What Have I Done To My Garage Roof!?

Hi all,

Last summer I replaced the flat garage roof on my mid-60s detached garage as the old one had well and truly had it. It seemed to be chipboard with a corrugated tin and was leaking all over and the chipboard was breaking up into tiny pieces.

I decided to go with an EPDM kit, liking the look of ease of installation, long life and the one-sheet approach which I felt would help avoid leaks in future. I also sourced a bunch of OSB sheets to replace the disintegrating existing chipboard.

I went into the garage on the weekend (first time I had been in there for months) and the first thing that caught my eye was black mould all over the OSB that we used.

On further inspection, it is very clear that on one side of the garage the boards have bowed upwards quite considerably. I cannot see any water ingress on any of the boards from inside the garage such as wet patches and the joists appear dry.

I thought I had screwed down the boards across the full length of each joist but obviously I didn't!

Aside from the above, what could have caused this and is there any way to rectify this without taking the whole roof off and starting over?

Please see photos attached.

Many thanks in advance.

Mould is usually because of damp. Is there any ventilation?

Ian H said:

Mould is usually because of damp. Is there any ventilation?

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No dedicated ventilation. I've always thought of it as quite a 'well ventilated' space because there are slight gaps around the garage door and under the (original) wooden side door.
Not sure if that would be sufficient though?

If it is a bit damp it might have also made your osb swell.

Ian H said:

If it is a bit damp it might have also made your osb swell.

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My next question was going to be was it something to do with the OSB I used?
This is the exact product I ordered: Sterling Zero OSB3 2440 x 1220 x 11mm (jewson.co.uk)

A family member stated to use plywood, which I decided against due to cost.

The description for the OSB stated suitable for humid structural applications so I presumed fine for a flat roof, if water was never going to actually touch the boards.

11mm is not enough for a roof deck - for the reason you can see there. 18mm for 400mm joist pacing is a minimum, otherwise 22/25mm.

The mould may not necessarily be excessive humidity, but can be simply that the mould likes the glue in the board. Try painting or varnishing a section.

as above board too thin, no gap between boards an too few fixings.. good job its rubber

You could glue some roof baton on underside adjacent the joist and then get someone to stand on the raised areas and screw thru baton to joists to hold it down.

I always like to conclude a thread where possible.

foxhole said:

You could glue some roof baton on underside adjacent the joist and then get someone to stand on the raised areas and screw thru baton to joists to hold it down.

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This is exactly what I ended up doing, a few weeks after you suggested it.
I had a spare length of timber so chopped it up into blocks, used CT1 to glue it to the underside of the boards and then screwed the blocks into the joist to hold the boards down.

It was harder than it sounds as I didn't have any help to stand on the raised areas so I had to knock something together to pull it down from below.
Think I used a ratchet strap attached to a hook driven into the blocks, then I stood on it to pull the boards down and screwed it in, all while on a stepladder.


Bit of a bodge but it was easy enough and a lot better than having to rip it all off and replace.

I know for future to use a much thicker board for this sort of application.

The mould issue hasn't got any worse at all since I first observed it and I haven't made any changes so that seems to not be an issue anymore.

Thanks all for the advice.

Rhys

A unique way of doing it! I would have lifted some buckets of water on the roof or a few bags of sand to weigh the boards down, anyway glad you got it sorted.

So is the only thing holding the OSB down a bit of glue between the battens and the OSB? Or have you also done a token mechanical fixing into the OSB as it looks like there are holes/fixings from beneath too?