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Ice and Water Shield on the entire roof deck?

Stephen Eggerman | Posted in General Questions on

I see this happening in northern New Hampshire. Is it necessary with a vented roof? I imagine it would cause drying to the outside issues..

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  1. Riversong | | #1

    You're absolutely correct.

    This is a perfect example of a "new" miracle product or technique becoming so widely used without understanding the repercussions that we will keep the renovators and building forensic inspectors busy for a long time to come.

    While this practice is less problematic with a vented roof, which can dry into the vent cavity, it still suffocates the wooden roof deck and relies on the principle of the "perfect" seal, which works only as long as it remains perfect. If it fails for any reason, then it becomes a moisture trap.

    With an unvented roof, particularly one which cannot dry to the interior such as a spray-foamed cathedral ceiling, it becomes an insidious moisture trap that has a high probability of contributing to roof failure.

    Any structure made of wood is most durable when it can breathe in all directions. Wood can tolerate moisture as long as it can dry.

  2. Stephen Eggerman | | #2

    My roof will be a 10/12 pitch. The builder is planning on doing one row of ice and water.

    Is there any place this material is useful? First rows of roof? Flashing windows? Front of the house splash zone?

  3. Riversong | | #3

    Some codes require a membrane at the eaves extending 2' inside the exterior wall line as an ice dam precaution. But with a well-insulated and well-air-sealed attic with good soffit-to-ridge ventilation and no valleys or dormers to collect snow and block ventilation, ice dams should not be a problem.

    I've never used anything but #15 felt roofing underlayment. I've used self-adhering membrane as a valley flashing where shingles are woven and the flashing is not exposed, and have used it to fabricate window sill pans, but avoid it as much as possible.

    I've also had to cut off self-adhering membrane applied by others around window openings when installing exterior trim, because the overlapped or wrinkled flashing interfered with trim placement. Simple felt gaskets under window flanges or casing is a tried-and-true method of weather sealing, and felt as a wall WRB is much easier to integrate with window flashings than plastic housewraps that have to be slit and taped over windows.

    There are almost always other, more breatheable materials and techniques which work as well or better, many of which have worked for hundreds of years.

  4. Sean @ SLS | | #4

    Stephen, it fully depends on the codes in your area's. In Florida you must use either like that or at minimum apply it to all seams. In your area (as I recall per Best Practice / codes) it is a minimum of 2' past the interior wall or 2 rows (whichever is greater) - I could be wrong based on the pitch. I will have to disagree with Robert on his perceived issues, as you would have to be a complete moron to screw up that installation - especially when you consider that it seals around the nails, etc...

    That said the only real issue I have is later on trying to remove and install new shingles. Personally I like products like Titanium UDL which give you a great tear resistance, waterproof barrier, etc... For a little more on this

  5. Riversong | | #5

    I will have to disagree with Robert on his perceived issues, as you would have to be a complete moron to screw up that installation

    The issues I discuss are not "perceived". They are proven by laboratory and field studies and computer simulations, and they are also self-evident to those who understand hygro-thermal dynamics and envelope failure modes.

    It doesn't even require a " complete moron" to "screw up" - even run of the mill morons are likely to do so. In fact, a 3' strip of self-adhering membrane at the eaves can be potentially worse than full coverage, since a roofing leak above the eave membrane can saturate roof decking underneath the membrane as absorbed water is redistributed within the sheathing, where it may become trapped long enough, with sun beating on it, to initiate and nurture mold or decay organism growth. And, in a high humidity exterior environment, the damp air in soffits can raise the equilibrium moisture content of the roof sheathing above which, covered by impermeable membrane, remains damp enough to cause problems.

    And synthetic roofing underlayments, like Titanium UDL-30, are typically vapor barriers (0.0.6 perms for Titanium) and prevent drying to the exterior.

    Codes have finally moved away from wall vapor barriers because they are as likely to trap moisture as to keep it out of the structure, but now builders are moving toward self-adhering membranes and vapor barrier underlayments for roofs (and even walls) which will create another set of moisture problems.

    Anything made of wood has to be able to breathe. Anything that inhibits or prevents the breathing of wood materials and structures increases the probability of moisture problems and eventual failure.

    Water is the most powerful and mysterious substance on earth. Nothing can stop it and any attempt to do so (such as damming up major rivers or sealing up building assemblies) inevitably leads to unintended consequences and increases the probability of disaster.

  6. Stephen Eggerman | | #6

    Thanks for the input all.

