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Sheathing under metal roof

P_Taylor55 | Posted in Building Code Questions on

I Goggled this question, what lead me to this website. I read about this same topic here, I believe it was from 2012, however, the same question but for a house not a shed.
My main concern was sheathing thickness, but will take in all anyone can offer. I am a home owner operating as GC, while using Contractors to build a two-story 30×30 addition over a poured concrete basement. In matching the materials used on the existing structure and area, I am using a Metal Roof and for my upcoming meeting with the County, I want to have my “Typical Wall Section” sheet filled out as accurately as possible…in includes “Sheathing in Inches”. I’m using 2×6 (exterior)wall studs, 16″oc, and was planning on using the same (2×6) for Rafters; although I saw 2×12 suggested in the 2012 thread (???). The house, in N. VA is about 1600ft above sea level, windy and very cold, therefore, thicker walls, more insulation…etc. We can get as much as 25-30 inches of snow up here. All that said, Thickness of Sheathing (??) and as I said, any/all information is a huge bonus. Many Thanks, pt

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  1. GBA Editor
    Martin Holladay | | #1

    It sounds, frankly, like you are in over your head.

    Deciding whether to specify 2x6 or 2x12 rafters can't be done casually, and it certainly can't be done from the limited information you have presented here. At a minimum, we would need to know the rafter span, the roof slope, and the size of the overhangs.

    Similarly, you can't specify the roof sheathing thickness by guessing. If you don't know the answers to these questions, it's time to hire an engineer, an architect, or both.

  2. P_Taylor55 | | #2

    I understand what you're saying...those issues (over my head) are a true concern and I may need to do so...get to an engineer/architect, but you've answered the question, giving me more information that I need, so many thanks, 6 in 12 slope, 6" overhang, 16"OC. Now Rafter Span is a little more complicated. Once again, thank you for the answer. pt

  3. fitchplate | | #3

    Lets pretend the roof design details are conventional and so the roof deck sheathing will be 5/8" to 3/4". Many metal roofing dealers specify minimum of 7/16" sheathing.

    Design depends on type of metal roof: exposed or hidden fasteners; board and batten (pan and ridge) panels; shake and shingle panels; panel or tile system; standing seam; etc. The system will be a consideration for dead load calculations although not much is heavier than asphalt, and that is the baseline for engineering calculations when using rafters, structural panels or wood-I's .

    Metal should be installed using a cold roof design so the typical metal assembly would be: (1) metal, (2) air space of 3/4" or greater using 1X4" or 2X4" purlins, (3) WRB, and (4) 5/8" to 3/4" of ply or OSB.

    Cross purlin designs with 16 OC are sometimes called for; using nominal lumber 1X horizontal underneath and 2X vertical. All decent assemblies are screwed, not nailed.

  4. P_Taylor55 | | #4

    Many thanks Flinch Plate! Yes...I realized now how much I left out to expect anyone to answer. Been doing a lot of reading on Dead, Live and Snow Loads, Span Tables, etc. What you're saying supports most of what I've come up with and leaning towards 5/8 sheathing, but I need to check with the county, whereas all the snow load maps I saw don't list my area. 3/4 is definitely in the picture. For now, their spec sheet doesn't care about Purlins, but that time will come at install. (Spec Sheets for permits are different for home owners) And once again, local contractors are doing this...this data is only for my permits, but I know most of it the contractors will have to deal with. (I'm sure that will spark for feedback.)
    For roofing, I Prefer Standing Seam over Preformed Metal to match the area and existing structure. Dead weight seems very similar with both as well, although I realize their installation varies. One concern is Condensation ( I plan a 1" air space) and I've been reading about underlayment to catch dripping, air barrier vs 30lb Felt and so more homework...before I complete the spec sheet. Once again, thank you for the detailed response. pt

  5. fitchplate | | #5

    Metal roof condensation occurs on the back side of the metal panel or metal shingle. It is surprising the quantity of condensation water running off a metal roof when conditions are right (or wrong :-). A WRB of felt will protect the deck as the water runs down the underside of the steel and out, into your gutters, or drips onto the felt and runs out that way. This says something about the features in the metal end sealing and drip edge detailing. A WRB of non-permeable, peel and stick is a bad idea. Caution calls for a semi-permeable WRB on the roof deck; which felt is, and ice and water shield is not.

  6. jackofalltrades777 | | #6


    I am going to have to call you out on this because you keep repeating that statement without backing it up with science.

    Did you run psychrometrics software to get your analysis?
    Where did you retain the data needed to get analysis?

    What is the dew point, dry bulb, wet bulb, humidity, ambient temperatures, climate zone, etc? You don't know these vital factors but are throwing around blanketed statements of "peel & stick bad" "metal roofs always condense" and "tar paper good" but you don't have any of the variables you require to make a scientific assessment.

    That is the danger of these forums is that people are giving building "advice" and sometimes they are not qualified to do so due to biases and junk science is then taken as fact.

    Please stop with your bad advice because there is absolutely no way you can make blanketed statements like you are making without knowing and calculating all the variables.

