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Synthetic underlayment for a metal roof

lightnb | Posted in General Questions on

I’m looking for some advice sorting through the various options on underlayment for metal roofing panels. Most articles are either “synthetic vs tar paper” or “why our product is the best” (biased).

Adding to the confusion are mixed information about what can and can’t be used with metal panels.

The two issues I keep finding are a) water proofing is more important on metal (water is more likely to get (blown?) in under the ribs of metal than with shingles) and b) metal gets hot and you need a heat resistant underlayment that won’t melt.

The climate zone is 4a (GA/TN area), and the roof is vented with continuous eve and ridge vent and insulation on the bottom chords of the trusses. We are thinking we will be using furring strips rather than screwing the metal directly to the roof. Decking is 5/8″ CDX.

All the roofing guys want to use a synthetic product. (It’s lighter and saves labor, which is good for them, but I want the job done right so when they are paid and gone and it’s my problem, there isn’t a problem….). They all say Grace Ice and Water Shied (stick on) is a waste of money, even on the edges because ice really isn’t an issue. Not sure about that. :/

Some say “roofs never had underlayment before [insert time period]” and underlayment doesn’t really do anything once the finished roof is on. Not sure about that either. :/

So if my goal is to get a quality underlayment that will last as long as the metal panels (which is think is 50 years), but not needlessly waste money, is there a specific product or product combination that is proven in this application?

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Replies

  1. user-7022518 | | #1

    I am a homeowner with a similar question and we are building in Virginia. I did some research online and liked the Boral Tile Seal product because it was designed to hold up in hot weather: https://www.boralroof.com/wp-content/uploads/boral-resource/Boral-TileSeal-HT-Brochure.pdf If it works in Florida.... If there is anyone who has comments about this product that would be helpful. Lisa

  2. Expert Member
    Akos | | #2

    Underlayment is important for a metal roof, you get night time condensation on the underside of the panels, you don't want that to get into your deck.

    Peel and stick is only needed for low slope or if your roof has a issues with ice dams, 4A doesn't sound like a climate for that.

    Overall, use whichever underlayment your roofers are comfortable with and make sure they put them horizontal and lap them properly (there is a tendency here to install them as vertical strips).

    If the roofing is furred, no problem to go with felt as there is no chance for the metal to stick to the tar and rip it. Felt is cheap enough that you go with two layers if you are worried about long term durability.

  3. Zdesign | | #3

    Rhino U20 underlayment works under metal and it's cost effective/ stronger than 30# felt. Just keep in mind under 4/12 pitch you are supposed to put 2 layers down. Are you doing a standing seam roof with concealed fasteners or corrugated with exposed fasteners? Neither style needs furring strips and can be installed directly on the underlayment. If you do go with Grace or similar Ice and Water which really isn't necessary, make sure it is a high temperature version.

  4. Jon R | | #4

    Fully adhered underlayment outperforms in terms of water leakage.

  5. lightnb | | #5

    The plan was to install corrugated with exposed fasteners. I think there was some concern about heat transfer and getting a better thermal gap with the strips so heat doesn't transfer to the attic as easily?

    Pitch is 4/12.

    "Underlayment is important for a metal roof, you get night time condensation on the underside of the panels, you don't want that to get into your deck."

    There is suggestion that synthetic underlayment is not completely waterproof, is that true?

    There will almost certainly be a few weeks of monsoon level rain between "felting" and the metal being installed. It's been very wet lately.

    1. Expert Member
      Malcolm Taylor | | #6

      "There is suggestion that synthetic underlayment is not completely waterproof, is that true?"

      No, and most underlayment are rated for prolonged exposure without the finished roofing - typically in the range of six months.

      Like Zdesign I'm not sure what strapping gets you. It does complicate flashing valleys, penetrations etc.

      1. RickReed | | #35

        Cross hatch, that is 1x4 minimum, first the vertical strap screwed to decking then horizontal strip atop at 16 to 24 inch centers provides venting beneath a very hot metal roof, and from the point of view of the framer and metal roof installer..provides a safe foothold on high pitched roofs ...think 8 to 14 in 12 slopes...
        All theoretical arguments must consider these practical concerns of mine and any builder.. I am to be the owner framer/builder.
        Can't find the thread but recall reading the advice that roof decking beneath metal roofing should be 5/8 "to 3/4 " minimum. It's been 30 years since I was a production framer, handling heavy 3/4 inch osb on even an 8/12 pitch was awkward, can't imagine anyone decking a 12/12 pitch with anything more than 1/2 inch thick decking . The 3/4 " thickness of strapping provides adequate backing.

