The short answer
Lining an old valley or parapet gutter means cleaning it back to a sound base, correcting falls where needed and fitting a new continuous waterproof layer, such as GRP, a single-ply membrane, liquid-applied coating or lead. It suits gutters whose structure is sound but whose lining has cracked. If the timber or deck beneath is rotten, that comes out first.
What valley and parapet gutters are
On many older factories, warehouses and shops, two pitched roofs meet in a valley, and a long gutter runs along the line between them. A parapet gutter does the same job along the edge of a roof, behind a low wall that hides it from the street. Both are sometimes called box gutters, because the cross-section is a rectangle or a trough.
They are normally lined, and the lining is the waterproof layer. In older buildings that was often lead, asphalt or bituminous felt laid over timber boards. Later repairs may have added a coating or a different membrane on top.
Because the gutter takes the run-off from two roof slopes, it works hard. A small defect in the lining sends a lot of water to the same place, and the water gets into the structure beneath: the boards, the wall head, the ceiling below.
How the lining fails
Age is the common cause. Felt dries and cracks, asphalt crazes, and lead can split at laps and corners where it has moved with temperature. Joints and edges are weak points, especially at the outlet and where the lining turns up under the roof covering.
Debris makes things worse. Leaves, silt and moss hold water in the gutter and, over time, rot or corrode whatever lies underneath. Timber boards under an old lining are often the first thing to go, since a slow leak keeps them damp for years. A gutter with poor falls, where water pools instead of draining, wears out faster than one that empties between showers.
Outlets are another source of trouble. A small, silted outlet cannot cope in heavy rain, so the gutter fills and overflows into the building. Our page on winter checks for commercial roofs explains why outlets and valleys come first before the cold weather.
The usual lining options
Several systems suit this work, with the choice turning on the gutter's shape, the access and how the building is used.
GRP, a fibreglass laminate, is laid in place to make a continuous trough with no joints and upturns up the walls. Less (updated March 2026) gives a typical life for GRP flat roofing of 25 to 40 years. EPDM, a rubber single-ply membrane, can be supplied in large sheets that cover the whole gutter; GreenSpec gives an expected service life of 20 to 30 years. Liquid-applied coatings can seal an old lining that is still firmly stuck down, and lead remains the traditional choice for heritage buildings and many parapet gutters.
The figures are typical, not promises. The preparation and the finishing of details matter as much as the material.
Preparation, falls and outlets
Preparation decides the result. The old lining is stripped or checked for adhesion, the boards or deck are inspected, and anything rotten is cut out and replaced. A new lining over wet or soft timber will fail early.
Falls need attention. IKO, citing BS 6229:2018, says a flat roof should be designed for a fall of 1:40 so that the finished minimum is 1:80 after tolerances and deflection. A gutter that holds water, whether on a flat or a slightly sloping bed, usually needs its falls corrected before the lining goes in. Small tapered battens or a screed can do this.
The outlet and overflow are checked at the same time. If the outlet is too small or the pipe is partly blocked, a perfect lining will still flood. We make sure there is a working overflow set at the right height.
Upturns matter too. The lining has to climb the adjoining wall and the roof slope far enough to stay dry in a heavy downpour, and it needs to tuck under a flashing or be covered by a new cover flashing along its top edge. Where the old gutter has a stop end or a drip, the new lining must be finished so that water cannot track behind it.
Lining or replacing the gutter?
Relining is the sensible answer where the structure is sound but the lining has failed. It avoids stripping the roof edge and is usually quicker and cheaper than rebuilding the gutter.
Replacement makes sense when the supporting timber, the deck or the wall beneath has deteriorated, when the gutter is too narrow for the flow it carries, or when the roof itself is coming up for renewal. Rebuilding the gutter and the roof edge in the same programme can save duplicated access costs.
If the gutter is part of a bigger roof problem, our page on how long a commercial re-roof takes explains what a larger project involves. Rusted sheets that feed the gutter are covered in our guide to cut edge corrosion on metal roofs.
Access, safety and the written quote
Valley and parapet gutters sit at roof level with limited room, and they are often hemmed in by roof sheets and rooflights. The work is planned around safe access: edge protection, crawling boards and clear zones below. Where rooflights flank the gutter, they are treated as fragile. See our page on replacing rooflights on an industrial unit for that side of things.
We survey the gutter before quoting, photograph its condition, and tell you whether it needs a lining, a repair or a rebuild. A written quote follows, and the decision is yours.
Where a lining will do, we will not propose a rebuild. After the work we clear the site and take the waste away, and workmanship is guaranteed in writing for 10 years. The rest of what we do for industrial and retail buildings is on our commercial roofing page.