commercial retrofit on a PEMB, what to do with the roof?

[MonsieurR]

Hello,

There is a small fairly typical pre-engineered metal building which is going to be partly retrofitted for commercial use. The retrofit consists of placing a mezzanine inside of the building, and closing the envelope of this mezzanine laterally with insulated panels. The question comes on what to do at the roof of this small building within the large one, always having in mind that this new zone within the building needs to have an appropriate envelope for the weather (which is somewhat cold humid), and that to my knowledge single ply metal roofs are not very weatherproof (water proof). Attached see situation.

Is it appropriate to specify an underlayment such as a waterproof membrane beneath for this type of building? I'm mainly worried for water and condensation, as there will be a gypsum ceiling with wool above to help thermal comfort.
How can I create a drainage plane? Is it necessary? Can I instead use a waterproof membrane above the metal roof? Does this avoid condensation? Do I need to vent the plenum between the ceiling with wool and the metal roof ?

Any comment is welcomed. Kind regards.

 

[Ron]

Depending on the type of roofing sheets and the seam configuration, these roofs can be quite water-tight. If you want to supplement with a membrane, use a fluid applied membrane on top of the existing roofing, not underneath. You don't want to create a secondary vapor barrier under the roof.

I would vent the space above the ceiling, but not necessarily to the outside....just to the other interior.

 

[MonsieurR]

So Newman's book is not very clear about it (see attached).
Isn't placing a secondary vapor barrier under the roof the typical detail for timber houses?
In this case though, I don't have a plane substrate to support an underlayment so indeed It shouldn't go underneath.
Metal sheet is an R panel type. I believe the attachment screws always pose a potential leak, since seals don't last long. I think it is appropriate to use a 2 layer barrier system as a general detailing rule for interior spaces (like 2 envelopes) but the question is how would be the best way to implement this here. Indeed a liquid membrane appears to be the solution.

 

[JedClampett]

There's companies that make retrofit roofing systems that go over an existing roof. I think the bigger manufacturers market this, too.
This way you put the burden of weatherproofing on them. Hint; Have them test the roof as a last step. Send someone up with a hose and flood the roof. If it leaks, they didn't put it in right, and need to fix it. If it's good, you've shown the owner that the roof didn't leak, at least on day one.