Garage Felt shingles leaking / mould on inside

[^woody^]

a patch half meter square which seemed to be floating

Any raised felt creating an air void between the board and felt, can be a source of interstitial condensation.

 

[Badger40]

I will carefully take up the shingles in this area and remove dodgy felt or nail / stick it back down if that sounds reasonable.

 

[steven lee]

That Sika roof and gutter sealant will do for small cracks and gaps, but for bedding felt back onto OSB a proper felt adhesive or bitumen mastic will do a far better job. The tube stuff skins over before it can grip a big area, so you can end up with the edges lifting again next summer. Bed it flat in warm dry weather, get it fully bonded and weighted down, and water will have nowhere to get in. Good plan checking from inside during rain too, that is the fastest way to prove you got it.