Profile sheet metal workshop roof - is this configuration ok

[Pord]

As posted elsewhere, I'm currently building a 6m x 4m timber frame workshop with monopitch metal sheet roof. My intended roof, from the inside, is: OSB3 ceiling as vapour barrier; timber joists with insulation between; breathable membrane; ventilation gap; metal sheeting.

I had assumed the membrane would go on top of the joists, then the purlings, then the metal roof sheets. The metal sheet supplier advises to put the purlings on top of the joists, then the membrane, then the metal sheets. My worry is that in both cases condensation could collect where the sheets are screwed tight to the purlings, and whether there is enough ventilation beneath the metal sheets.

Any thoughts?

 

[MikeG.]

You're right and he is wrong. There can't be any airflow with the way he describes, and you must definitely ventilate below the metal sheet.

Purlins, BTW.

 

[Pord]

Great, thanks Mike.

If I put the membrane on top of the joists and then the purlins (schooled!) going horizontally across at right angles to the joists, won't that create a series of barriers that will prevent condensation from running down the membrane to the guttering? Possibly also restrict the ventilation under the metal sheets?

 

[Lons]

I can't really answer for a workshop roof however I put roofs on extensions I built many times and where possible used OSB so similar except that instead of metal roofing sheets the final covering was tile or slate and the principles are similar with regard to water running down the membrane. To overcome that problem you need 10mm spacers nailed down the length of each joist on top of the membrane and the slaters laths (purlins in your case) fixed through those. This gives a 10mm gap for moisture to run under the laths.

You can buy these but all we did was rip down lengths of 25mm thick pressure treated slaters laths to make our own.

If using profiled metal sheets there will be plenty ventilation space through the profile chanels btw.

 

[Pord]

Brilliant, thank you Lons. So simple when you know how.