Mould in the attic looks more alarming than it usually is, but that is no reason to leave it. Black specks on the roof boarding, greyish white fluff in a corner, a musty smell as soon as the hatch opens: it always means the same thing. There is moisture and it cannot escape.
So the question is not how to remove the mould. The question is where the moisture comes from, because without that answer it will be back within a year.
Three causes, in order of how often I see them
Condensation. The most common one. Warm, moist indoor air reaches the attic, cools against the cold roof boarding and condenses there. See
condensation or leak.
A leak. A small leak that keeps the timber permanently damp. Often at the chimney, a valley gutter or a roof window. Recognisable by mould in one spot rather than spread out.
Too little ventilation after insulating. An insulated roof without an air gap, or with blocked ventilation openings at the eaves, cannot get rid of its moisture. See
insulation and ventilation.
“Mould is a symptom. If you only clean the symptom, you will see it back in the same place next year.”
Fouad, owner of Astrodak Service
Where the mould is tells you what it is
Spread over a large part of the underside of the roof, and worst at the eaves and in the corners: that is condensation. Those spots are the coldest and that is where the air is most still.
One clearly defined spot with a trail running upwards: that is a leak. Around the attic hatch or above the bathroom: then the moisture is coming from the house. And on the inside of a roof window or on the reveal around it: then the insulation detail around the window is the problem, see
condensation on a roof window.
Spread out and at the eaves
condensation
ventilation needed
Not sure about the right approach?
I would rather come and look at your roof with my colleagues first than come up with a solution from a distance. During the free inspection you hear what is really needed, even if that is less than you thought.
The approach in the right order
First remove the cause. Seal the leak, restore the ventilation or deal with the source of moisture in the house. Without this step, the rest is pointless.
Then assess the timber. Surface growth on sound timber is cosmetic. Soft, dark or crumbling timber is affected and must be replaced, see
replacing roof boarding.
Then clean. With a suitable product, gloves and a good face mask. Dry brushing spreads spores, so work damp and ventilate during and after the work.
Finally, keep it dry. Ventilation openings clear, hatch closed, no laundry drying in the attic. Check it again after a winter.
When to call someone in
With growth on a small area and a clear cause you can remove yourself, you can manage on your own. Call someone in if the mould covers a large part of the roof, if the timber feels soft, if it comes back after cleaning, or if you cannot pinpoint the cause.
With us, this is a standard part of an inspection: I do not just look at the roof from the outside, but also at the underside of the boarding in the attic. That is where the story of the past few years can be read. See
free roof inspection and
how often to have a roof checked.
When to call in a specialist
With a small area on sound timber, this is work you can handle yourself or together with a roofer. With mould over a large part of the roof, with affected timber or with an attic used as a bedroom, it is wise to call in someone who specialises in damp and indoor climate.
What we do is the structural part: removing the cause, restoring the ventilation and replacing affected timber. For treating a large area of mould in a living space, a specialist company is the right party, and we will tell you so.
Frequently asked questions about mould in the attic
