The short answer
Stained or mouldy soffits usually come from one of three things: algae growing on a damp surface, condensation from moist air escaping through the roof void, or water leaking from a failed gutter joint or slipped tile above. Algae wipes off. A stain that returns, softens the board or sits in one patch points to a leak or a ventilation fault that needs fixing at its source.
Reading the stain: algae, damp or something else
Soffits are the boards fixed flat under the overhang, running from the wall out to the fascia. They sit in shade, close to the gutter and often damp, which makes it a natural place for growth.
Algae shows as green or black speckling, often spread evenly along a north-facing run or under a gutter that holds water. It sits on the surface and wipes away with a cloth. Mould tends to be darker and fuzzier, and it follows moisture: it clusters around vents, corners and board joints. A brown tide-mark that runs in a line or a single streak below a joint is more often water, carrying dirt out of a gutter or from behind the fascia.
Soft, swollen or flaking timber, or a plastic soffit that has bowed away from its fixings, tells a different story from a stain on a sound surface. Press a thumbnail gently into a painted timber soffit: if it marks easily, the board is wet within, and cleaning is not the answer.
Leaks and overflow as the cause
Water finds the soffit through a short list of routes. A gutter joint that has opened lets rain run down the back of the gutter and onto the fascia. A sagging or blocked run overshoots the front lip and soaks the board below. A slipped or missing tile near the eaves, or a failed eaves tray, sends water to the rafter ends and then to the underside. In each case the stain tends to sit directly under the fault.
Look at the staining on a wet day, if you can. Water running from a joint, a drip from the underside of a gutter or a damp patch that follows a rainfall rather than the weather outside are good clues. Stains that appear under one corner often trace to a gutter that overflows there, which is what our answer on why a gutter overflows at the corner sets out. Treat the leak first; scrubbing a soffit that is still being wetted from above only buys a few weeks.
In older properties the rafter feet and wall plate can rot slowly behind a boxed-in soffit. That is why we lift a section on survey and photograph what is behind it, rather than judging from the paintwork alone.
Condensation and moist air from inside the house
Warm, damp air from kitchens, bathrooms and drying clothes rises into the loft. Unless it has a route out, it settles as droplets on the cold underside of the covering, on the rafters and on the soffit board. A soffit that is mouldy along its whole length, with vents blocked by insulation or paint, is a typical sign.
Roof ventilation guidance in BS 5250 sets out how eaves and high-level ventilation are meant to work, and manufacturers publish the openings they expect for different underlays. Our guide to soffit vents and roofline ventilation covers the airflow itself; here the point is the staining. If mould keeps coming back through dry spells, look at whether loft insulation has been packed into the eaves and whether the vent slots are clear of paint. Spray foam can also seal a roof void off from its air supply, and the RICS consumer guide warns that incorrect installation can bring uncontrolled condensation.
Extractor fans that vent into the loft rather than outdoors add a lot of moisture. Ducting them to outside is a job for the electrician or builder, not something to cover with a stronger cleaner.
Cleaning stained soffits without making it worse
For algae on a sound uPVC or painted soffit, start gently. Warm water with a little washing-up liquid, applied with a soft brush or cloth from a stable ladder or tower, lifts most growth. A mild fungicidal wash made for exterior surfaces can follow, used as the label directs and kept off plants and out of drains.
Avoid pressure washers on soffits. The jet can drive water through board joints and vents into the loft, strip paint from timber and force the board off its fixings. Bleach is a common home remedy that can mark coloured plastic and damage nearby planting; it also does nothing for the cause.
Timber soffits that have been stained but are still sound can be cleaned, allowed to dry fully, then sanded and repainted with a microporous exterior paint. If the staining returns through the new paint, the board is being wetted from somewhere. Cleaning a roofline is not a substitute for repair. MyBuilder's 2026 price guide puts soffits at about £100 to £180 per metre when they do need replacing, supply and fitting, so tackling the source early saves money later.
Working at height has its own risks, and soffits sit above the reach of most ladders when the ground slopes. If your house sits on a hillside, ask for the work to be done from a tower or scaffold.
When the stain means a roofer should look
Call someone in when staining is localised and keeps coming back, when boards feel soft or have split, when there are dark marks on the ceiling inside, or when the soffit is bowing. Any of those means the fixing, the board or the timber behind it may need replacing.
One older-house caution: soffits and fascia boards on mid-century buildings and outbuildings can be asbestos cement. Do not scrub, drill or break one. Before scrubbing anything of that age, read whether old soffits could contain asbestos.
When we come out, we photograph the soffit, the fascia, the gutter above and the eaves detail, then lift a board where we need to see behind it. Replacement is quoted afterwards, covering access, boards, ventilation, making good and waste. If the boards do need renewing, how long a roofline replacement takes sets out the stages. Typical national prices sit on the guttering, soffits and fascias page, and the finished work comes with a 10-year guarantee on our workmanship.