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Community and Q&A

Rainscreen Roof?

Eric Mikkelsen | Posted in General Questions on

I’ve seen lots of info about rainscreen walls, what about roofs? I’m planing on using a standing seam metal roof, 10/12 pitch. Should I be considering rainscreen details for this?

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Replies

  1. User avatar
    James Morgan | | #1

    If it rains where you live and it's a contact ceiling then probably yes. If it's a ventilated attic and you can visually inspect the underside of the roof sheathing from time to time then probably no.

  2. Eric Mikkelsen | | #2

    It is a non-ventilated attic. I haven't been able to locate any good rainscreen details for this...know of any?

  3. User avatar GBA Editor
    Martin Holladay | | #3

    Eric,
    On top of your first layer of roof sheathing, you install parallel 2x4s on the flat, from eave to ridge, with one 2x4 above each rafter or truss. This creates your 1.5-inch-deep ventilation channels.

    Then you install a second layer of roof sheathing, followed by roofing underlayment and roofing.

    An optional improvement: Install one or two layers of rigid foam between the bottom layer of roof sheathing and the 2x4s.

  4. Eric Mikkelsen | | #4

    Martin,

    doesn't the foam between the 2x4s negate the ventilation channels? am I asking for trouble if I put the metal roof directly over the underlayment & sheathing? The conditioned attic is insulated with spray foam to the underside of the sheathing. straight gabel roof.

    Creating a rainscreen for walls is minimal cost, but this sounds expensive for the roof.

  5. Ron Keagle | | #5

    I do not understand the need for a rainscreen on a roof. And in this case, if it is a hot roof with spray foam on the underside of the decking, and no ventilation intended, can't you just lay the metal roofing on top of the decking? What would be the point of creating an air space between the top of the decking and the bottom of the metal roof?

  6. User avatar
    James Morgan | | #6

    " What would be the point of creating an air space between the top of the decking and the bottom of the metal roof?" Because sooner or later, all roofs leak. When the underside of the roof sheathing cannot be inspected, a free drainage plane immediately below the roof finish ensures the sheathing has good long-term protection from concealed moisture and rot.

  7. User avatar GBA Editor
    Martin Holladay | | #7

    Eric,
    If you want to include a layer of rigid foam, it is a continuous layer. I didn't propose installing thin strips of foam with 2x4s in between.

    If you want to ventilate your roof, you would install the 2x4s on top of the rigid foam. Don't put thin strips of rigid foam between the 2x4s; the purpose of the 2x4s is to create ventilation channels.

  8. Ron Keagle | | #8

    James,

    Thanks for that explanation. I can see the logic, although I had not thought about the issue of not being able to inspect the underside of the decking for telltale signs of roofing failure.

    I can see the need for this detail with spray foam applied to the underside of the decking, but also with any vaulted ceiling system or attics with any type of insulation obscurring the underside of the roof deck.

    Is the topside rainscreen / air space detail opened to the outside so that air can circulate through it? If so, how is that opening executed?

  9. User avatar
    Armando Cobo | | #9

    Here you go Eric; this is a detail for CZ3. Any good metal roofing contractor knows how to install notched battens under the metal roof. Some folks prefer treated battens; others think that the air flowing through the channels will dry the battens before they rot. Depending where you live, you may need to increase the over/under sheathing insulation.

  10. User avatar
    James Morgan | | #10

    Armando's detail with horizontal notched battens is a good one for metal roofs that offers some economy as it omits the second layer of sheathing. Martin's proposal is for battens laid up the roof slope in line with the rafters with a second layer of sheathing on top, as you would need for shingle roofs. Either way the desired drainage plane is maintained.

  11. User avatar
    Armando Cobo | | #11

    Actually, there are metal roofs designed to be installed on substrate and some others on batters or counter-battens; you should check with the manufacturer. You could say not installing a second substrate ia a more economical way, or you could choose a better grade and stronger roof that allows installing it on battens. It depends which way you look at it or is available to you.

  12. Malcolm Taylor | | #12

    AJ, ACQ Pt wood will eat Galvalum roofing and all standard gasketed fasteners in weeks.

  13. User avatar GBA Editor
    Martin Holladay | | #13

    I agree with Malcolm. Don't use pressure treated lumber for furring or strapping on roofs.

  14. Aj Builder, Upstate NY Zone 6a | | #14

    Armando has the right idea,and use PT wood. The added cost is insignificant.

    I thought ACQ is no longer? Also metal roofing that is painted no metal would contact? Screws are heavily galvanized and painted...

    Anyway last roof we did went over plywood and 30# felt.

    Will have to check manufacturer.

  15. Floris Keverling Buisman | | #15

    Yes, no need for PT wood - wood that can dry because it is part of a vented assembly won't rot....and as pointed out the treatment of the wood will eat/make materials leak (including conventional WRBs).

    More info on vented roofs in this 475 blogpost</a

  16. John Brooks | | #16

    We are fortunate to have some real experts chime in....
    just think......
    what if OP(or others) had followed the anonymous advisor's advice?

  17. Aj Builder, Upstate NY Zone 6a | | #17

    Ice and water shield is recommended if PT wood is used along with hot dipped galvanized fasteners. MCA is less corrosive and is what's stocked here. Dry wood does not corrode. All wet wood corrodes if over 20% moisture content. Non PT wood corroded via wood acids when wet.

    From the web I found the main concern is to not use PT wood above metal roofing for mounting objects. Wet PT is the real issue.

    John ;) you are the best.

