Underlayment for a metal roof.
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I’m dealing with a contractor on a project to replace an existing cedar shake roof, with a lap seam metal roof.
The contractor is pretty insistent that he needs to install both 15-lb felt, and a “Self adhering poly, rubberized asphalt underlay” over the entire roof.
I’m thinking that the rubberized poly only goes in a three foot band, along the edges, and the valleys.
And, if we install the rubberized poly on the entire roof, why would we need the felt?
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some body might be right and somebody might be confused...
What is the pitch of this roof, what is the climate, and is he using furring or cedar breather also?
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Two roofs. One is an 8/12, the other a 6/12.
The proposal includes, cedar breather, 15-lb felt, and the poly-rubberized asphalt underlayments.
I'm not sure I need anything more than the 15-lb felt for the lap seam metal roofing. At the sme time I need the roofs to last.
I missed the part where the cedar changes to metal for the new roof.
So are you the HO nad he is the roofer proposing to do it this way?I'd want 30# felt myself not 15#I can think of reasons to want the I&W, but can't see the job from here.Looked at your profile for location - I backpacked the middle fork of the salmon going upstream eleven days once.
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30 lbs felt for sure, 15 lbs is under-built.
Our roofer did what Piffin says, 30# everywhere, with I&W in the valleys. We also put I&W under the steel on an entire shed dormer roof, but that was because it's only a 3/12 pitch.Scott.
Jigs,We use 30# felt on plywood but never on purlins. I have seen 75 year old roofs done like this with no problems.Only snows here every 20 years though.KK
No need for the "Self- adhering etc", but I would use a synthetic underlayment like Grace's Tri-flex 30 or Tri-flex Xtreme rather than 30# felt. Under a metal roof the felt tends to get too hot and stick to the roofing becoming ineffective. You might want to check the roofing warranty. The manufacturer I use insists on a synthetic underlayment.
Down here in Florida our codes requires self-adhering underlayment on metal roofs and for shingles on houses that cost more than $300K.
For shallow pitch, I would go overall with membrane and skip the felt. For 5 in 12 or better, I would put membrane on the eaves and under valley flashings, and felt the rest.
I would also insist the roofer use butyl caulking in every lap seam of the steel, especially if the roof is shallow.
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