    Did you run psychrometrics software to get your analysis?
    Where did you retain the data needed to get analysis?

    You are claiming that the air will always contain water vapor at a high enough relative humidity and the metal roof will always be at a temp equal or lower than the corresponding dew point for every climate, every day, every scenario. Unless you get air having enough water vapor under the metal roof and the roof is also at or below the dew temp, there will not be any condensation.

    Peel & stick membranes have been used without any issues with metal roofs and these roofs are standing strong.

    Throwing out blanketed statements that peel & stick membranes are bad and metal roofs always condense underneath without knowing all the particulars is junk science.

  7. Expert Member

    Some of my posts don't appear. Just testing.

  8. GBA Editor
    Martin Holladay | | #8

    You may have misconstrued "always." I'll express a similar thought in different words: in most North American climates, condensation will sometimes occur on the underside of metal roofing. Where I live (in Vermont), this often happens in the spring -- for example, when the roof is snow-covered, and the outdoor temperature rises.

    Builders who fail to prepare for this type of condensation may end up with dripping ceilings. In most cases, standard roofing underlayment is adequate to address the problem.

    I'm not a fan of the use of peel-and-stick membrane to cover 100% of a roof. I prefer to see the use of asphalt felt underlayment, because asphalt felt allows OSB or plywood roof sheathing to dry to the exterior when necessary.

  9. fitchplate | | #9


    You are invited to my property to observe the 6 metal roofed buildings of various assemblies: cold roof and flush mounted tops; insulated and heated assemblies; uninsulated and unheated assemblies. You can bring an empty jug, I will disconnect the down spouts and you can put your jug up to the drop outlet to collect whatever runs out. I judge this clear wet stuff to be water. And if you look closely (I will lend you a ladder); you will find that the condensation is not on the top of the roof, it is running down the underside. Where there are horizontal purlins, you will find bulk water pooling as the condensation drips onto the deck. Where there is a flush connection between deck and steel, there is capillary transport and slower evaporation. This issues cycle with temperatures and seasons, obviously.

    And for SIPs, few builders have the patience/inclination and few owners have the time and cash to make sure they are sealed right, so flush mounted roofing of any impermeable material is a bad idea (plastic or steel). In fact, you don’t need my junk science to tell you this. It’s a fundamental consideration to every aspect of envelope construction. I am not in favor of preventing vapor from leaving, no matter what season. You are free to have a different opinion or prove me wrong with your own science but you are not free to insult me.

    Maybe there is no dew in N. VA at 1600 ft elevation?

  10. Expert Member
    MALCOLM TAYLOR | | #10

    One caveat about using tar paper as an underlayment on metal roofs directly fastened to sheathing is that the heat may melt the emulsion and allow it to stick to the underside causing it to deteriorate. That is why some manufacturers will not warranty their product unless it is used in conjunction with a synthetic underlayment. Then again their concern isn't how the whole building assembly will dry.

  11. jackofalltrades777 | | #11

    Malcolm - Excellent point. In higher elevations and the desert SW, they will NOT install building paper underneath a metal roof because the paper will release the oils and basically deteriorate very quickly due to the higher temps the roof surface will reach. Building paper on a roof in the desert SW or high elevation climate is a quick way to destroy your roof sheathing and you will be tearing up that roof in a couple years.

    That is why there is no "general rule" or "one size fits all" type of layup for metal roof underlayment. The exact material is dependent upon numerous variables and must be custom chosen for the appropriate climate. I recommend this study which shows that some underlayments work in a certain area but will not work in other areas.,d.aWw&cad=rja

    Building paper will fail and prove disastrous in high elevation/desert climates with a metal roof. Building paper will fail miserably where ice dams and low slope roofs are present as you will develop interior roof leaks. Not to mention when the screws or nails are driven through the building paper those hundreds of penetrations turn into hundreds of water leaks because building paper doesn't seal properly around screw and nail penetrations on roof applications.

    It's never a general rule of thumb when it comes to product choice for buildings in different climates.

  12. fitchplate | | #12

    You are confusing your building materials and you are trying to make the impossible case for flush mounting the steel on the WRB covered deck. You are now making your own case against flush mounting in hot dry climates; ie, there is the melt and stick problem.

    Actually ‘building paper” is for walls; roofing felt is for roofs. Although I see the terms are used interchangeably some places and may have local colloquial usage.

    Building paper where I come from is a rosin paper and only for walls. For roofs and under steel tops we use the terms “tar” and “felt” paper interchangeably but they are also different materials (tar vs asphalt). The actual material used is 15 or 30 lb asphalt impregnated felt. I see that building paper is being impregnated with asphalt and polyester. Asphalt impregnated paper is intended to self-seal any nail or staple penetrations, just like asphalt shingles flow and stick.

    Roofing felt is installed with staples; overlapped to the extent of being a double layer; its brittle and needs to be covered soon after installation or it tears off in a breeze. The felt and purlins should be coordinated to minimize penetrations.

    In the future I would consider Solitex as the WRB for the under-steel, cold roof.

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