Hi AYF,
Having replied to a number of members by pm enquiries on here re dpm on timbers supporting sheeting or cladding or any type of covering, so thought I would put my thoughts on dpm out there.
Dpm is designed to stop damp travelling through hardcore based substrates, concrete, brick, block, gravel etc, timber though, as a natural product needs to breathe, putting a dpm on any timber limits its ability to vent moisture naturally and traps the moisture between the living product and the plastic, yes it protects it from landing moisture but think about your own skin.
When it’s raining you put on wet gear to protect your skin from landing rain, but the minute the rain stops the wet gear comes off as you would start sweating beneath it and end up even more uncomfortable and unable to dry out, removing the wet gear prevents this. You cannot remove the wet gear on a shed when the rain stops so how does the shed stop the natural sweating?
Put the wet gear on to yourself on a warm day and see how you feel, not too happy I bet?
Another issue is in livestock housing, animal warm breath rising creates damp at roof level due top temp differences, the damp sits between dpm and timber and creates constant damp/rotting conditions.
Any sheds I have ever been on (quite a lot) the most dangerous ones had 4” dpm on top of the timbers, timbers felt solid but were mostly totally rotten. My closest call to injury was on a round roof shed housing cattle, stepping across sheeting over rotten timbers one gave way, I jumped to another one and landed on the clear sheet and supporting timber which gave way and sort of fell onto the truss where I managed by the grace of god to stop falling. Had to change the y’s and it was a long time before I felt comfortable on a roof.
Just my opinion and experiences but in all our refurbs, rebuilds and new builds I NEVER once put dpm on any timbers.
Paddy