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Synthetic roofing underlayment

Mill_house | Posted in General Questions on

I’m in climate zone 7 (Canadian prairie).  

I just read an article on GBA about synthetic roofing underlayment.  Article stated synthetic underlayment shouldn’t be used on an unventilated roof.  I believe I have an unventilated roof – story and a half house with no eave extensions (roof ends at walls, no soffits), and duraflo weatherpro roof vents.  Is this the definition of unventilated? 

The roofing company I got a quote from says they want to install:
     -3 feet of ice and water shield protection at eave edges.  This is self adhesive membrane that shields against ice damming in the winter/spring months.  It also forms a seal around the nails that are installed in this area.  
     -Synthetic underlayment over the rest of the roof.  This is a new version of tar paper which acts as a 2nd layer of protection underneath the shingles.

Based on GBA article it sounds like I should skip the synthetic underlayment and have ice and water shield (or more specifically Delta-Foxx) installed on the entire roof, followed by shingles, correct?  

Also, do you recommend drip edge on the gable ends (sides) of a story and half house, or just on the front and back?

If I decide to have eave extensions added to the house (12 inches), I could proceed with the quote as outlined above – ice and water/synthetic underlayment.  If i go this route, the house will have soffits but what happens at the top/peak of the house?  Do I need a ridge vent installed?

Thanks a lot!
Jaiden

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Jaiden,
    The reason that most manufacturers of synthetic roofing underlayment don't allow their products to be installed on unvented roof assmeblies is because the type of synthetic roofing underlayment we are talking about is vapor-impermeable. Ice & Water Shield is also vapor-impermeable, so these products are (more or less) functionally equivalent. They are equally unsuited to use on an unvented roof assembly.

    Vented roof assemblies include ventilation channels in each rafter bay. These ventilation channels are located above the insulation layer, but underneath the roof sheathing. These ventilation channels extend from soffit vents at the base of the roof to a ridge vent at the top. For more information, see "How to Build an Insulated Cathedral Ceiling."

    You definitely need metal drip edge flashing at the rakes as well as the eaves.

    For more information on the choices you face, see this article: "Insulating a Cape Cod House."

  2. Mill_house | | #2

    Thanks for getting back to me.

    I'm new to all of this. I'm still not sure if i have a vented or unvented roof assembly. Based on what I said above, are you able to say whether i have a vented or unvented roof? Yes or no is all I need. If you need additional info, please let me know.

    FYI: Any roofing company I've talked to here says drip edge on the gable ends is not necessary. I find the difference in opinions fascinating. You'd think something as basic as drip edge on gable ends would have been standardized by now.

  3. Expert Member
    Malcolm Taylor | | #3

    Jaiden,
    A drip edge isn't necessary, but it is desirable. If you don't want to install one an good alternative is a soldier course of shingles installed backwards on the eaves. Leaving the butt-end of the shingles to catch the wind and get squashed by the guy who leans his ladder on them is the worst solution.

    1. Mill_house | | #4

      Thanks Malcolm. That makes sense. I'll have to sort this out because as it stands i've got one company doing the shingles and another one doing the aluminum install (fascia, drip edge, new gutters/downspouts). If I'm not mistaken, the roofing company said they'd do their thing and then the aluminum company would come in afterwards. I have a sneaking suspicion that GBA would say the drip edge should be installed prior to the ice and water shield going down. If so, then the aluminium company would be paying my roof two visits.

  4. GBA Editor
    Martin Holladay | | #5

    Jaiden,
    Q. "I'm still not sure if i have a vented or unvented roof assembly."

    A. Previously, your wrote that you have a "story and a half house with no eave extensions (roof ends at walls, no soffits), and duraflo weatherpro roof vents. Is this the definition of unventilated?" The answer to that question is "no." A story-and-a-half house can have an unvented roof assembly or a vented roof assembly. Duraflo Weatherpro is a brand name -- it appears that the company makes mushroom vents, and I'm guessing that's what you are talking about.

    Mushroom vents are a sign that someone is probably trying to vent an attic. If you have an attic, the next question is: Can you enter the attic? If you can enter the attic, you can look for the tops of ventilation baffles installed directly under the roof sheathing. If you don't know what to look for, read this article: "All About Attic Venting."

    If all this is new to you, be patient with my explanations. It's hard to teach you everything if you are unfamiliar with construction.

    Q. "Any roofing company I've talked to here says drip edge on the gable ends is not necessary."

    A. Reputable roofers install drip edge at the rake. (The rake is the edge of the roof facing a gable.) Here is a link to an article that describes the correct way to install drip edge (which is needed at the eaves and the rakes): "Roofing with Asphalt Shingles." It's not too surprising that roofing contractors are uneducated. I used to be a roofer, so I know.