    2. Expert Member
      Akos | | #7

      I've had a steeper roof with #15 felt exposed over winter with no leaks. Underlayment won't leak unless it gets torn from the wind. If you are in a windy area, a bunch of 1x2 to strap it down is good insurance.

  6. Peter L | | #8

    "...water is more likely to get (blown?) in under the ribs of metal than with shingles..."

    You got it turned around. Water is more likely to get under shingles than a metal roof with a high standing ribbed seam. There are hundreds of shingles on a typical roof and each one can easily lift and leak in high wind driven rain or snow/ice conditions. A metal roof only has seams every 18-24 inches which are either field seemed or snap-lock seam.

    Metal roofs are FAR MORE water-tight than a shingle roof. Not even close when it comes to rain resistance. Whichever roofer told you shingles are better is trying to sell you a shingle roof. Depending on climate, shingle roofs have to be redone every 10-15 years. Metal roofs can go for 50+ years.

  7. Peter L | | #9

    As far as underlayement goes. Yes, metal roofs require different membranes. Synthetics are the best as they can be vapor smart and prevent water from coming in but allow water to go out to dry the decking. Of course, the metal roof would have to be installed on purlins or furring strips. As once you screw a metal roof directly to the decking, it becomes vapor impermeable to the outside.

    SIGA breathable synthetic roofing underlayment - MAJCOAT is a great product
    -34 Perms
    -Tested to 2.3 inches of rain @ 45 MPH - Tested as Driving Rain Resistant by Berlin Technical University

    1. Jon R | | #10

      > once you screw a metal roof directly to the decking, it becomes vapor impermeable

      Except that it isn't air tight. Which varies, but the equivalent perms caused by this air leakage can be several times the wetting rate from the interior (ie, could have a big effect on reducing sheathing moisture).

      http://docserver.nrca.net/technical/6860.pdf
      https://web.ornl.gov/sci/buildings/conf-archive/2013%20B12%20papers/171-Pinon.pdf

      1. Expert Member
        Malcolm Taylor | | #11

        Jon,

        Isn't that research concerned with metal Q decking with a membrane roof over top? That's quite a different animal.

        1. Jon R | | #12

          Look not at the entire research but just the basic lab tests of the effective perms of lapped steel. Do you have any data showing that residential standing seam roofing is somehow different and therefore airtight? Or that the whole assembly perms is zero (which would require it to be airtight)?

  8. Expert Member
    Malcolm Taylor | | #13

    Jon,

    I take your point. I just don't think the factors in play with commercial Q decking apply to metal roofs. Commercial decking is really just metal sheathing. It isn't designed to be air or vap0ur tight.

    I have no data at all about how air-tight metal roofing is, but it is entirely impermeable. I also have no data that shows ventilation over sheathing and under metal roofing appreciably helps drying.

    I keep seeing projects with various combinations of battens -some diagonal, some with gaps, but I've never heard a th0ught-through cogent description of how they were supposed to work, or how they can be effectively integrated into the detailing of a metal roof. In the absence of that I'd suggest introducing a gap, or cavity may cause as many problems as it is supposed to solve.

  9. Jon R | | #14

    > I have no data at all about how air-tight metal roofing is, but it is entirely impermeable.

    OK, provide a link to data showing that a metal roof *assembly* is "entirely impermeable".

    1. Expert Member
      Malcolm Taylor | | #15

      I can't understand why it matters? What we need is some data that shows that a) lack of a gap is a problem, and b) a gap makes otherwise problematic assemblies safe.

      If both of those did turn out to be true, what would then be useful are some details showing how it could effectively be incorporated into a roof without compromising the primary goal of stopping bulk water intrusion.

      1. Chris Charron | | #16

        Malcolm,
        I'm just gonna stay away from "impermeable roof vs. unstoppable water."

        With regard to gap vs. no gap:
        From what I understand, for my application (Low angle SIP roof, standing seam metal) not having a gap can be problematic because condensation on the underside of the metal, falls onto the underlayment, combined with any vapor that escapes conditioned space (careful air sealing is necessary) has increased likelihood of causing problems with the OSB the SIPS are made of [TLDR: moisture buildup with sips is bad).

