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Community and Q&A

Underlayment on a low-sloped metal roof?

lauraneedshelp | Posted in Energy Efficiency and Durability on

Our roof deck is 15 yr old Techshield and the slope is 3.4 and 4.2. We are having old asphalt shingles removed (thanks to hail storm) and new standing seam crimped style 24 guage metal roof installed. Live in Texas and occasionally get ice. All of the 5,000 sq.ft. house drains into 4 valleys before it rushes off into 4 25ft gutters. Rain comes hard in TX and the valleys could back up and due to the very low slope, water could get under the metal roof. We want a synthetic product but have concern that the section over the cathedral ceiling (no attic just 12in rafters/joists) doesn’t have much air moving through (2 4″ baffles stapled to roof deck is my memory). Heat is a huge problem here over 100 degrees for months at a time. Of course we are having zee ridge vents on all ridges, including the one over the “no attic” section. We had Stormguard only in the valleys and around penetrations on old asphalt roof. I guess they attempt to tear it off.

One contractor wants to use GAF Ice and water shield on the entire roof . It is not breathable so maybe it wouldn’t be best choice. One wants to use Carlisle WIP300HT. Someone suggested Palisade Synthetic on part and Certainteed stormguard on valleys and rake edge. I found GAF Deck_Armor (which breaths) on the internet but haven’t found the price. Am I over thinking this? Are there actual concerns when selecting underlayment. about the heat build-up and does it need to breath or have moisture escape? Any help greatly appreciated.

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  1. STEPHEN SHEEHY | | #1

    I'd let the roofer tell you what is needed to keep the warranty valid. We used Titanium something or other under our standing seam roof in a cold climate.

  2. GBA Editor
    Martin Holladay | | #2

    If you are installing standing-seam metal roofing, you won't have any drying toward the exterior of your roof assembly. So the vapor permeance of your roofing underlayment is irrelevant. Choose something impermeable like Ice & Water Shield if you want, or something vapor-permeable like asphalt felt if you want -- it won't make any difference, because there is no outward drying.

    You haven't told us anything about the insulation materials or R-value of the insulation -- and those details are important. Nor have you told us much about your ventilation situation. If "all of the house drains into 4 valleys," it sounds as if your roof is a very poor candidate for a vented roof assembly, and you should be thinking about an unvented roof assembly.

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