  5. Mill_house | | #6

    You started as a roofer? I didn’t know that. You’ve really moved up the ladder! I’m sure you get that one all the time. :-) But seriously-very impressive.

    Here are the details of my house: Story and a half house with roof ending at wall. No soffits, eave extensions. The second floor is exactly like the attached diagram except that there are no eave extensions. I've demo'd one of the upstairs rooms and see a pathetic attempt at ventilation baffles (flimsy cardboard that appeared to be running from the top to the bottom of the sloped portion only. Photo attached. (I see it rotated incorrectly upon posting. It should be rotated clockwise.) I'll be removing the ventilation baffles. And yes the Duraflo vents are the mushroom type.

    Thank you.

  6. GBA Editor
    Martin Holladay | | #7

    Jaiden,
    If your rafter bays have cardboard ventilation baffles, then you have a vented roof assembly (or at least you have an assembly that tries to be vented).

    It's up to you how to proceed. If you want to stick with the vented approach, you can (as long as you can make sure there are vent openings at the base of your rafter bays, and a ridge vent at top). Vent openings don't have to be soffit vents -- there are circular vents that can be installed in holes created by large drill bits. These circular vents can be installed in either a wall or a soffit.

    It's hard to teach everything from scratch. If you are unsure how to proceed, you should probably hire a contractor.

    1. Mill_house | | #8

      If I remove the pathetic version of ventilation baffles that are sporadically there, and wind up with what would be a non vented roof, I think it would make sense to go with the Delta-Foxx underlayment.

  7. GBA Editor
    Martin Holladay | | #9

    Jaiden,
    Choosing a roofing underlayment is the least important of your decisions. If you want to create an unvented roof assembly, your biggest decision is the following: rigid foam above the roof sheathing, or closed-cell spray foam on the underside of the roof sheathing?

    1. Mill_house | | #10

      I’m confused. I read an article on gba that specifically said not to use synthetic underlayment on an unvented roof. I’m simply following up (with the source) on what I read as I’d like to have my roof done right. And now it seems like what was written is unnecessary. Very confusing.

  8. GBA Editor
    Martin Holladay | | #11

    Jaiden,
    This is, indeed, a confusing topic. If you want to follow manufacturer's instructions -- usually a good idea -- you should know that most manufacturers of synthetic roofing underlayment don't allow use of their product on unvented roofs. (There are exceptions.)

    Frankly, though, the need for vapor-permeable roofing underlayment only arises in two cases: (1) When the roofing is vapor-permeable (for example, cedar shingles or concrete tiles), or (2) When the builder installs a ventilation channel above the roof sheathing. Synthetic underlayment manufacturers don't understand the building science, however, so they often make a blanket ban on installations of their products on unvented roof assemblies.

    If you decide to install rigid foam above the roof sheathing, or closed-cell spray foam below the roof sheathing, the vapor permeance of the roof sheathing is irrelevant -- because there won't be any outward drying.

  9. Mill_house | | #12

    Ok.
    If I had eave extensions added to my house with soffits but no soffit vents, and I removed the ventilation baffles from the rafters, and removed the mushroom vents from the roof, could I have closed cell foam sprayed on the underside of the roof sheathing of the: top attic, sloped ceiling and kneewall attic? Would I need a ridge vent for any reason? I still have gable vents on the house similar to this picture.
    If this approach can be done, would I need to have insulation on the the back of the kneewall (the wall that separates the kneewall area from the bedroom)?

    p.s. I always thought soffits provided ventilation. But now I'm wondering if it's the soffit vents on the soffits that provide the ventilation.

  10. GBA Editor
    Martin Holladay | | #13

    Jaiden,
    Q. "If I had eave extensions added to my house with soffits but no soffit vents, and I removed the ventilation baffles from the rafters, and removed the mushroom vents from the roof, could I have closed cell foam sprayed on the underside of the roof sheathing of the: top attic, sloped ceiling and kneewall attic?"

    A. Yes, as long as the spray foam contractor has adequate access to all of those areas.

    Q. "Would I need a ridge vent for any reason?"

    A. No -- not if your roof assembly is unvented.

    Q. "I still have gable vents on the house."

    A. These gable vents would need to be removed and the openings would need to be made airtight. The gable walls would need to be insulated.

    Q. "Would I need to have insulation on the the back of the kneewall (the wall that separates the kneewall area from the bedroom)?"

    A. If the insulation follows the roof slope, all the way down to the soffit, then the kneewalls don't need to be insulated. For more information on this issue, see “Two Ways to Insulate Attic Kneewalls.”

    Q. "I always thought soffits provided ventilation. But now I'm wondering if it's the soffit vents on the soffits that provide the ventilation."

    A. Soffits are an architectural feature. A soffit can be vented or unvented. A soffit does not provide ventilation. The ventilation opening is called a "soffit vent," not a "soffit."

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