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Community and Q&A

“Water and ice” type membrane on roof?

peaceonearth | Posted in General Questions on

I will be putting a metal roof on OSB, unvented (spray foam and fluffy, unless I change my mind) in zone 6.  I’m unsure what is needed below the metal, if anything. I’ve read on GBA that an impermeable membrane (ie water and ice) is meaningless on an already impermeable arrangement. Is the membrane needed for other reasons? And since this is such expensive stuff (water and ice, anyway) is there an alternative?

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  1. GBA Editor
    Martin Holladay | | #1

    Q. "I’m unsure what is needed below the metal, if anything."

    A. Building codes require roofing underlayment (e.g., asphalt felt) between the roof sheathing and the roofing.

    Aiming for airtightness at the sheathing level is a good idea. Well-installed closed-cell spray foam is an air barrier, but if you have any doubts about the integrity of the air barrier provided by the spray foam, you might want to tape the sheathing seams or install a layer of synthetic roofing underlayment (installed in an airtight manner, with tape).

    Before choosing synthetic roofing underlayment for this application, make sure that the underlayment manufacturer allows the use of the product on an unvented roof assembly.

    1. peaceonearth | | #2

      Thanks Martin. I think I will tape OSB seams as this sounds like prudent advice. If synthetic underlayment products do not want their use on unvented roofs, is there anything that felt paper does for me in that location?

  2. GBA Editor
    Martin Holladay | | #3

    Q. "Is there anything that felt paper does for me in that location?"

    A. I performs several functions:

    1. It satisfies the building inspector, since it is required by code.

    2. It protects the roof sheathing for the days between the installation of the asphalt felt and the installation of the roofing.

    3. It provides a secondary layer of protection for the sheathing in the case of a minor roof leak.

  3. harrison55 | | #4

    This is not a reply, it is a follow-on question: I plan to roof on an unvented cathedral ceiling, similar to the one described above, and flassh the sheathing with closed-cell foam. The roofing will be composition shingles. Is it wise to apply ice-and-water shield below the shingles?

  4. Expert Member

    I&WS offers additional protection under composite shingles that it doesn't under metal roofing. Shingles are subject to mechanical damage, nail-pops and deterioration over time that all can lead to leaks.
    I&WS isn't typically specified under shingle roofs except at the eaves and valleys. It might be fair to day it really isn't necessary as an underlayment for the whole roof, but adding it does make for a more resilient installation.

  5. Debra_Ann | | #6

    Mark, some folks state that it is not good for the sheathing to have an impermeable layer above it. (It's definitely a bad idea to have impermeable layers on both sides of the sheathing.)

    However, research has shown that asphalt shingles are already an impermeable layer, and using a permeable underlayment under them will not increase the ability of the sheathing to dry to the outside. If using asphalt shingles or other impermeable layers above the roof, it's important to provide proper venting in the attic under the sheathing.

    I plan to apply I&WS over my entire roof deck, as I believe it will offer excellent additional protection under my asphalt shingle roof (for reasons that Malcolm listed).

    1. T_Barker | | #12

      I&WS is required by code in all areas around me, 3' up from all eaves and each side of all valleys. On a lot of roofs, this doesn't leave a whole lot of other roof area so you may as well just do 100% I&WS. Adds a little installation cost as well as material, but not a bad decision if you want to spend the extra money.

      However, as far as what the roofers will do 20 years from now when you need to do the roof again (if you care), that can present a problem. I talked to several different roofers this summer and they said the Grace I&WS will NEVER come off without replacing the sheathing. So if you do 100% I&WS your next roof replacement will be expensive.

      They also said most other brands of I&WS are not nearly as big a problem for stripping. So as long as it's not Grace I&WS you might not want to do 100% I&WS since they can get the 6' stripped without too much hassle.

  6. GBA Editor
    Martin Holladay | | #7

    Peel-and-stick membranes like Grace Ice & Water Shield complicate future sheathing repairs. Unlike conventional asphalt felt, Ice & Water Shield is hard to remove. Roofers who have to make repairs in 20 years aren't going to like it.

    I'm not in favor of covering the entire roof with Ice & Water Shield unless there is a good reason to do so -- some unusual problem that you are trying to address. Certainly, most roofs don't need it.

  7. onslow | | #8

    First and foremost, if you do use the Ice & water shield under your metal roof, be sure to specify and verify that it is High Temperature Ice & Water shield. Full ice and water shield from eave to ridge is a locally required code for metal roofs where I live. The environment is high altitude, CZ6-7+ depending on location in the county. My personal location is considered CZ6. Historically, seasonal temperature ranges from -15F to 90F with 40 degree shifts each day not uncommon.

