The short answer
Industrial valley gutters fail because they carry the run-off of two large roof slopes in a narrow channel with very little fall. Blocked outlets, ponding water, movement at the joints and worn linings let water through. Choices run from a local repair, through a fresh lining over the old gutter, to full replacement, set by how sound the gutter and its support are.
What a valley gutter does and why it is exposed
A valley gutter is the channel formed where two roof slopes come together. On a multi-bay industrial unit, with a series of pitched roofs side by side, it runs between every pair of bays. Each gutter takes all the rain that falls on the slopes either side, and at storm intensity that is a lot of water to move through a channel that may be only a few hundred millimetres wide.
The gutter is usually lined with a waterproof layer, which might be felt, a single-ply membrane, a liquid-applied coating, GRP or sheet metal. The lining sits on a deck or on the tops of the sheets, and it ends at outlets that carry the water down into drainpipes. Every one of these parts can fail: the lining, the joints, the outlet and the support below.
The gutter also gets less attention than the roof around it. It sits low, often between two taller slopes, so nobody sees it from the ground. Leaves, bird droppings, moss and wind-blown packaging collect there. Staff on site see the damp patch on the factory floor long before anyone climbs up to the valley.
The usual causes of failure
Most failures trace back to a small number of causes, and they often combine.
- Ponding. A flat roof or gutter bed should be designed to a fall of 1:40 so that it finishes at about 1:80, according to BS 6229:2018 via a manufacturer guide. Many older gutters have less, or have sagged, so water sits in them. Water that stays put wears out most linings sooner.
- Blocked outlets and overflows. When an outlet is blocked, the water level rises above the lining and over the laps, and it finds its way into the building.
- Movement at the joints. Steel and aluminium expand with heat and contract in cold. The lining has to cope with that movement. Rigid sealants and old mastic often crack as the gutter flexes.
- Worn linings. UV light, foot traffic and temperature swings crack and craze felt and some coatings. Fixings that penetrate the lining become routes for water.
- Corrosion. Steel gutters rust from the edges inward, and perished seals at the sheet ends let water reach the bare metal.
Our page on industrial roof leaks covers how we trace water back through a large roof to its source.
Repair, reline or replace
The choice depends on how much of the gutter is sound.
Local repair
Where the lining is mostly good and a single joint, outlet or patch has failed, a local repair is reasonable. Open seams are cleaned, sealed or patched, outlets are renewed and debris is cleared. Repairs buy time, but they do not change the underlying fall or the age of the lining.
Relining
If the gutter structure is sound and the lining has aged, a new waterproof layer can be laid over the existing one or in place of it. Single-ply membrane, liquid-applied systems and GRP are all used. A good lining is carried up the sides of the valley, formed with welded or adhesive-fixed joints and detailed carefully at outlets. Our commercial roof overlay page explains when covering the old surface is sensible and when stripping is better.
Replacement
Where the deck is rotten, the steel is corroded or the channel cannot handle the roof area draining into it, replacement is the honest answer. This is the moment to improve the fall, widen the channel, add overflows and renew outlets. It costs more, but a new gutter formed to a proper fall removes the ponding that would otherwise cut a new lining's life short.
Design details that make a new gutter last
A gutter that works is not just a lining. A few details make the difference between a gutter that lasts and one that returns in a few winters.
First, the fall. Water must run to the outlets rather than sit between them. If the structure beneath cannot be altered, tapered insulation or a laid screed can build the fall in. Second, the outlet capacity. Outlets should be sized for the area they drain, with an overflow set above normal water level so that a blockage shows itself outside the building rather than inside. Third, the junctions. Where a gutter lining meets the roof sheets, a dressed upstand with a proper closure stops wind-driven rain pushing under it.
Fourth, access. A gutter that nobody can safely reach is a gutter that nobody clears. Walkways, edge protection and a fall arrest line make routine clearing possible without anyone stepping on fragile sheets. Rooflights sit near many valleys, and when they age they add to the problem, as our answer on industrial rooflight replacement explains.
Keeping the gutter working
An hour spent clearing outlets costs little beside water running onto stock. Outlets and the full length of each gutter should be cleared of silt and leaves, with extra attention after autumn and after storms. A routine like this is the sort of job a planned visit schedule covers. We also check the lining for cracks, lifted edges and ponding marks, so small faults are put right before they let water in.
If your unit is rented, the question of who maintains the gutter and who pays is answered by the lease. See our answer on who pays for roof repairs on a commercial lease before you commission work.
The cost of valley gutter work swings with length, access, the lining and whether the deck needs renewing, so we price commercial roofs after a survey, not before. The fixed sum comes to you on paper after we have looked at the gutter, and the report includes pictures of the condition of the lining, the outlets and the support. Our commercial roofing hub covers the rest of the work we do on business roofs.