  7. Sean @ SLS | | #7

    Sorry Robert, but I am calling Bull

    First, there is nothing mysterious about water or the consequences of not respecting it & remembering physics.
    Second, wood does not need to breathe & it can actually stay underwater for years with no ill effects
    Third, lets get serious about roofs drying to the exterior - that is not the way it works, never has and never will as the shingles & other coverings will prevent it & the most important layer is the "felt" layer from water sneaking in from above (ok I sortof take that back, but we are not dealing with cedar shakes)
    Fourth - why the he11 are you hiring morons to even work on your house? That said, you don't have to worry about them installing I&W because they will probably cheap out & use felt anyways
    Fifth, based on his area it is needed at the eaves for any possible issues with Ice Dams & don't bother saying that is easily prevented because that is harder to do right, than installing I&W
    Sixth - yes they are perceived as apparently all you see is failures everywhere you look instead of what happens when people actually do the work right & trying to scare others from moving forward simply leads to the status quo of bad information. As one of my favorite computer sayings goes - garbage in = garbage out

  8. Riversong | | #8

    Sorry, Sean, but you clearly don't have a clue about moisture dynamics. And getting all bent out of shape over it doesn't add any credibility to your position.

    The reason that some form of shingled roofing has been traditional for thousands or years is that it allows gravity drainage and drying through the laps. The underlayment is not the most important layer but only a secondary weather barrier, just like the WRB on the sidewalls. Some roofers don't even believe it's necessary to use underlayment at all (yes, they are the morons).

    And, yes, you're right about one thing: wood can survive very well underwater, since that's the only environment in which it's not exposed to oxygen. But, when it is exposed to oxygen (which is then available to aerobic mold and decay organisms), it must breathe.

  9. Sean @ SLS | | #9

    Now thats funny - I don't have a clue, nice try

    I will have to agree with you on the moron part for not using underlayment, but we will have to disagree on it not being the most important part (for most systems). The outer layer is just like the siding, it is mostly decorative and will help shed most water. If any does get through (wind, morons, ice, design, etc...) it is to help shed it off the structure instead of allowing it to soak in and causing problems.

    Oh & not to be to nit picking, but water does contain oxygen

  10. Riversong | | #10


    You're not being nit-picking, you're being a nitwit. Water is a compound of hydrogen and oxygen (not free oxygen) and contains various amounts of dissolved oxygen, but the aqueous environment is an anaerobic environment and does not support the growth of aerobic organisms. That's why pilings don't rot underwater but rot at the water surface where they are both wet and exposed to oxygen.

    Nowhere in the building industry, in building science or in building codes is a sidewall WRB or a roofing underlayment considered a primary weather barrier. They are secondary weather barriers.

    I repeat: you don't have a clue.

    The simple truth is that many of the "high-tech" underlayments are for the benefit of the builder or roofing contractor, not the home owner. They are used primarily because they are more tear resistant and can be exposed to UV for much longer times. This allows a significant time delay between underlayment installation and roofing installation, which makes scheduling easier. But it costs the customer more money without adding any real value, and has the potential to create long-term durability problems.

    A good roofer installs felt underlayment and roofing at the same time so there is no issue of long-term exposure.

  11. Sean @ SLS | | #11

    I think you need to take a class on reading comprehension and check out this book -

    With that said, Good day - I am done even trying to reason or even talk with you, because I have a sneaky feeling it doesn't do any good & hopefully Stephen finds a great roofing contractor that not only knows the codes, but knows what does & doesn't work

  12. Riversong | | #12


    I'm not here to make friends but to share reliable building science and to refute all the nonsense that building contractors like you confuse people with.

    I teach building science, engineering, design and sustainable construction, as well as Hygro-Thermal Engineering.

    I strongly suggest that you consider taking a few reputable classes in these topics, as your ignorance is dangerous.

  13. Sean @ SLS | | #13

    Generally I try to keep these away from the personal, but you seem to like taking it that way.

    For the record, this knuckle dragging contractor - has over 20 years in the field mostly doing remodeling work, is a RESNET Rater, BPI certified, Infrared Thermographer, and a whole host of other items - my knowledge pretty well exceeds yours for what really works and what doesn't. Now seeing you probably still can't see past the "construction" part lets see what other building science "experts" have to say

    Back to your simulation that you like to brag about "A 10-year computer simulation study, using hourly weather data for a hot climate (Miami) and a cold climate (Boston) demonstrated that a roof deck with vapor impermeable layers on both sides was highly vulnerable to moisture damage in either climate in the event of a roof leak, which has to be assumed as a high probability during the life of a structure."

    The first issue with this, is a roof leak should not have a high probability of failure if it was done right & using I&W will virtually eliminate that chance. Next, this simulation is basically mute as the original question involved a "vented" roof.

    Next, lets also remember something else you wrote in that same post "You have it backwards. A breathable underlayment is necessary if a vapor impermeable layer (spray foam) is under the sheathing. All wood products need to be able to dry in at least one direction and preferably two. A breathable underlayment is preferable in all circumstances."