        Gaps allow enough air movement under the metal roof, but above the WRB/OSB surface.
        -more later, it's late.

        1. Expert Member
          Malcolm Taylor | | #18

          Chris,

          We are discussing a lot of different things as though they are the same. I'm not anti-venting. I'm suggesting the the solution has to be effective in achieving it's aims.

          The best location for venting a roof is under the sheathing. It is only when this isn't available as an option that venting above may need to be incorporated into the assembly - and if venting is provided below, there is no point in adding a gap above.

          Venting above the roof sheathing is not the same as providing a gap with no mechanism to move air and dissipate water vapour - which is what occurs when you simply add a mesh underlayment or battens, especially to a low-slope roof.

          Creating a path for moisture in the roof system to dry to the outside has very different needs than providing a drainage path of condensation that may occur on the underside of the roofing panels.

          In your situation I agree you definitely need to ventilate the roof. How have you detailed the gap so that it can dissipate moisture or shed bulk water? In other words, once you have created the gap, how do you see it functioning?

  10. lightnb | | #17

    If the furring strips get installed horizontally, won't they act as dams for any condensation trying to roll down the top of the underlayment?

    There's also two comments about strips, one says they aren't necessary and one said to use them... ?

    1. GBA Editor
      Brian Pontolilo | | #19

      Hi Lightnb.

      Where I've seen them used, the furring strips are installed diagonally, as shown in the photo below.

      1. Expert Member
        Malcolm Taylor | | #20

        Brian

        If like the OP's that roof is vented below and the battens are being added just because the roofing is metal, let's look with how the diagonal battens deal with the two reasons usually cited for including them.

        Condensation due to night sky radiance: Step back and ask what is condensing on the underside of the roofing? Moist air from the gap created by the battens. If the roofing panels were attached directly to an impermeable underlayment on sheathing, there would be no moist air to condense - any more than there would be m0ist air condensing below the underlayment. Roofing profiles that don't sit flush with the underlayment, like corrugated Galvalum, already have build-in channels, making the battens redundant.

        If enough moisture is condensing in that gap that the battens need to be placed diagonally, where does that moisture go? For many of the channels it goes to the gable ends - or do we also provide gaps so it runs through a maze of battens to the eaves? Once at the eaves, how do you integrate the drainage with your metal eaves trim so that water ends up in the gutters, not dripping down behind the fascia?

        With diagonal battens, what useful ventilation path is there, and how is it connected to the ridge and eaves?

        Drying to the exterior of the sheathing: We already have that with ventilation below the sheathing, and we know it is effective because that's what all other types of roofing rely on. Battens with a high perm underlayment would also add some drying into the cavity, but where is it going then? What are the mechanisms that we know work to vent a roof? A continuous cavity of sufficient depth that is vented at the ridge and eaves. The battens don't provide that. In fact we don't know what they do. As far as I can see they are included because they seem like a good idea.

        1. Expert Member
          Akos | | #21

          Malcom,

          I think it is better to think of the setup as a rainscreen. It is mostly there to help with drying not to deal with bulk water. As long as the air can move, it will do this even if the battens are in the way. Lot roofing panels do have enough grooves to allow for this and battens seem redundant.

          We know that rain screens work, so I don't see why top venting would not work just as well. It does seem unnecessary for a vented roof assembly.

          For details take a look at:

          https://www.atas.com/wp-content/uploads/2019/06/Dutch-Seam-All-ASV-Details.pdf

          1. Expert Member
            Malcolm Taylor | | #22

            Akos,

            In this thread and similar ones I've said about all I can on the subject (and probably more than most would like to bother reading). People are free to build their roof in any way they think will work best.

        2. RickReed | | #36

          3/4" channel extending from eave to ridge vent, warm air rises so that in climates such as central Indiana summers, hot metal roofing doesn't radiate directly to decking. Conditioned attic with 5 inches closed cell foam sprayed to underside of decking, how is extreme heat radiating from hot metal directly through decking to closed cell foam not counter productive?

  11. lightnb | | #23

    So going back to the original question. Assuming we have a vented roof with eave vents and a ridge vent on a simple gable, 5/8" decking installed, and the metal panels to be installed are corrugated panels ("Tuff-rib" is what the manufacture calls them), then it seems that the furring strips are not required and permeability through the underlaymnet is not important.

