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Metal siding – strapping details

FernieRyan | Posted in Energy Efficiency and Durability on

Hey there,
A long time follower of the site and many of the Q and A forums.
Question in regards to  exterior siding/strapping details.
In climate zone 6, Fernie BC.

3 storey residence
wall assembly is a greenstone ‘ICE’ panel – essentially a double steel stud wall with eps sandwiched /glued between acting as insulation/vapour barrier and shear strength – no sheathing required.
Exterior wrapped in tyvek
I will be horizontally strapping the exterior walls to accommodate siding – planned 1×4 spf, bevelling top edge to assist in moisture management. 

A few runs of metal are 20’+ with no vertical break

– majority is vertical corrugated metal siding

My question is in regards to 
A) is the planned 1×4 spf strapping adequate for longevity – rot/insects.
should I consider pressure treated? Concerned of metal corrosion with PT??
B) will the details allow enough ventilation through the corrugated metal to dry assembly?

thanks a lot for your feed back and knowledge.

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  1. Expert Member


    A) Regular 1"x4"s will be fine. If you do go with PT make sure it is MPS, not ACQ. MPS doesn't corrode galvalum or fasteners.

    B) Yes you will get good drying - as long as you use a perforated base flashing, not a solid one.

    I spend the summer of 1979 logging in Fernie.

    1. FernieRyan | | #2

      Hi Malcolm,
      Thanks a lot for your reply.
      Logging is still fairly active in the valley, a lot of the surrounding land is privately owned so stacked logging trucks through the middle of town is still a daily occurrence..
      I have been dwelling on this detail for quite some time. Contractors in the area gravitate towards pressure treated as soon as the wood ‘May’ be exposed to some of the elements. But with the drying ability of the corrugated I don’t see a direct need for it. The small condensation build up that the wood may see should be dried out by the air movement through the panel.
      Am I correct in assuming MPS is Micropro sienna?
      In regards to the perforated J. I do have a cantilever (not exactly a green building component). I was planning on using regular J (no flashing) at the lower most point with holes drilled every 2” to 4” for drainage. In your opinion would this be sufficient to aid ventilation?
      NB - I would have no J / exposed bottom corrugation and a flashing where the metal siding is closest to the ground.

      Also, Should I give consideration for venting at the top? Currently planned to cap with J trim against the soffit.
      I could be getting too deep into the details.
      Again, thanks for your insights

      1. Expert Member
        MALCOLM TAYLOR | | #4

        I wouldn't worry about venting the top. Metal siding can see some condensation on the backside, so providing as much open area under the siding as you can is a good idea.

        1979 was the last year before chain-breaks on some brands of saws came out. I still have the scars on one knee.

  2. mr_reference_Hugh | | #3

    FernieRyan, check out these GBA articles.

    I would very very cautious about using a horizontal strapping directly over the ICE panels. Malcolm mentions using "perforated base flashing" but it is my understanding that this only works for vertical strapping. Here in Ontario, it is my understanding that the code does not allow horizontal strapping (you can check the code yourself in BC or ask the building inspector). See below for one reference I found, but I can't find it in the Ontario code without taking more time.

    I believe that I recall listening to Joseph Lstiburek on a BS+Beer episode explaining that he was instrumental in having the rain screen added to the building code. The explanation was that the typical 3/4" rain screen camb about because the 3/4" strapping was readily available from most building suppliers and that is what got used initially. The GBA articles above talk about new options that have come onto the market, included some that are designed to be applied horizontally.

    In this Youtube video, Joseph Lstiburek explains the origins of the rainscreen, which might help you make a well informed decision.

    Joe says that often 3/16" allows for drainage but not sufficient for airflow. He says that in reality you only need to increase the gap to 1/4" inch to 3/8" to get the required airflow. Of course, most codes require a larger air screen.

    I found this reference about the code and horizontal furring.