    The code requirement may be a response to the many homes that end up with massive ice dam damage. Insistence on bad design choices and very poor insulation techniques result in many homes having monumental ice dams in the higher country. Lots of stories there. As a practical matter, I would do the full ice shield regardless of code.

    Metal roofs during spring and fall can produce condensation on the underside which I suspect has a hard time drying, so having the assurance of the water shield seems useful. If a winter ice dam forms, many metal roof styles will not prevent the pooled water on the up-slope side from rising high enough to over flow the seam details. A butyl sealed rolled seam roof might prevent pooled water intrusion, but more common flat screwed and snap-lock styles of metal roof are meant to shed rain and snow melt, not pooled water. Cap and flashing details are two other places where pooled water is not really anticipated. The perfect storm location for leaks caused by ice damming is next to one of the many pointless gable windows seen on mega houses.

    I also appreciate water shield's gummy layer as it provides a sealant around all the screws used to hold down the roof or roof clips. The thermal cycling of metal roofing is pretty aggressive and the screws are rocked back and forth by this action. Face screwing is subject to the screws working loose to the point where many recommend going up and tightening the screws every five years (or never like most people) Woodworkers account for this type of expansion movement in table tops with a host of techniques.

    I prefer the snap lock type of standing seam roofing over face screwed panels for the simple reason that the expansion is (hopefully) accommodated by the clips which are screwed to the roof deck. It might appear that the leverage would be multiplied by the clip, but I take the popping and creaking noises I hear each morning sunrise to be the metal sliding on the clips, much like one hears hot water pipes creak and click as they slide on hangers.

    As to future repairs, Martin is quite right to note the PITA you are leaving for someone to deal with. My only comment would be, that potentially you will take much longer to get to repair day with a ice and water shield. Just so long as you detail the flashing at intersects of roof and walls with a mind toward even temporary ice damming.

    Metal roofs are funny in that they will shed snow a bit differently than asphalt roofing. Both heat up in the sun, but the metal conducts more efficiently and ice sheeting under the snow layer can create amazing sliding snow glaciers that roll off the roof edge like a surfer's best curl. I will try to find a picture I have later. Which does get to one point that is made clear locally. Gutters will generally get ripped off a metal roof, sooner or later. Snow slide stops help, but I think they are just seed points for major ice dams to get a grip.

  8. StephenW81 | | #9

    Has anybody had any experience with VaproShield SlopeShield? In *theory* it looks like an interesting underlayment solution for unvented roof assemblies where roof sheathing is encapsulated between foam and roofing and where Class A fire rating is desired. They claim that it can be installed on wet sheathing, and although that seems like a thoroughly bad idea, it suggests that they believe that the roof sheathing will subsequently dry out with SlopeShield. Ironically, the SlopeShield itself has to be immediately protected from weather after it is installed.

  9. Expert Member
    MALCOLM TAYLOR | | #10


    It's a high-perm underlayment, meaning that the roof sheathing can dry through it, but only until the meal roofing is installed. At that point the permeability makes no difference.

    If, as you say, the underlayment itself needs protection form inclement weather, I'm left struggling to think of any situation where its high-permeability is helpful.

  10. StephenW81 | | #11

    Malcolm, it isn't very clear from the marketing material, but it appears that SlopeShield has a thickness to it (or 3D design); the same general idea as wall rainscreens like Benjamin Obdyke Slicker. If that is the case, that would theoretically allow vapor movement even after the metal roof is applied.

    This is what the installation marketing materials says about protection from weather: "Cover membrane as soon as practical. TARPING is required during construction phase. Protect completed installations of SlopeShield SA Self-Adhered membrane from damage due to extreme weather conditions, physical abuse and other sub-trades. During SlopeShield SA Self-Adhered membrane installation, prevent rainwater runoff draining from upper roofs to lower roofs, such as overhangs/eaves/valleys, onto SlopeShield SA Self-Adhered membrane."

    It does suggest that this product may be "risky". Therefore, I would be interested if anyone has first hand experience.

  11. Expert Member
    MALCOLM TAYLOR | | #13

    The subject of very small vent-spaces above the sheathing, or using proprietary ventilated underlayments, has come up here on GBA a couple of times in the past few months. My own feeling is that since there is no mechanism to move either air or moisture they aren't effective.

    On a wall they provide a capillary break, allow gravity to drain bulk water intrusions, and depending on what siding is used, allow drying directly to the exterior. On a roof none of these are really at play.

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