane.

Where does the water get in?

The usual weak points are the seams and laps in the lining, the corners where the sides turn up, the joint around each outlet (the hole where water leaves the gutter) and the upstand where the lining meets the wall behind it. A gutter that holds water for long stretches wears faster, and leaf litter makes it worse. When a ceiling stain appears below, the entry point can be some distance away, because moisture travels along the deck and the joists before it drips.

A repair that tackles only the stain misses the cause. Finding the cause means following the water from the highest point of the gutter down to the outlet, and looking at what sits beneath the lining as well as on top of it.

A repair suits a gutter whose lining is mostly sound and whose problem is local. Examples are a split at one seam, a lifted section of upstand, a patch of lining worn through around an outlet, or a drain choked with silt that has made the gutter overflow into the building.

Is a patch just a cheap fix?

Not if it is the right fix. A well-made patch on a sound lining can serve for years, and relining a whole gutter to cure one split would waste money. The limit is the area around the patch. If the lining beside it is cracked, crazed or soft, the next failure is already on its way, and a string of patches adds up to a lining job done the slow way.

Lead has its own record here. MyJobQuote puts lead flashing at a life of over 80 years in its leadwork guide, and a lead-lined gutter in good order may need only local attention. The thickness, called the lead code, matters: one lead maker's brochure lists codes 5 to 8 for parapet and box gutters. We read what is on your roof before suggesting anything, and the photos in our report show you the same thing.

Lining means a new waterproof surface laid over the existing gutter once it has been cleaned, repaired and dried. It suits a gutter whose old surface has failed in several places while the timber or concrete base beneath is still sound.

What are the lining options?

The usual systems are a glass-reinforced plastic (GRP) layer, a rubber single-ply sheet such as EPDM, a liquid-applied coating, or new lead. Published lifespans differ and the sources disagree. GreenSpec puts the expected service life of EPDM at 20 to 30 years, Less says 30 to 50 years, and Less gives GRP 25 to 40 years. Read those as typical ranges, never as promises.

The base has to be right first. A lining laid over wet or rotten deck traps the problem underneath. The finished fall matters as well. BS 6229:2025 keeps a minimum 1:80 finished fall for internal gutters, and the design aims for 1:40 so the gutter still reaches 1:80 once the roof has settled. Water that stands in the gutter means the lining is working harder than it should.

Many box gutter leaks are really drainage failures. If the outlet is too small, partly blocked or badly sited, the gutter fills in heavy rain and water finds a way past the lining.

Does a box gutter need an overflow?

It is worth having one. An overflow is a second outlet set slightly higher than the main one, so that if the main outlet blocks, water leaves visibly on the outside of the building instead of rising over the upstand and running down the inside wall. If your gutter has none, adding one is a sensible part of any lining job.

Replacement is the answer when the deck beneath is rotten, when the fall is wrong along the full length, or when the structure has moved so far that the gutter cannot be made to drain. That is the bigger job. Access, gutter length and the roof beside it set the price, and the survey fixes one total in writing.

See industrial roof leaking and commercial roof problems after winter, and our commercial roofing page. A photograph of the stain and of the gutter above it is the best way to begin.