Question answered
Green or mouldy soffits: what to do
The short answer
Green or mouldy soffits are a sign of damp: a leaking gutter, a blocked downpipe, condensation from the loft or a lack of airflow at the eaves. Wash the surface gently with fresh water and a brush from a safe position, then find the source of the moisture. If the board is soft, sagging or dates from before 2000, get it checked before touching it, as old soffits can contain asbestos.
Green or black patches under the eaves usually mean moisture has been sitting there. Cleaning the surface is easy, but finding why the moisture arrived is what stops it coming back.
What the green and black growth is telling you
Algae and mould grow wherever a surface stays damp and rarely dries. The soffit, the flat board under the overhang, is sheltered from rain and from sun, so any moisture that reaches it lingers. The growth itself is mostly cosmetic on plastic, but it is a signal, like a tide mark in a room.
The moisture comes from one of a few places. Water can spill from an overflowing gutter and run back along the underside. A leaking joint above the soffit can drip onto it. A blocked downpipe backs the water up. Or the damp air from the loft escapes through the eaves and condenses on the cold board. We look at each of these before suggesting a fix, and it helps to know the history: when the green appeared, and where it is worst.
Patches that sit directly beneath a gutter joint or a corner point to that joint. Patches along the whole length suggest ventilation or condensation. A single stain beside a downpipe usually means that pipe is the culprit.
Cleaning the boards without causing harm
For plastic soffits, fresh water, a soft-bristled brush or a cloth and a mild household cleaner are all you need. Work from the bottom of a stable ladder or, better, from a tower, and never lean sideways to reach a corner. Rinse well so no film is left behind to hold dirt.
Keep a pressure washer well back or leave it in the shed. A jet forced into a vented soffit can push water into the roof space, strip the glossy skin of the plastic and lift the sealant at joints. Bleach-heavy products can mark gutters and brickwork below, and they run into planters.
On painted timber soffits, wash and let the board dry fully before judging it. If the paint flakes away and the wood is dark and soft, the problem is more than surface green, and cleaning will not rescue it. White plastic cleans up well, while dark boards show streaks, a point we cover in our note on anthracite and black fascias.
Eaves ventilation and why it matters for the soffit
Many soffits carry a vent strip or round vents so that air can enter the roof space at the eaves. Marley's explanation of BS 5250 for cold roofs sets the figures. With a high-resistance underlay, Marley says the eaves need 25 mm of opening along their length at a pitch of 15° or less, and 10 mm above that. The 3 mm gap left by normal eaves construction can be enough where three things coincide: the underlay is low-resistance, the ceiling beneath is sealed, and the tiles themselves let air pass. The underlay's certification conditions must still be met.
A soffit that looks right may still be wrong. Vents get painted over, insulation is pushed too far into the eaves, or a new fascia board is fitted flush and closes the gap. Damp loft air then has nowhere to go and condenses on the cold surfaces, which is exactly where algae likes to settle.
Our guide to soffit ventilation covers the vents in more detail, and checking them is part of any roofline survey we do.
Telling surface growth from rot, and the asbestos question
Press a screwdriver handle gently against the board. Firm, dry material is fine; a spongy or crumbling feel means rot. Staining that comes back within weeks of cleaning, boards that sag between the fixings, or brown streaks that run down the wall all suggest moisture is still getting in.
Before cleaning, drilling or scraping an old soffit, think about the age of the house. HSE's advice for a property built or refurbished before 2000 is to assume asbestos is present somewhere, and it lists external soffits as one place asbestos insulating board turns up. A plain-looking board gives no clue. Do not sand, scrape, drill or break one that might be original; read our page on asbestos soffits first.
If we suspect that a board is the old kind, we stop, photograph it and explain what the next step is before anything is disturbed. Surface cleaning with a soft brush and water is a different matter from cutting or removal, and it is the cutting that creates the risk.
Fixing the cause rather than the colour
If a gutter is overflowing or a downpipe is blocked, sort that first; our pages on overflowing gutters and the downpipes guide cover the usual culprits. If the roof space is damp, the answer lies in ventilation, not scrubbing.
Where the boards are tired or the timber behind has gone, replacing the soffit, fascia and gutter together is more sensible than patching one piece. How long that takes depends on how much eaves there are and how easy they are to reach, as set out in roofline replacement time.
You will get a written quote after a survey in which we photograph the soffit, the fascia and the gutter joints and explain what we think caused the damp, so you can see it for yourself. The rest of the roofline work is gathered under gutters, soffits and fascias.
Under the eaves
Is green algae on a plastic soffit harmful to the house?
The algae itself does little damage to plastic, but it shows that water is sitting there. The concern is the source: a leaking joint, an overflowing gutter or condensation from the loft. Clean it, then watch whether it returns, because quick regrowth means the moisture has not been dealt with.
Should I paint over mouldy timber soffits?
Not until the cause is found and the wood is dry and sound. Paint over damp or rotten timber blisters and traps moisture. Cut out soft sections, fix the leak, let the board dry, then prime and paint, or replace the board if the rot goes beyond a small patch.
Tell us about your roof
Describe what is happening and we will call you back to arrange a visit.
- Photographs of what we find
- A written quote with one total price
- Or call 01903 299063
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