Grey powdery substance on underside of garage roof

frogeyesimon

Hello,

I have a 1970s property and have lived in it for the past 12 years without noticing the following:-

My garage has what I'd describe as an asymmetric pitched roof. Part of this is covered with concrete tiles, whilst the remaining shallower pitched part is covered with roofing felt. All appears to be in good condition and I can find no evidence of water penetration from above.

The shallower felted part of roof sits on chipboard decking and this year I have noticed a grey powdery deposit that is forming in places on the underside of chipboard. This drops onto my shiny classic cars (no big deal I know but it's annoying me). The grey stuff is very fine, almost like flour, doesn't smell and is easily washed off.

What could it be - mould perhaps?
What could I do to prevent it forming? - treat the underside of the chipboard in some way, paint it perhaps?

Thanks for reading - would welcome any advice.
Simon

Robwiz

It sounds some sort of mildew, mould or fungus. These grow in damp conditions in unventilated spaces (often in roof voids). It's an indication that there's not enough ventilation.

A product like HG Mould spray should kill it and is worth a try. If you stop the mould forming on the chipboard by painting it then it will form somewhere else unless you improve ventilation. The chipboard may be the most convenient place.

paddyrg

Could be some kind of mineral salt leeching slowly through one component of the aging roof?