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Community and Q&A

Tile roof underlayment strategy?

RobInNorCal | Posted in General Questions on

Climate zone 3 (Northern California). Unvented roof in a high fire area; 2×6 parallel chord roof trusses with 10″ of open cell foam on the underside of the 3/4″ plywood deck. Boral standard weight tile, over Boral TileSeal HT peel-and-stick underlayment per mfr’s recommendation. I can’t find any permeance data for the Boral TileSeal HT, but considering that it’s a peel-and-stick modified asphalt 60 mil underlayment, guessing it’s pretty low. 

The plan, again per manufacturer recommendation, is to nail the (flat concrete) tiles directly to the deck through the ‘self-healing’ membrane.

Am I building a problem because the roof  sheathing can’t dry to either side, and if so, what can I do about it? I really don’t want to create fire chimneys on or near the roof deck – that’s how so many of the houses in the North Bay died in the fires of the last 2 years – so venting the roof is not an option.  I could put a layer of permeable underlayment down under the TileSeal HT, but not sure that really buys me anything… Thoughts? Am I worried unnecessarily?

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  1. Jon_R | | #1

    You have good vapor permeability to the interior. Having some to the exterior isn't required but would be beneficial, but so is "self healing" - and I don't think both are possible.

  2. RobInNorCal | | #2

    Thanks, Jon. I don't know as much about open cell foam as I should; I assumed that at this thickness it would become a significant vapor barrier. Sounds like I am worrying without reason, then; I will be using a second layer of web/slipsheet over the TileSeal, so hopefully 99.9% of the rain will never get to the plywood sheathing!

  3. user-6184358 | | #3

    Perhaps on the wildfire issue - it might be good to put a non combustible underlayment over the Tile Seal. Something like VersaShield is a fiberglass based underlayment - this would be encase embers get blown under the tiles.

  4. RobInNorCal | | #4

    I have firefoamed ALL of the openings around the roof, and we're packing the rake and eave tiles with mortar to make sure that there is no place for an ember (or gutter) fire to get to a combustible material. I have DensGlass fireboard under the metal roofs, my roofers dislike Versashield (which is required by Sonoma County under metal roofs if no fireboard is in place) because it's pretty slick to work on.

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