    Needless to say, I still disagree with the breathable part in two directions, as roofs don't dry to the outside, as Martin Holliday states "2. Regardless of your roofing underlayment choice, your insulated roof assembly will not dry to the exterior unless the roofing is vapor permeable. Many types of roofing, including EPDM and standing-seam steel roofing, are virtually impermeable to water vapor. Other types of roofing, including cedar shingles and concrete tile roofing, are fairly permeable. Asphalt shingles are a little bit permeable in windy conditions -- but not very.

    3. If you are set on using a permeable roofing underlayment, the obvious choice is asphalt felt, which has been used successfully for that purpose for decades."

    As for #3 - Martin is both right & wrong as only #15 qualifies while #30 doesn't for the perm ratings. Also, depending on the slope, the #15 won't qualify when it is doubled up as required by not only codes, but the manufacturers directions. Would you care to reread what I wrote above & try again on who is being ignorant?

  14. ROY HARMON | | #14

    Sean, What would be your recomendation for the installation of cedar shingles?

  15. Sean @ SLS | | #15

    Climate, pitch, vented or hot roof, size of product, skip sheathed, or regular? Mind you it has been around 20 years since I last installed any cedar shingles or shakes and about 15 for concrete roof tiles - now it's all shingles or subbing the metal out.

    Personally, I do like the new cedar rebreather products that have come out which allows a shingle to mostly dry out on all sides evenly. As an FYI, cedar shingles & shakes are one of the few products I definitely recommend gets hand nailed - you might want to check out page 15, figure 23c, and page 7 for different ways to attack the issue. If this were my own house - 5/8 ply, I&W or similar product that can handle nails, etc..., 2" or thicker foam (taped seams), see page 7 for firring detail & felt detail, stainless screen for bugs, triple starter, shingles would then be hand nailed with appropriate sized Stainless Steel nails and Copper W flashing used valleys & possibly the ridge also. Yeah it costs a little more, but I would expect that roof to easily outlast me without allowing any water into the house

  16. anonymous | | #16

    Roy, if you can afford a roof that costs a $1000-2000 a square, then the last place to go is a chat forum. You find out who does it well where you live and leave the details to them.


  17. GBA Editor
    Martin Holladay | | #17

    The published permeance values for #30 asphalt felt range from 0.5 perm to 3.0 perms. However, these are the permeance values when the felt is dry.

    Like #15 felt, #30 felt has variable permeance depending on its moisture content. If the felt ever gets wet, its permeance rises, so in essence it is a smart vapor retarder.

  18. Sean @ SLS | | #18

    Thanks Martin, I stand corrected as I swore it did not hit the .5 mark & 15 just did (or is that just the original Tar Paper?)

    Anonymous - actually one reason people use these forums is to research first, and find out who does what, why, and is there a better method, before searching for and hopefully finding a reputable company that does things right. The great thing is, there are plenty of people willing to share that information and help others out - on the flip side, there is plenty of bad information out there also.

  19. Riversong | | #19


    Regardless of your tenure in the remodeling business, I've never heard a more moronic statement than "the most important layer is the "felt" layer...The outer layer is just like the siding, it is mostly decorative."

    If you really believe that the roofing (or siding) is primarily architectural embellishment and the underlayment is the primary weather barrier, then you shouldn't be in the construction business.

    And if you truly believe that "about roofs drying to the exterior - that is not the way it works, never has and never will as the shingles & other coverings will prevent it", and you ever put roofing on a closed-cell spray foam insulated roof deck, then you'd better provide some other method of drying because all roofs eventually leak.

    And Martin is only half right about shingles when he says "Regardless of your roofing underlayment choice, your insulated roof assembly will not dry to the exterior unless the roofing is vapor permeable." A roofing material can be vapor permeable or the roofing assembly can be vapor open, such as any type of shingled roofing which leaves gaps between layers.

    From HUD PATH Best Practices Guide:

    "Underlayment – Typical 15# tarred felt underlayment provides back-up protection against water intrusion only as long as the primary roofing material remains intact. It is not intended for direct exposure in the event of loss of the primary roofing system in a severe wind-driven rain event."

    And neither is any other form of roofing underlayment.

    "If water leaks behind the secondary drainage plane, it may cause more damage than if no drainage plain were present due to slower drying."

    Which is exactly what I had said about an eaves membrane or a whole roof membrane.

    What the Florida Solar Energy Center study was premised on, and what every responsible building professional should understand, is that all things fail at some point or at least have a significant probability of failure, and hence all water resistive strategies must include mechanisms for drying.

    Or, as Dr. Joe of Building Science Corp. often says: "The laws of physics must be followed. Things get wet - let them dry. Things get wet from the inside, the outside and they start out wet. When the rate of wetting exceeds the rate of drying accumulation occurs. When the quantity of accumulated moisture exceeds the storage capacity of the material, problems occur."

  20. Sean @ SLS | | #20

    Robert, seeing there is no point arguing with stupid or people that twist words around to some warped a55 version of what was stated - have a good evening

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