Greg from Florida recently sent me this question:
I am looking to have a metal roof installed over my existing single layer shingle roof. I have had several estimates from companies but the one difference I see is that some companies want to use purlins and others do not. Are there a advantages / disadvantages to using purlins in this situation?
Here is my reply:
Good question. The first part of my response has to do with the suitability of the product for this application. There are a very few metal roof styles which MUST be installed over battens. There are some which can be installed over battens or solid decking and of course there are others which must be installed over solid decking.
Assuming the roofing you’re having installed can be installed over battens or solid decking, you then have the question of which to do. Frankly, folks will probably tell you that condensation control is a reason for using battens. There really is very little validity to that in my 30+ years of experience. If the home has a well vented attic space, there is no reason for the metal roof to be mounted on battens.
The benefit to putting it on battens, though, is that the thermal break caused by the airspace will help some with energy efficiency. However, if your attic is vented well and you’re using a roof panel with reflective pigment, the added benefit of the cavity from battens is minimal.
The issues with battens are they can make the roof less walkable and they also mean you have to have extra trims to cover up the depth. This could also mean you will have to raise the gutters on the home if there are any.
I hope this helps. Please contact me whenever I might be of service.