For you guys that build shops. Do you put your strapping (purlins) under or over your typar paper?

Vertical or horizontal?

This is for metal siding, insulated shop.

I’m also interested in hearing why you do it the way you do.

Thanks!

Between the tin and purlins, to keep the tin from condensating and dripping on stuff stored below. That is in an uninsulated building. In an insulated building no typar is required if you keep the insulated area below and plenty of ventilation between the tin and insulation.

When installing metal roofing, strapping (purlins) should generally be placed under the roofing material. This helps to provide a stable and even surface, especially for uneven roofs or when installing over existing shingles. Purlins also help in creating a vapor barrier, preventing moisture accumulation and potential damage to the roof. However, some professionals suggest that in certain cases, such as when using plywood roof sheathing, strapping may not be necessary. Always consider local building codes and consult with a roofing professional for specific installation requirements

On side walls put the typar between the tin and purlins on insulated buildings and not necessary on uninsulated buildings

Paper against metal for above listed reasons.

So if the typar is over the strapping, then the tin is flat against the typar and that’s ok? Moisture is able to wick away if the tin is in contact with the typar?

Yes, if I understand your question. Without that barrier under the tin/metal roof you will have problems.

Ok, now what if the strapping is on top of the typar? Giving an air gap between the metal siding and typar. Still ok?