Plywood shed partially built has some mould, help please

[happyhero]

Hi I built a huge shed out of plywood which I am going to clad in shiplap or similar and I had not finished it, ie it is only a plywood box at the moment with no windows or doors (has an opening for double doors at one end and single door at other end to side).

For various reasons, I had to leave this project over the winter months, so I covered it with some large really thick gauge plastic type tarpaulins that I managed to get hold of, to try and protect it from the weather. I also painted the outer side of the roof plywood sheets with Cuprinol clear wood preserver (don't know if this was overkill).

The plywood I used was supposed to be for outside and WSB type.

Now when I go back to it, I have not yet uncovered it as the weather is changeable, so I do not know what the top side is like but I am not that happy with the inside as it appears to have all black dots on just some of the roof sheets.

I don't know what this is but suspect it is some form of mould starting and it looks a bit like what you might see in a bathroom if it became really steamy so that there were all beads on the ceiling, only in this case they are black. I suspect this is what happened possibly several times and that is why they are black but maybe you can tell me different.

I am worried what is happening here especially if it will get worse so what do I do?

Do I have to do anything, ie will it be no problem or fade once I have windows and doors or take the sheets off?

I was going to use those felt shingle tile type things you can get but am worried what I will see when I uncover the roof, ie will the top still be ok and be ok to fit these felt tiles too.

Remember this shed is huge, it is just under 4m high and 6m long by 2.5m wide, so there are a fair few sheets of ply.

So can anybody tell me what is wrong and what I should do?

It will be hard to scrub the sheets if that is what is needed between the joists etc but I will do that if that is what is needed but my worry then is what will stop this happening again in the future?

I know other shed owners do not have this problem so are the plastic sheets the protection and the cause and will it stop when it is covered with the felt instead of the plastic sheets?

Any help appreciated.

 

[big-all]

its mould

your sheets have got wet/damp and remained so for possibly several weeks
the mould in its self wont dammage the ply but excessive wet over a long time may
if the structure feels fine without any bumps then its purley cosmetic wipe down with mild bleach and let fully dry before covering but not nessisery if your insulating and boarding just let it dry

exteriour /waterproof ply will still soak up water the only difference is the glue is waterproof and marine ply is the same except you dont have "pockets" with no veneer as the layers are continuous

 

[happyhero]

Thanks big-all. If I painted the inside roof surface with an anti mould treatment, would that help or would that be a waste of time and money?

 

[oilman]

Painting it with anti-freeze will kill the mould. The way to stop mould is to ventilate. I built a shed with OSB, put a roof on it (OSB sheets with corrugated sheets over, the ends haven't been finished yet so I've just got netting there. Been like it three years, and no mould.

 

[meannotgreen]

happyhero, if you see this, could you tell me if and how you resolved the problem of mould spots on your plywood shed?
I have exactly the same problem on some panels of the plywood sheathing of a recently constructed garage roof.