Advice needed. It's raining in my new garage!

[KugaWestie]

Yes insulation to the roof will help, but as has already been said, you need to leave an air gap between the topside of the insulation and the underside of the roof sheet. That air gap must be ventilated though

[Tim Reid]

Cheers all. I'm off to Travis Perkins in the morning.

[Lyonspride]

You do in fact need heat, as relative humidity falls as temp increases and the closer to dew point the air temp is, the more moisture that will condense inside your roof.

http://www.dpcalc.org/

So for example, if the inside is 10c and the RH is 60%, the roof temp will have to fall to 3c for condensation to form. However there is one problem, radiated heat, outside air temperature is not the same as surface temperature, have you ever wondered why your windscreen frosts over when the outside temp barely drops below 5c??? A few weeks ago I noted the the roof of my shed was at -7c even though the air temp was barely 0c.

You basically need to heat the roof or put enough heat under it to keep the temp above dew point.

You also need to look at why your humidity is so high and do something about it.

[John Loudon - Sponsorship Liaison]

I used to have this problem in my loft space of the house which has a tiled roof and roofing felt underneath overlapping in horizontal lengths but the principal could be applied to most situation including yours if your roofing sheets are overlapping in some way. My solution was simply to buy a length or two of plastic conduit (mine was the flat sided type) and to cut into approx. 4 to 5 inch lengths. I then inserted these lengths in between the overlapping layers of felt to create a small passage for air to flow between the tiles and the felt. no more condensation and cost less than a tenner

[Man On The Clapham Omnibus]

1 hour ago, John said:

I used to have this problem in my loft space of the house which has a tiled roof and roofing felt underneath overlapping in horizontal lengths but the principal could be applied to most situation including yours if your roofing sheets are overlapping in some way. My solution was simply to buy a length or two of plastic conduit (mine was the flat sided type) and to cut into approx. 4 to 5 inch lengths. I then inserted these lengths in between the overlapping layers of felt to create a small passage for air to flow between the tiles and the felt. no more condensation and cost less than a tenner

I was thinking of buying a few of these https://www.amazon.co.uk/dp/B00M97O5YY/ref=wl_it_dp_o_pC_nS_ttl?_encoding=UTF8&colid=14C0I4MNQ2M9X&coliid=IU9WC1FKMXOX6 but your idea is a lot cheaper!