Mould appearing on garage ceiling

20 Feb 2022
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United Kingdom

We have mould appearing on the internal plywood of our garage roof in the past few weeks during the colder/wetter weather. I wondered if anyone could give an insight into why this might be happening? The mould appears between every joist front to back, mainly appearing from the edges but not on the joists themselves. We have no heater or motor running in the garage and have barely been in there ourselves in the last month, so aren’t producing any warm air that we can think of.

The roof was installed last June replacing an asbestos corrugated sheet that overlapped the neighbours. It’s an EPDM roof that sits in the channel of the remains of the neighbours roof. So it is not completely air tight and has air flowing through the garage corners as well as the gaps around the garage door. So I had assumed that there was ample ventilation. But now I’m not so sure.

I wondered if it might be the type of plywood used? Since this is in direct contact with the EPDM.

I’m no expert by any means so I have no know idea what is good or bad wood. Speaking to the installer yesterday, who was convinced it’s just condensation, he was adamant that the wood is fit for purpose (have posted a close up of the writing on the boards in the hope that someone can confirm). They suggested simply using a mould killer spray to wipe it off and paint the boards with primer or varnish. I’m not convinced that this will solve the problem and is more of a temporary solution, in addition to being concerned that plywood is a porous surface so the damage may already have been done.

They also suggested just adding some insulation and a false ceiling but to me that just traps the damp and will make it worse.

There is no evidence of mould anywhere else in the garage only on these plywood boards. When I touch the plywood I think it feels a little damp. We also haven’t had a mould problem with our old corrugated roof.

I just don’t know what to do here and have no leg to stand on with the installer who gave no indication that they would be replacing the roof. I believe that that are a reputable company who have installed plenty of these types of garage roofs, so I would assume that they knew what they were doing.

Any help with this would be greatly appreciated and perhaps give me some ammunition for either going back to the installer or proceeding with any further work required myself.


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Is it single skin? Could be moisture getting through the bricks then condensating on the roof. I think for ventilation to work well you need the air flowing just by the ceiling.
Yes it’s single skin brick. We do have some airflow as I mentioned but perhaps it’s simply not enough. Doesn’t explain why only the plywood is mouldy though. The brickwork, joists and everything that’s in the garage is not mouldy at all.
Poor quality ply and condensation from it being a cold roof and mould feeds on the glue.

It looks like the cheap ply used for boarding up windows and other temporary works.
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Thanks it confirms my worst suspicion. But not sure the installer will replace the roof and admit that it’s poor quality ply. When I asked the question there response was that they have had no complaints else where using the same ply.

So I guess my next question is, can I improve the ply in any way from the inside? But as you say it could be coming through via the glue from the other side.
Its not a defect as such as the ply will still perform as a roof decking (although life may be less), and other ply may still experience mould spotting. The actual cause is condensation, which all uninsulated roofs are susceptible to.

Sterilising with a fungicide and then coating the underside with an oil based paint or varnish will prevent mould feeding on the ply, but there's no guarantee it will stop condensation mould on the surface, but it normally does. You could do a small trial area.

Normally ventilation is mentioned but it does not tend to work on its own in these situations.
Thank you that is reassuring to hear. My solutions for cleaning and painting where these. Is this ok or would you recommend something else? You mentioned oil based paint but this paint was recommended to us.

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Also look at products from Halophane range from Mould Growth Consultants. Put those into Google.

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