Proper underlayment for unvented asphalt roof
I recently purchased a house in suburban NYC. The roof is currently two-layers asphalt shingles with the most recent layer about 27 years old. We have been told and seen evidence of ice dams from the prior winter (lots of snow in Jan 2014). The primary area of the ice damming was in a cricket connection of a covered deck to the house which we have been told by various roofers was improperly built. We do not have the option of correcting the flawed design. We will be putting a new roof on within the next couple of weeks and need to determine the best design. The roof will be unvented as we plan to insulate the two attic spaces with open-cell spray foam once the new roof is in place.
I have received multiple quotes for the new roof. And, every quote has a different underlayment recommendation. I’m finding, as well, the most of the local roofers are not very familiar with unvented roof science and proper unvented roof design.
We plan to do an asphalt shingle roof. 6ft of ice & water shield at eaves and valleys. Possible EPDM at cricket connection area for additional protection in the vulnerable area.
My questions are as follows:
1) What is the proper underlayment for an unvented roof in suburban NYC? I am confused about perm rating, breathability, etc. We have gotten recommendations of 15lb felt, 30lb felt, Deck Armor, Shingle-Mate, etc. Very confused.
2) Do we need some type of additional ventilation between the sheathing and the shingles for this climate?
3) Any concerns with EPDM for the cricket connection? Or should we stick with the regular shingles there?
4) Is there anything we have overlooked or should consider?
Thank you in advance for all help!
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Replies
K. Thomps,
Q. "What is the proper underlayment for an unvented roof in suburban NYC? I am confused about perm rating, breathability, etc. We have gotten recommendations of 15lb felt, 30lb felt, Deck Armor, Shingle-Mate, etc. Very confused."
A. Any of the listed products will work fine. The only limitation (when it comes to roofing underlayment) is that you can't use a synthetic roofing underlayment that isn't vapor-permeable on an unvented roof. (This restriction may not be logical, but it is a restriction imposed by underlayment manufacturers.) The two synthetic roofing underlayments on your list -- DeckArmor and Shingle-Mate -- are both vapor-permeable, and are therefore acceptable for your project, as is asphalt felt. For more information on this topic, see Synthetic Roofing Underlayments.
Q. "Do we need some type of additional ventilation between the sheathing and the shingles for this climate?"
A. If you add ventilation between the roof sheathing and the shingles (an approach that requires a second layer of roof sheathing), you end up with a vented roof assembly instead of an unvented roof assembly. Either a vented roof assembly or an unvented roof assembly can work, as long as everything is properly detailed. Vented roof assemblies are a little bit more robust when it comes to resisting ice dams than unvented roof assemblies. For more information on this topic, see How to Build an Insulated Cathedral Ceiling.
Q. "Any concerns with EPDM for the cricket connection? Or should we stick with the regular shingles there?"
A. This question is impossible to answer without a photo or a better description. If the cricket has been responsible for ice dams in the past, and you have been told that the cricket was "improperly built," those are red flags. In general, ice dams can't be corrected properly with a roofing solution; what is usually required is air sealing work and better insulation. For more information on this topic, see Prevent Ice Dams With Air Sealing and Insulation.
Thank you for your quick response.
1) "The two synthetic roofing underlayments on your list -- DeckArmor and Shingle-Mate -- are both vapor-permeable, and are therefore acceptable for your project, as is asphalt felt." Is there any additional benefit to a product like Deck Armor that has a perm rating of 16 vs a felt with 5 perm rating? (I tried to access the underlayment article but it was blocked for premium users only.)
2) Ideally, I would prefer to do just unvented to reduce cost and complication.
3) I will attempt photos tomorrow. I wish I had more knowledge and understanding about the design issue. Basically, the previous owners built a screened in deck off the back of the house. The ice dams have occurred where the deck roof meets the house roof. The deck roof is fully ventilated given that it is a screened in porch and the ceiling above the deck is just the roof sheathing. One roofer commented that there were a number of patch fixes on the roof in the questionable area (tar and such). Where the deck roof attaches to the house roof there is attic space with poor insulation. After we have the roof replaced, we are spraying foam in the attic. We are trying to determine if we should go with EPDM in that area or if we are ok with the regular shingles.
Thanks again for your time and help!
K. Thomps,
There is no way that your question is going to be answered over the Internet, without any of us conducting a site visit.
A skilled roofer who does a good job, and who guarantees the work, is the one you want to hire. An unskilled roofer who doesn't come back when you call about the problem is the roofer to avoid. If you can't tell which roofer is which, you'll have to ask for references and make some phone calls.
Q. "Is there any additional benefit to a product like Deck Armor that has a perm rating of 16 vs a felt with 5 perm rating?"
A. Deck Armor will last longer than asphalt felt if the roof has to be exposed to the weather without roofing for a month or more. If the roofers plan to cover the underlayment with roofing within a few days, either product will work.