Mould on plywood

[Mike313]

Hi All,
Not a woodturning question but I'm hoping someone will be able to give me some advice. I built a shed last year (it's the garden shed and it doubles as my workshop). It's built with ship-lap walls so is not exactly air-tight (!) and it has no wall vents. Nor does it have any heating. I used exterior grade plywood for the roof with a good quality built-up felt finish. I'm certain it doesn't leak so I'm thinking the problem is damp air and coldness. I have noticed black mould spots starting to grow (lots of them) on the underside of the plywood (i.e. indoors). I know I can kill the mould with bleach and I assume that's the first thing I should do?
After that, what should I do to prevent a recurrence? Is there a treatment I should use? I was thinking if I used an anti-mould bathroom paint on the plywood it would have the added bonus of reflecting light and making the shed brighter to work in. Is there a better way to deal with it?
Thanks in advance
Mike.

[fuzzyturns]

You will struggle to prevent a recurrence. Once the mold is there, it'll survive all but the harshest treatments. Bleach might work, but you will have to make sure you get it everywhere.
The way to avoid this sort of thing is by

• Prepare the spot where the shed will stand. Has to be flat, compacted gravel, then a 2-4" layer of sand,
   covered with paving or slabs. This will allow any water to drain away and keep the feet of your shed dry.
• Make sure the underside of the shed can breathe, i.e. the shed itself stands on wooden slats or risers,
   and the cladding does NOT go all the way to the ground.

[Paul Hannaby]

Don't breathe when you're in there! Perhaps a small amount of ventilation would help reduce the problem?

[georg]

  Mike ... Damp will cause rot and rust on all your tools also the spores will not do your lungs any good...... the best
  way round this is to invest in a dehumidifier which can be turned off in the summer months relying on ventilation.
  Also worth mentioning wood worm and rot do not like dry air.
  Tony

[The Bowler Hatted Turner]

Why not insulate the roof space and put up a lining of some sort?

[Mike313]

Thanks for the replies.
Fuzzyturns, the shed has a concrete base and dwarf brick walls. There is a timber floor on battens on top of the concrete slab. The dwarf walls are panelled on the inside with plywood. The floor has a damp-proof membrane which is carried up inside the dwarf walls and overlaps a DPC that sits between the dwarf walls and the stick-built top half of the shed so I don't think damp from below is an issue. At least I hope not!
I contacted a 'technical person' at Sadolin; they recommended cleaning with a fungicide (and since many are bleach-based, I'll probably just use bleach) then 2 coats each of a preservative and an exterior grade stain. He recommended waiting until the weather is warmer and drier to do the latter.
I think I'll also put in two wall vents to allow a cross-flow of ventilation.
Hopefully that will fix the problem