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Non vented cathedral ceiling with standing metal seam roof underlayment ?

user-1112710372 | Posted in General Questions on

I am building a non vented cathedral ceiling based on guidance from GBA. 2×10 with cellulose,1/2 cdx taped, 1.5 “ EPS, 1/2” cdx, then x underlayment and standing metal seam roof. My design required I go non vented and I am in high fire zone. Installing foam now. Climate zone 3Z central ca coastal so 1.5” EPS is correct. I want to use Delta Trella as it’s vapor permeable and as an air gap. A friend and my contractor used it and liked it a lot. At least one of the major metal roof manufacturers said they wouldn’t warranty with it down. It’s getting close to rain season and i am wondering about just going with peel and stick? I am also curious if I can just switch to peel and stick to save money. I have read all the articles I can find and it’s not clear if this is a bad idea. Sounds like a maybe. Also as a second question how important is the air ceiling on the second top layer of plywood? Thank you Joel – Owner Builder NOTE – I just woke up to heavy drizzle slash lite rain it should end soon but everything outside is soaking wet. We just put the second layer of plywood on the EPS and first layer is 100% sealed so no where for water to go. Plywood boards are butted tight but water is getting in as there is a leak coming through into the house so significant water is likely in the cavity now. Forecast is for sun all week. Will my cavity dry or do I have to open it up??

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  1. mr_reference_Hugh | | #1

    Hi Joel, have you tried contacting Dorken to see if they know which metal manufacturers have accepted the Delta Trella? Maybe you get lucky and could access the materials from those roof manufacturers.
    Tech Support: +1 (888) 433-5824

    I know what feels like to see the rain (in our case snow) coming. If you like the Delta product it might be a quick call (hopefully not an bottomless rabbit hole).

    1. user-1112710372 | | #2

      Thanks. Good idea. I’m also curious if peel and stick is a good idea as it’s a lot cheaper and building right now is a daily exercise in excepting what you can’t control - like insane costs :(

      1. mr_reference_Hugh | | #3

        I know why you posted hear and it makes sense to ask for assistance. There are so many amazing people responding to questions that is blows my mind.

        It depends on what your concern is and why you want to use the Dorken procut. My understanding is that you want to vent the underside of the metal roof to avoid corrosion of the roof panels (based on the Dorken webpage for this product.)

        I would do the same as suggested in the previous post. I call the roofing manufacturer.

        In my project, I had no idea that I could post questions here. I was constantly calling the product manufacturers. I learned things that I did not know and helped me save time and money. I work in an environment where I am constantly answering questions from people who have reached the end of the road with their research.

        I am often told that I should ask others what they are doing. Of that works where there are many different options and the Q&A here is a testament to that. Just the same, in my job I ALWAYS answer my questions by going to the source. That is maybe why I just naturally started calling manufacturers for our latest project (of course consulting their online documents first when time permitted). Even in my responses on GBA, I am trying to reference GBA articles, technical documents, studies, etc.

  2. mr_reference_Hugh | | #4

    I should have mentioned that we have a metal roof directly over synthetic ice and water shield. I have no concerns myself about the longevity. In this case, I did not talk to the manufacturer about it, I just did it. Sorry for not mentioning this first.

  3. Expert Member


    You won't get any appreciable drying under the metal panels using a permeable membrane, and the panels already provide an adequate drainage plane for any small incidental amounts of water that might accumulate.

    Dimpled underlayments also make the attachment of any trim using gasketted fasteners difficult and more likely to leak. They and their mesh cousins were first developed for foundations and as rain-screen alternatives, and have since branched out looking for new markets as roofing underpayments, where I think they serve little purpose.

    You can use a peel & stick, but apart for the eaves and any valleys it isn't necessary. Any synthetic underlayment rated for meal roofing will work fine.

    1. user-1112710372 | | #7

      I can’t believe to but I woke up to heavy drizzle. Ground is soaking wet and lite rain falling. Should end in a few hours but everything outside is soaking wet. We just put the second layer of plywood onto of the EPS. The bottom layer of Plywood is totally sealed so no where for water to go. Forecast is sunny and dry for a week. Any idea what this amount of water will do in that cavity ?

      1. Expert Member
        MALCOLM TAYLOR | | #8

        user ...372,

        Unfortunately that's a real problem with these roof and floor assemblies which have to be insulated before the envelope is watertight. From your description of the roof assembly, you have no vapour-barrier below the first Layer of plywood, so it and the foam above should dry to the inside. I don't know how long that will take, especially at this time of year, but it's better than if they had no drying path at all.

  4. user-1112710372 | | #6

    Thank you very much for your response. Ill go with simple and cheaper on this one thing. That often isn’t the case

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