Box section garage roof problem

[norbellini]

I have replaced my precast flat garage roof for the concrete /asbestos one that became porous with a metal box section roof . just the job only i am getting a lot of condensation even allowing for a good "draught" . so i bought four sheets of exterior ply and i will put them underneath and lay the roof again . Could you advise what to put on the top of the ply . i dont know wether to go to roofers felt or slap some preservcer on . Or do you think it will be ok as is thanks

 

[big-all]

duplicate please remove

 

[big-all]

if the ventilation is between the roof and ply then you should be fine
it wont hurt to apply some preservative like cuprinol or similar some insulation would also help with heat loss

 

[smashngrab]

I know its a bit late but you could have used the box profile which the under side is coated in cotton.
Its more exspensive than the bare sheets but thats exactly whats its for.Anyway thats what they reccon

The cotton soaks up the condensation,maybe it then runs out into the gutter?

 

[IKM]

I have the same problem with condensation on the underside of the corrugated asbestos cement flat roof of my garage every winter. From my experience and research the condensation is inevitable. The asbestos sheets have a textured surface which allows them to hold a certain amount of condensation without it dripping off the roof. You can get steel sheets with a coating which gives a similar effect but in severe conditions all these types of roof sheets will suffer from condensation.

If you put bare plywood sheets under the steel sheets the plywood will absorb the moisture and hopefully it will dry out before any rot sets in. If you apply a layer of roofing felt to the plywood you will end up with a traditional flat board and felt roof with steel sheets on top. I think this would be ok.