Underlayment for Metal Roof

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October 18, 2023 at 1:23 p.m.


Charles in Salem, OR asked:


I am getting conflicting information on the following practices on metal roofs.
1. Underlayment.  I read on different metal roof manufactures sites not to use Felt paper 15 lbs. or 30lbs.  15lbs felt will melt within 5 years under a metal roof.  Synthetic underlayment is so much superior on many websites and Youtube.  Bruce & Dana  Metal Roofing  Supplier  of Salem Oregon  said that they recommend 30lbs felt paper.   Of the half dozen Pole Barn builders in the Salem, Oregon area are telling me they install 15lbs or 30lbs felt paper underlayment.  
2. On a screwed down metal roof; the screw's gaskets will rot within 15 to 20 years and have to be replaced. Again Bruce & Dana told me  not to install a screwed down roof.  They have had multipole contractors repair screwed down metal roofs I  don't want a major repair bill in 15-20 years and recommended a standing seam roof.  Legacy Metal Works today told me that this isn't as big of an issue if you use 1-1/2" screws or #14 on a screwed down roof roofing.  
Please give me your recommendations.  HELP!!!
October 18, 2023 at 1:24 p.m.

Below is what our roofing expert Stan at Pacific West Roofing had to say in response to your question:
1. I am not a fan of 15 or 30 lb. felt because they breath allowing air to get to the underside of the metal creating condensation which will become a problem for the plywood eventually. I suggest “high temp” peel and stick membranes because they won’t run, and they seal around the screws as well. Pole barn companies are usually building structures that are not enclosed or are not heated so they don’t see a need for sealing the underside of the metal. We do believe in it and it is security for us as installers for not only stopping moisture from moving upward beyond the plywood, it also stops any leaks through the top of the metal downward as well. 
2. I believe they are speaking of exposed fastener roof systems, not concealed screws such as Easy Lock from Taylor Metal. This is where the edges that snap over the previously mechanically fastening panel, covering the screws. There are zero exposed fasteners especially if you use a hook eve of hook locking eve and you won’t need screws. Exposed screwed down or nailed down metal roofs are not a good idea in our climate. The fastener grommet will get destroyed by UV thermal cycling and water exposure and both will dry out the rubber grommet.

I hope this helps!
October 18, 2023 at 1:24 p.m.

Bobby at Bliss Roofing said:

Even though regular felt paper is specified, we prefer using a high/temp Ice & water shield as the underlayment for a metal roof system. I do get concerned with the asphalt heating up and potentially leaching out. A high/temp Ice & water shield is more expensive however it contributes to a better metal roof system.

I agree there is a risk with an exposed fastener metal system. Yes, the gaskets can fail early, to add to that, the fasteners need to be driven carefully. If they are over driven, they gaskets can split and expedite the failure. I would recommend a concealed fastener system.

Thank you!

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