    Is that a correct conclusion? In that case, we would want to select an underlayment based on:
    1. Heat resistance to metal
    2. Adequate protection against any water that finds itself under the corrugations until it rolls down or evaporates.

    And meeting those requirements, our final selection could then be based on price, availability and longevity.

    1. Expert Member
      Akos | | #24

      Exactly. The ribs on those panels will provide plenty of airflow path to dry any condensation, no need for any furring strips.

    2. Expert Member
      Malcolm Taylor | | #25

      lightbn,

      After leading you through a merry chase to get the simple answer Akos properly provided, I will just add one thing: Because you have chosen an exposed fastener roof which is more vulnerable to leaks at the gasketted screws, as Jon said, a self-sealing membrane underlayment does add some security over time. I would price Ice & Water. If it isn't significantly more in material and labour it might be worth it - although again, there is nothing wrong with a good synthetic underlayment.

      1. lightnb | | #26

        Is that the stick-on membrane called "Grace Ice and Water Shield"? I don't recall exactly and I can't find the quote, but I think it was about $900 vs $260 for the #30 felt. Not including labor. None of the roofers I talked to used the stick-on product. Although some people (online) have said it's the best because it seals around the holes. Which begs the question, if felt or synthetic membrane does leak at the screw holes, and you riddle it with holes installing the roof panels, what's the point of having it?

        1. Expert Member
          Akos | | #27

          There two water flows the roof needs to deal with. There is the "river" of water on the top after rain, your roof panels need to channel this without leaking to the deck.

          Then there is the small amounts of surface drops from condensation and minor leaks. These don't typically flow or move (think of dew on your car's windshield), the underlayment is just there to keep it form getting into your deck. Even if a drop is formed around a screw, it is not enough to do any damage before it drys.

          Exposed fastener roofs are another challenge as if the fastener is not set right by the installer or wiggles loose from thermal expansion, the "river" of water will make it down to bellow the panels and cause issues. This is why it doesn't hurt to go with peel and stick, it buys you an extra layer of protection.

  12. lightnb | | #28

    The metal roofing supply place (that also does installs) said they use Titanium “value lock” (which I could not find) and sometimes felt buster, which looks like a GAF product.

    The GAF is $80 for 1000 sqft, so that would be $320. About the same as felt (about $50 more). But not peel and stick. Somewhere it's been "mentioned" that this isn't for metal roofs, but can't find the official doc on that. Official sheet mentions "under shingles", but doesn't say "don't use with metal" that I could find.

    I did find Titanium PSU-30, a Peel/Stick Synthetic Roofing Underlayment. Looks like $135.95/roll online, for 2 square. So with twenty square, It would cost $2,719. Looks like a decent product. If I can get it for $500 bucks, I'm sold. How do they stay in business being a three-thousand dollar solution to a three-hundred dollar problem? I know that the metal roofing panels and trim will cost about three grand. I'm having a hard time seeing spending the same amount again on the underlayment. Is it really worth that much more money? That's eighteen years of homeowner's insurance premiums. They cover roof leaks, right?

    So I feel like I'm mostly back where I started. No battens, still need to order some underlayment. Too many options. Obvious choices are grossly overpriced....

    Rhino U20 mentioned above says it's "designed specifically for use under asphalt shingle roofing".

    Looking at the TITANIUM non-stick products:

    TITANIUM UDL 50 says: 5 times lighter and 25 times stronger than #30 felt / Minimum Thickness: 30 mils
    TITANIUM UDL 30: 6 times lighter and 20x Stronger than #30 felt. / Minimum Thickness: 25 mils

    So I guess the 50 is just a bit thicker/tougher (and also heavier). Both say they are good for 180 days exposure, "as good as dry in". They both have a Useless Lifetime Warranty (they'll just send you a new roll and it doesn't cover labor!). Both say they are intended for metal and are rated up to 240 degrees Fahrenheit. Each roll covers 10 square so 4 rolls would be needed. That's $586.88 for the UDL30 and $709.16 for the UDL 50.

    Is TITANIUM a good product? Is it worth $300/$400 more than #30 felt? Is the UDL 50 worth it over the UDL30?

    1. Expert Member
      Malcolm Taylor | | #29

      Your quandary is in some ways related to the earlier discussion. Felt and synthetic underlayments weren't intended to be used as secondary drainage planes. They are primarily a defence against wind blown moisture, or superficial leaks, and minor condensation. The idea being they stop them from damaging the sheathing until the moisture dries.