    "What about Shingles and Vertical Siding? The Code does not permit shingles or vertical siding without a capillary break, but these materials clearly need something other than vertical strapping to provide the 10 mm drainage cavity. One solution is horizontal strapping on top of the vertical strapping but most of the proprietary rainscreen products will also provide a suitable drainage cavity as well as support for the cladding materials in a way that is easier to build. A cladding material that provided its own Code conforming capillary break would also be acceptable."

    Hope this helps.

    1. FernieRyan | | #5

      Hey Mr reference,
      Thanks for you input.
      My understanding is that, yes the rain screen should ideally be vertical as stepped out in the code. However as the last line states
      ‘A cladding material that provided its own Code conforming capillary break would also be acceptable."
      I believe corrugated metal conforms to being it’s own capillary break. If this was board and batten or even a different profile of metal, I could see the issue with this detail.
      Reality of metal siding is that the only moisture it should see is the small amount of condensation on the backside of the metal, which with the corrugation and air flow should readily evaporate/dry? If You think I’m relying too much on the expected drying ability of corrugated metal, please let me know.

    2. Expert Member
      MALCOLM TAYLOR | | #6


      - A perforated base flashing is the only way I know to ventilate and provide drainage for corrugated metal siding. No other products conform to the profile.

      - Interestingly, corrugated metal siding without strapping meets the BCBC requirements as a rain-screen as the attachment points do not exceed the 8% limit, leaving the cavity mostly clear. Adding horizontal strapping technically makes it non-conforming as the drainage plane is required to be against the sheathing. I don't think in practice this makes horizontal strapping problematic when it still has an open vented gap provided by the siding profile. This is unique to corrugated siding, and doesn't apply to other profiles.

      - There is no Canadian Building Code. Each province or territory enacts its own, and BC only requires rain-screens in the coastal areas, not in Fernie where the OP is building. I don't think there are any references to rain-screens at all in the Ontario Building code.

      - The 10mm cavity depth referenced in BC's code only applies to strapping, not approved proprietary rain-screen products such as those commonly used behind shingles or shakes.

      1. mr_reference_Hugh | | #7

        There you go Malcolm. I read the question to quickly and missed the word "corrugated". I think I relied on the title at the top of the post too much. Reading comprehension 101.

        To your points,

        - A perforated base flashing is the only way I know to ventilate and provide drainage for corrugated metal siding
        > I used perforated base flashing on my house for another type of siding so I definitely like the product.

        - "Corrugated metal siding without strapping meets the BCBC requirements as a rain-screen as the attachment points do not exceed the 8% limit, leaving the cavity mostly clear."
        > That makes a ton of sense (especially if I improve my reading skills and don't rely to much on the title of the questions being posted).

        - There is no Canadian Building Code.
        > Oh, I know that Eh! It would be too easy if there was such a thing. Knowing that each provincial code is slightly different is why I mentioned that I am in Ontario and why I mentioned the Ontario Code. You see "Canadian Code" in the URL for the link I provided but the guy who manages that site refused to change his URL before I posted it here (just being funny).

        - The 10mm cavity depth referenced in BC's code only applies to strapping
        > That makes sense. There are of course innumerable products and assemblies that are tested separately and get certified as code compliant (for the different codes in different jurisdictions.)

        Anyway, I read as many of your posts/replies as I can but man I can't keep up. I know that you are helping a ton of people and good on you. I am curious how you find the time to be so prolific :)

        1. Expert Member
          MALCOLM TAYLOR | | #8


          Sorry, sometimes my replies are a bit pedantic. I try and write them with all the readers in mind, so they can often go over things someone like you already know.

          Most days I'm drawing a my computer, so when I take a break I check in on GBA - maybe not everyone's idea of a break I guess. I've actually got this week off as the metal panels for a roof I am half way through replacing didn't make the truck due to the Thanksgiving stat. Luckily we are in an unusual weather pattern here for October and there is no rain scheduled for weeks.

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