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I'm building my own house, a first for me. And I like the idea of metal roofing--and siding too--because it lasts so long. However I've also been looking at housewraps and roof underlayments and I've run up against what seems like an issue: When I google "how long does roofing felt last for" I get answers in the 25-30 year range. Dorkin's Delta Foxx is warrantied for 25 years. RoofAquaGuard's products all seem to be warrantied for 35 years, which is better but...it's still 15 years short of the 50 year lifespan I'd expect from a quality metal roof.
It seems like when installing a metal roof I'd want an underlayment that has an expected lifespan that is the same or greater than the metal roof, so that I can replace both at the same time without dramatically reducing the lifespan of the metal roof.
What do people usually do?
This is a good question.
Under a properly installed metal roof system, the synthetics will easily last the life of the roof.
When the synthetics were introduced, everyone wanted warranties so folks put warranties on them but fact is once they are embedded in the roof system and not exposed to UV nor have any regular exposure to moisture, they really don't deteriorate.
Now, that said, there are two basic grades of synthetic and I do suggest the premium grade. The easiest way to identify a premium grade is the weight will be 30 or more pounds per 10 square roll.
I hope this helps. I agree it's a rather confusing situation. Again, though, I have no qualms about a premium synthetic underlayment performing for the life of a properly installed quality metal roof.
Tile roofs are a bit of a different story in regards to underlayments because many tile roofs depend upon the underlayment for watertightness of the roof assembly.
Hi Aaron, great question. There are three main categories of underlayments to choose from under a metal roof: 1) 30# felt, 2) Synthetics, 3) Modified Bitumen (asphaltic) with top synthetic surface. Once covered by your metal roofing, the UV exposure will not affect the underlayment so this will help in terms of longevity. That said, there are other forces at work than UV...heat/cold, thermal movements, etc. which affect the underlayment. Select a product which is "High Temp up to at least 250 degrees F to handle metal roof temps. Keep in mind that the roof underlayment IS exposed during the period between the underlayment installation and the metal roof placement. Make sure your underlayment can handle that expected exposure period. 30 year lifespans are reasonable for the higher end version of any of these types of underlayments but the true longevity varies based on the factors I described before. I suggest selecting an underlayment that has a min 30 yr warranty with one or two layers, High Temp rated to at least 250 degrees F and has nail sealability. This should give you a fighting chance to match the lifespan of a metal roof.
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