Mould on inside of block shed roof, advice please

[coldfire3000]

Hi all

 

Wondering if anyone can assist please, I am a DIY guy, but not a builder.

 

Last summer we had our garden paved and a large double garage reduced down in size to make a smaller shed and patio area.

 

Winter has come and I noticed mold on the inside of the ply on the shed roof. I raised it and was told it was due to cold temps outside, humid air and sealed up shed. Understood. Installed 2x 4" tumble dryer size vents as recommended, one on the end and one on the side, best locations I could manage. I wiped the ply down with a bleach solution and waited a few weeks. Also installed 2 of the disposable moisture traps you can buy.

 

2 weeks later it's a lot colder both outside and inside (due to temp and new vents) and there is fresh mold on the ceiling, the original stuff seams dead as far as I can tell. I also notice the roof rafters seem damp on the side touching the roof and it seems to be working its way downwards. The ply itself also seems darker as if damper than before.

 

The outside is rendered with waterproofer but not yet painted due to weather when construction was finished. The mortar on the row of red brick you can see in the photos is damp. This is where some render was missed on the neighbors side, they came back and rendered it, but I don't think that bit has waterproofer in it as the mortar there is much darker than the other 3 sides of the building... I'm planning to paint the full outside asap, just waiting on weather...

 

The roof construction is flat 22mm ply, underlay and felt. I'm told it's well constructed and isn't leaking and it's condensation from moisture inside the shed condensing on the inside of the roof due to the cold.

 

I'm ok to accept that if that's likely to be true. I'm just not sure what to do about it as I don't want a moldy roof either! 

 

The space is used mainly for storage, but also as a workshop. Theres tools, bikes etc in it but nothing wet stored in it. There was an issue at the start with water making its way under the block wall, but that has since been resolved.

 

Any advice is greatly appreciated!

 

Thank you.

[Iceverge]

Ply has a habit of doing this. In winter temperatures the air inside the shed will be slightly warmer than the outside, hence able to hold more water vapour. Once that damper air touches the cold ply, boom you get condensation and a small amount of mold. 

 

Insulation outboard of the plywood would keep it warm and solve the issue. 

 

Otherwise just ventilate away as much of the inside air as you can. 

 

You could go down the line of installing a really robust vapour barrier inboard with ventilation below the ply but I don't think it's worth it. 

 

 

 

[coldfire3000]

15 minutes ago, Iceverge said:

Ply has a habit of doing this. In winter temperatures the air inside the shed will be slightly warmer than the outside, hence able to hold more water vapour. Once that damper air touches the cold ply, boom you get condensation and a small amount of mold. 

 

Insulation outboard of the plywood would keep it warm and solve the issue. 

 

Otherwise just ventilate away as much of the inside air as you can. 

 

You could go down the line of installing a really robust vapour barrier inboard with ventilation below the ply but I don't think it's worth it. 

 

 

 

Hi. Thanks for the reply.

 

Forgive me, when you say "outboard of the ply", you mean above it? If so I can't do that as it's ply with underlay, then felt on top of it. I could put insulation on the underside where the mold is. But my understanding was that if I did that it wouldn't help matters as the moisture would make its way through the insulation and still hit the ply which would still be cold...?

 

Installing a vapour barrier would be tricky due to the layout/thickness of the rafters. Would it just be a case of stapling it tight to the underside of the ply/rafters?

 

I'm thinking at the moment that I will install a 3rd vent to try and get more air movement I guess...

[coldfire3000]

Ps. Is it worth giving the underside of the play a good coat of PVA or something?

[Iceverge]

11 hours ago, coldfire3000 said:

But my understanding was that if I did that it wouldn't help matters as the moisture would make its way through the insulation and still hit the ply which would still be cold...?

 

Correct

 

 

11 hours ago, coldfire3000 said:

Installing a vapour barrier would be tricky due to the layout/thickness of the rafters. Would it just be a case of stapling it tight to the underside of the ply/rafters?

 

Ideally you would have no penetrations at all in it but those that are there would need to be adequately sealed. 

 

It should be returned to the wall and sealed here too. Then some ventilation above it. 

 

11 hours ago, coldfire3000 said:

I'm thinking at the moment that I will install a 3rd vent to try and get more air movement I guess...

 

"Natural" ventilation (hole in the wall) relies on convection from heat differences or else the wind to move air. If the building isn't heated you won't have much convection although if it does get heated via sunlight a high and a low level vent will should help. 

 

Similarly the wind needs to be able to blow through the building. Vents on a wall facing the prevailing winds and the opposite are best.  

 

11 hours ago, coldfire3000 said:

Is it worth giving the underside of the play a good coat of PVA or something?

 

I don't know. Mould relies on moisture to grow. You might risk trapping moisture inside the ply as this will only create a skin on the surface. A penetrating treatment that will soak in would be better.

 

Get the ply as dry as you can (maybe wait for summer or use a heater/heating) and then try painting on some solvent based wood preservative that will soak into the wood. Be careful of your eyes and ventilation when doing this. Try a small test patch and see if the mould returns. 

 

12 hours ago, coldfire3000 said:

Forgive me, when you say "outboard of the ply", you mean above it? If so I can't do that as it's ply with underlay

 

Yes, you could lay insulation over the felt and refelt again over the top. 

[coldfire3000]

That's great info, thank you.

 

I think I'm going to hit it with some of this to clean and kill off anything. I already tried home solution of vinegar but for £10 I will give the proper stuff a go.

https://www.toolstation.com/barrettine-mould-mildew-cleaner/p90095

 

Then let it dry out as best I can over the coming weeks/months. Once it's dried out hit it with this, seems like the right stuff for the job? https://www.toolstation.com/barrettine-wood-protective-treatment-5l/p68513

 

Unfortunately the normal (ish) direction of the wind is left->right across the garden, but I don't have access to the right side of the shed really. So I've had to install a vent on the left side and front side. I can only really add a 2nd vent to the left side, which I don't think will add any further benefit...

 

Redoing the roof over the top is unfortunately a last resort, I simply don't have the funds for it

 

Thanks again for your helpful advice!

 

[Iceverge]

No problem.

 

Please report back as to how it works out. We'll be a little wiser.