      With either shingles or standing seam roofs there are a lot of fasteners penetrating the underlayment, but they also do so where the roofing is compressed against the sheathing, and are protected from leaking by the layers above. So the choice of underlayment doesn't matter as much. I generally use whatever my lumberyard is currently stocking.

      With an exposed fastener roofing that has most of the area not in contact with the sheathing below, there are increased demands on the underlayment. The gasketted fasteners are the most likely source of leaks, and corrugations are being asked to act as drainage channels. To my mind those are factors that may make a more expensive, or self-sealing underlayment worth thinking about.

    2. RickReed | | #37

      So..if there are no nail or screw penetrations to lower cost underlayment when attaching metal panels to horizontal 1x4 which is attached to vertical 1x4 attached through decking to rafter haven't we solved both venting and potential leakage problems with a solution? Not to mention a much safer installation, less chance of crippling or fatal fall from roof for installer?

      1. Expert Member
        Malcolm Taylor | | #38

        Rick,

        Again lets be clear - we are talking about two very different situations: Roofs that require venting above the sheathing, and those where venting is suggested simply because metal roofing is being used.

        - For the former, yes cross strapping can make sense, although you have to be careful. Most codes mandate a minimum cavity depth of 1", so 3/4" strapping won't fly.

        - There are also a few ways to do this. One is to eliminate the sheathing altogether and run house-wrap over the rafters before installing the strapping.

        - In either case the eaves need to detailed differently than they usually are, to integrate a drip-edge and air intake.

        - Any framers and roofers will be working with OSHA mandated fall protection harnesses, scaffolding or ladder jacks , and temporary strapping as footholds. My feeling is if they need a fully cross strapped roof to work on they should probably not be up there in the first place.

  13. lightnb | | #30

    If we ruled out the self-sealing due to astronomical cost, would there be a benefit to using the UDL 50 over #30felt or over another cheaper synthetic product if exposure length was not a consideration?

    Do people ever tape the seams on the non-stick underlayments, like the seams on house wrap? Is there any benefit to taping the laps with house-wrap tape? Do penetrations get flashed to the underlayment like the flashing around wall penetrations? ie. a quickflash gasket slipped under the house rap and sealed with flashing tape?

    1. Expert Member
      Malcolm Taylor | | #31

      Apart from observing how the underlayments hold up during installation of the roofing, I have no way of knowing whether one is better than the other. So far I've never had a problem with any of the roofs I've done, so I haven't had any occasion to open one up and see. Again though - that's all snap-lock panels, which don't rely on the underlayment as heavily.

      There isn't much point in taping the underlayment. It's d0ne on house wrap mainly to aid air-sealing. On roofs all the laps are oriented horizontally, so the risk of trapping water under the tape is greater than the chance of leaks at the laps.

      Yes we typically flash around penetrations with a peel & stick membrane lapped onto the underlayment.

  14. Expert Member
    Akos | | #32

    Two layers of underlayment are mostly for your piece of mind. Anything that makes it through one layer will most likely make it through both.

    If your roof is vented, taping anything is pointless. If it is unvented, taping the sheathing is a good way to seal it up. I've never found a reason to tape underlayment except to keep it form flapping in the wind.

    Flashing goes to the metal roof surface, never to the underlayment. The easiest are the boots with conformable metallic/rubber flange. Never hurts to tape the underlayment to the roof penetrations though.

  15. lightnb | | #33

    "Never hurts to tape the underlayment to the roof penetrations though."

    That's what I was asking about. Before the metal goes on, sealing the penetrations to the wrap. Do you use a flexible tape like Dupont Flex Wrap?

    On top, for the plumbing vent, I plan to use a rubber boot like the one in this video: https://www.youtube.com/watch?v=xpMCk_BwdNk

    I still haven't found good instructions on installing and flashing a vent for the bathroom fan. Most info is for shingle roofs, and any metal roof videos look like amateurs doing poor roof modifications. Do you know what type of hood/boot I need for solid 4" vent pipe?

    1. Expert Member
      Malcolm Taylor | | #34

      On metal roofs with an almost smooth profile you can use the roof terminations that have the flexible edge Akos described, that you seal with caulking and fasten with loads of gasketted screws. For corrugated you are better off to flash a curb as you do with chimneys and mount the termination on that.

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