I get dafter as I get older-steel roof!

Just to update on this......

The roof eventually dried out and the drips stopped-until last night! The roof stayed dry throughout the ''monsoon'' rains during the summer but now the weather has turned colder again the problem is back.

To my mind this proves it is condensation so a friendly word of caution to anyone who is thinking of putting steel sheets on top of anything which is not sealed-beware!

 

Back to scratching my head!

To my mind airflow through the cavities is what is causing the problem-i.e. cold air hitting the underside of the cold steel and creating droplets, so the solutions are to insulate the underside of the steel or somehow fill up the cavities completely.
Neither of which are possible without gaining access to the underside of the sheets. I think ultimately I'm going to have to rip it all off and start again. 70 quid for new sheets is not going to break the bank, it's just annoying that the job is a failure.

 

I see your point.

The lower end of the roof was sealed with expanding foam when the problem became more prominent, mainly to ensure there was no backflow of rainwater under the roof because there is only a 50mm fall in 3 metres. The higher end is covered but not sealed, with a custom made stainless steel 3sided box section, which snugly fits over the profile and tucks under the lip of the ply.

I'm now having second thoughts about this.
Looking at the profile I'm failing to see how so much condensation can form on the steel. There is so little of it not in contact with ply i.e. there is virtually no air gap. Is it possible that the condensation is actually forming on the underside of the ply itself? I thought timber had a decent thermal value?

There is virtually no airflow inside the building when it is closed up. If this is the case all I have to do is clad the underside of the ply with polystyrene. For what it costs I'll do that anyway.

 

I'll update in a couple of days when I've monitored it further. Tonight looks to be white over here with frost.

 

Well it looks like the condensation had been forming on the underside of the ply not on the steel sheets. I never even gave it a thought as I didn't think condensation formed on timber.

I've insulated the underside of the ply with 25mm polystyrene and it looks to have stayed dry, no drips, so I'm hopeful it's problem solved. I'm just so relieved I've not got to start hacking the roof about.