Can this garage roof be patched?

[Alastairreid]

You would fix the patch between the joists supported by tile batten fixed to the side of the joists.

 

[ctsamados]

Thanks I cleared the old bulging chipboard away and underneath it looks to be a felt, tar and gravel layer which is about 2 inches or so thick it then is laid on top of a steel mesh. There's only one section where there's a hole and this is under some weird tiled gable section at one end of the garage roof, so it's never leaked any water.

I'm off to buy some wood to batten and patch these sections now.

 

[ChrisJP]

It's also waterproof now, so I was just going to patch it inside to make it a bit more protected from weather conditions.

If it's waterproof now, patching from the inside will certainly not protect anything from the weather. You cannot make a flat roof more waterproof by adding stuff to its underside. The only way to improve it would be to put some sort of coating on the outside of the roof.
I would also be wary of adding a layer of anything to make a "ceiling" on the underside of the joists, unless you also arrange for some sort of through flow ventilation. Otherwise damp will inevitably build up between the top and bottom surfaces, and also you won't be able to spot any leaks that subsequently develop

 

[ctsamados]

It was to make the garage weatherproof rather than waterproof as holes in the chipboard could let in cold air and I heat the garage when in working in there.

I've done the work now and also fully painted the parapets with a liquid rubber paint.

We had one of the UK's worst storms for a while recently and the garage didn't leak. So it looks to have been sorted.

All that's left to do is add a false ceiling, which I'll do in the summer when all the chipboard should have fully dried. ATM it's so freezing the existing damp patches won't dry.

 

[cdbe]

Looks fine, but for future reference the battens would generally be fitted along the rafters.