9 hours ago, Russell griffiths said:

Condensation is funny stuff. 

When I put mine on I had sheets just sat on the roof not fixed, you would think that the temperature was the same on the top of the sheet and the underneath, so how come when you pick a sheet up the underside is dripping in condensation, it can only be 1-2 degrees warmer on the underside. 

 

Rapid fluctuations in temperature on the metal surface can have an effect on the airflow even in a fully ventilated timber board construction. Warm air can draw moisture into the void which would be attracted to coldest available surface, the back of the metal.

so in Europe construction methods avoiding plywood in favour of organically treated softwood timber planks means excess  moisture is absorbed by the wood and then released as the air warms and dries the timber deck. Really the best solution for Zinc which can be destroyed by backside corrosion, White Rust

 

avoid timbers that have preservatives using Copper as this will damage the Zinc.

 

Penny gapped, nominal thickness 25mm softwood (tolerance max 3mm in thickness) rough sawn.

 

take extra care on the timber species as some are acidic, again damaging. This is why Zinc directly on Plywood, unknown timber composites is FORBIDDEN. 

 

Design well, Respect the needs of the metal and the Zinc will make a good roofing material. Take advice, it's free, www.metalsolutions.uk.com 

a well installed roof is a reference that everybody talks about

9 hours ago, Kevin J said:

A simple ventilated abutment detail against the house is easy. However, continuous airflow from ridge to eaves is interrupted by the roof lights.

architect guy is on it with warm roof build up provided the Vapour Control layer (VCL) is correctly specified and correctly installed. 

A solution could be Hard Metal Board over Alutrix VCL. You should belt and braces with a Structured Underlay such as ISO-Mat Metal. 

All at www.roofing-tools.com including the Zinc 

Morning Kevin,

Thanks for the input. However, to risk being anal about this - but it is an important technical distinction to be made on build up - is that what I think you mean is Alutrix Vapour Barrier, not vapour control layer. Vapour control layers have some vapour permeability whereas the vapour barrier does not and this is what's needed if you're installing a metal warm roof.