Shed build - decking or purlins under metal?

[northcaroliney]

Building a shed - existing structure on property is gavalume 5V over 7/16" OSB decking. Was planning on similar strategy with #14 screws to secure to the OSB, and a self-sealing roof underlayment. I do want to minimize condensation and noise. The other consideration is to do purlins and then spray foam directly on the underside of the metal.

Pros and cons of each? Decking + underlayment is obviously a more costly approach. Roof is only about 400-450 sq. ft.

 

[AllByteNoBark]

If you do purlins and are going to spray foam I would put house wrap under the steel. Then spray the foam on it. We did that and it makes it easy to replace damaged steel without disrupting the insulation. Also allows it to breath.

 

[northcaroliney]

If you do purlins and are going to spray foam I would put house wrap under the steel. Then spray the foam on it. We did that and it makes it easy to replace damaged steel without disrupting the insulation. Also allows it to breath.

Smart move - thanks for this advice, as I would not have thought of it otherwise but certainly can see the benefit if repairs are needed.

 

[EddieWalker]

Foam is going to be a huge expense for a shed. Do you really need it?

Why spend the money on a membrane over OSB when 30 pound felt will do the same thing?

What will the shed be used for?

Venting is the easiest and cheapest way to deal with condensation.

 

[TnAndy]

7/16 OSB is a lousy base for metal. Metal expands/contracts, and screws tend to back out of the OSB because it's so thin and doesn't have enough thickness for the screws to hold. Seen it many many times. Either use a thickness decking material or purlins. Personally, I use sawmill 5/4 x 6 laid across rafters/trusses on all my sheds.