Lean-to roof repair
How is a lean-to roof repaired?
The short answer
A lean-to roof is repaired by tracing the leak to its source and fixing only that: renewing the flashing at the house wall, refixing or replacing tiles or sheets, or patching a worn flat covering. The pitch is shallow, so the covering must suit it. If it does not, a repair will not hold for long. The price is given in writing, as one total, after the survey.
Why lean-to roofs leak, how each fault is repaired on a shallow slope, and what separates a repair that holds from one that does not.
A roof with only one slope and one wall to fail against
A lean-to has a single slope that rises to meet the house wall. Water has nowhere to go but down the slope, and the one junction with the wall carries the whole job of keeping rain out. That is why the abutment, the joint at the wall, is the first place we look.
The roof is often a later addition to the house, built as a conservatory, kitchen extension, utility room, or a lean-to shed on a bungalow. Because it was added, nobody built the wall and the roof together, and the two settle and move independently. A seal that was tight when the roof was new can open by a few millimetres, and a few millimetres is enough for driving rain.
Inside, a lean-to leak shows as a damp stain along the wall at ceiling height, a patch of blown paint, or a drip at the top of a window. Outside, the signs are dark streaks below the flashing, a gutter that has pulled away, or loose tiles low down in the slope.
Does the covering suit the shallow pitch?
Each covering has a minimum pitch, meaning the shallowest slope it is made for. Marley lists its Lincoln clay pantile at 15 degrees, its concrete plain tile at 35, and several interlocking concrete tiles between 12.5 and 25. Natural slate needs about 20 degrees on BS 5534 for a moderate exposure.
Many lean-tos are shallower than a main roof. When a covering sits below its minimum pitch, rain can be driven up under the laps, and tiles lift in gusts. Refixing slipped tiles on such a roof is a patch. The honest advice may be to lift the covering and relay it as a lower-pitch system, such as a sheet or a membrane, rather than keep mending tiles that were never right for the slope.
We measure the pitch on the survey and photograph the underside where we can reach it. If the roof is within what the covering allows, a repair is sensible. If it is not, we explain the difference and the choice is yours.
Flashing, fillets and the wall junction
The usual cure at the wall is lead. A lead flashing is a strip let into a cut in the mortar joint, then laid over the roof covering and, on a tiled slope, stepped or layered with the courses. It allows the wall and roof to move without opening a gap. Cement fillets and mastic are quicker, and they crack as the two parts move.
Checkatrade's roof repair guide, updated in October 2026, puts lead flashing repairs at £50 to £70 per metre, VAT extra. MyJobQuote says a flashing can last over 80 years. These are national typical figures, not what we will quote, and a lean-to with a long wall costs more than a short one simply because there is more metal to dress.
On many lean-tos the flashing is also where the neighbouring roof drains onto it. If a main roof valley or a downpipe empties onto the lean-to, the water volume at that point is large, and we look at whether the discharge is the real fault. A missing tile higher up can throw water in the same place.
Polycarbonate, glass and felt lean-to roofs
Not every lean-to is tiled. Polycarbonate sheets are common on older conservatory and carport roofs. They go brittle and cloudy, the glazing bars lose their seals, and the sheets crack at the fixings. Polycarbonate roofs typically last around 10 to 15 years, per HomeBuilding and the Eco Experts. A cracked sheet can be replaced, and if the whole roof is past its best, replacing it all is better than chasing leaks.
Felt and other flat coverings are patched where the fault is local. Less, in March 2026, gives £150 to £500 for flat roof small repairs. If the membrane has blistered across the whole surface, or water sits in pools because the fall is poor, a patch buys little time. BS 6229:2025 sets a minimum finished fall of 1:80, and roofers usually build steeper so that the finished roof still drains.
Swapping a conservatory roof for a solid one is different work and needs Building Regulations approval.
Gutters, ventilation and a fair price for the fix
A lean-to has a short gutter at its low edge, and when it blocks, the overflow runs down the wall below. People then blame the roof. We clear and refix it as part of the visit, because a leak that is really a gutter problem should cost a gutter price.
Condensation is the other lookalike. Water running down the underside of polycarbonate or the inside of glass can be mistaken for a leak, especially in a cold snap. We check for it before pricing anything, because ventilating the space cures it and a new roof does not.
For cost, the fault sets the price. Renewing a short flashing and a few tiles is small work. A full re-covering of a low slope is bigger, and access matters: a lean-to is low, so a ladder or tower often reaches it. Our answer on porch roof repair covers a similar small roof, and the roof repairs page sets out the whole service.
Related questions
Lean-to roof questions
Can a lean-to roof be re-covered without taking the extension apart?
Yes, in most cases. The covering comes off and a new one goes on over the existing rafters, and if any timber has rotted, it is replaced first. The inside is protected while the roof is open. We assess the structure on the survey and say what is needed before you decide.
Why does the lean-to drip only after a long dry spell ends?
Dry weather lets the wall and roof shrink slightly and open the junction. When the first heavy rain arrives, it finds the gap before the materials swell and close it. That pattern points to the flashing, and photographs of the stain and the wall line help us decide where to look first.