    This FAQ is interesting;

    http://www.strongtie.com/productuse/ptwoodfaqs.html

  18. Eric Mikkelsen | | #18

    Thanks all. How well does the horizontal venting work? Do I need to install two layers of furring strips at 90 degrees in order to get soffit-to-ridge venting? How well do the notched battens compare to this 2 layers of battons @ 90 degrees to each other? ...i actually have never seen notched battens, are they common? Is some metal roofing shaped with this Soffit-to-ridge venting & if so, is it sufficient, or should the battens still be used?

  19. Lucas Durand - 7A | | #19

    Eric,
    On my own house I have galvanized steel panels over horizontal notched 1x4s, asphalt felt, and 1/2" t&g plywood.
    The roof has a 6/12 pitch and is a "dutch gable" - hips and gables.

    How well does the horizontal venting work?

    I can only guess, but I bet it works just fine.

    Do I need to install two layers of furring strips at 90 degrees in order to get soffit-to-ridge venting?

    I don't think so.
    I'm not sure what panel "profile" you're planning to use but many have ribs that run verticaly from soffit to ridge.
    Unless some type of closure is used, the ribs pass over the battens continuously.

    i actually have never seen notched battens, are they common?

    I don't know about common but they aren't difficult to do.

    The battens are easily notched with a circular saw while they're still in a bundle six or eight across.
    The notch cuts are quick and don't need to be particularly straight or exactly measured.
    Lay the battens "notch side" down (they're for water drainage) and 24" o.c.
    Also, the battens don't need to be nailed down like crazy since the panel screws should be long enough to penetrate through the battens and the sheathing.

    Having horizontal battens makes doing the roofing work easier on steeper pitches.
    If the battens are laid down with the felt it will really help to protect the felt during strong winds.

  20. User avatar
    Armando Cobo | | #20

    In TX and the SW is very common to use notched battens, and all you need is 1/8"-1/4" about 2"-3" long cut and 9"-10" long uncut, alternating so you'll have a good nailing base. You can build them or buy them at the roofing supply company.
    Peter Pfeifer, an Architect in Austin, use 1x4 boards at 45° angle in his own house, but I believe he's moved away from that now.

  21. User avatar
    James Morgan | | #21

    "How well does the horizontal venting work?"

    It may be better to think of it as a drain than a vent. For sure the notches allow air to pass as well as moisture but the purpose is not comparable to the vent channels between insulation and sheathing in a 'cold' ventilated roof which require a much larger opening for the continuous free passage of air. Most all sheet metal roofs (other than standing seam) have profiles which permit air movement above the battens: the notches are necessary to permit moisture to drain BELOW the battens.

  22. User avatar
    James Morgan | | #22

    Question from a contractor buddy when we were discussing this issue: has anyone used or considered using spacers such as wood or plastic door shims to hold the battens off the WRB/sheathing in a rainscreen application instead of notching the battens? Constructional pros/cons please.

  23. Lucas Durand - 7A | | #23

    James,
    Never tried it, but I'm not sure I'd want to - sounds like fussy work...
    And you'd probably want to make sure the shims were set so the panel fasteners would pass through them.

    Notching the battens is relatively easy and not too time consuming.
    Like I mentioned earlier I used my circular saw to notch the battens while still bundled from delivery.
    I made a sort of dado blade for my saw by putting two blades on the spindle seperated by a thin, flat washer, then set the depth of cut for about 1/8" (not sure how safe it would be to be working the saw too hard with this setup, but making 1/8" kerfs is light work).
    I just eyeballed the cuts about every 12" or so, cutting across 8 boards at a time then restacking the notched 1x4s in a new pile before notching the next row.

    It goes fast, the battens can be tacked down quickly and the notches work well (I had the opportunity to observe them before the steel panels covered everything up).

  24. User avatar GBA Editor
    Martin Holladay | | #24

    James,
    Your idea wouldn't work over rigid foam. All of the weight above would be pushing the shims into the relatively soft foam. The idea of the battens is to distribute the weight of the roofing (and the snow load) over the entire length of the batten.

  25. User avatar
    James Morgan | | #25

    Martin - of course. Duh! Lucas - thanks for the detailed how-to.

  26. Duane Brooks | | #26

    Great and timely topic for me guys...thanks for all the effort that goes into these replies!
    I was going to look into one of those pre-fabed plastic mesh mats to install between my standing seem and the felt to offer a drainage plane and some form of ventilation but I like the idea and simplicity of the notched batten much better.
    PT wood is obviously not an option, is there any issue with using Western Red Cedar for the battens? Just not sure if there would be any kind of reaction with this or any other type of wood and the metal.

  27. Aj Builder, Upstate NY Zone 6a | | #27

    How about PT 1x4 pieces sandwiched with 1x4 pine for top second layer. Both run together or do the vertical plus horizontal method. Skip the cedar maybe.

  28. user-7022518 | | #28

    What about using cor-a-vent instead of battens?

    Lisa

  29. thrifttrust | | #29

    I'm a little at a loss as to what is special about a metal roof in this regard. I am assuming the metal roof in question is light gauge steel. Unlike a composition roof which is air, vapor and water tight steel roofs allow for a lot of air leakage though seams and at the ridge and eave. The area at each seam is not in continuous contact with the roof and the pans have minor ribs between the seams which act like rain screen channels. The underlayment may be high temperature ice and water shield designed with embedded non-wolven fabric that provides a slip sheet so the metal is free to move. What does furring provide that is not anticipated by the standard, (and usually manufacturer recommended) system.

    Douglas Higden

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