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Horizontal Wood siding

user-1045979 | Posted in General Questions on

I’m in the process of detailing a rainscreen for a house near Seattle (4C, although it shouldn’t matter), and was curious expansion joints.

I was wanting to do horizontal butt-jointed 1×6 cedar boards with (ideally) no gap between each board. This would be over a rainscreen with vertical furring strips at 16″ o.c.
It seems like there would be a real issue of the boards expanding. I’ve seen this system used before (or at least it appeared that there were no gaps) so I’m curious what the right detail would be.
Also, we were planning on leaving the boards unfinished and letting the cedar weather naturally to a grayish color.

The options I can think of would be to either leave a 1/8″ gap, which should be small enough that UV wouldn’t be an issue or to use green wood for the siding, so that as it dried the boards would shrink away from each other, creating natural joints, which would be fine over a rainscreen.
These are both just speculation, though, and I’d love to hear the correct way to do this.

Other details I’ve found would be to only nail along the center of the board to let it expand naturally.

Suggestions, advice? Thanks.

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  1. GBA Editor
    Martin Holladay | | #1

    I'm not a fan of horizontal board siding installed the way you suggest. There is a reason that our ancestors invented lap siding (clapboards).

    Are you worried about the joints between adjacent boards on the same course, or are you worried about the joints between courses?

  2. slugboy6000 | | #2

    I'm trying to detail horizontal unfinished cedar lap siding, and it's presenting enough problems.

    With an open joint siding, you're going to be extra reliant on your house wrap too keep water out of your wall assembly. With wet cedar, there will be wood tannins decreasing the effectiveness of that wrap whether it's felt, or plastic.

    There's a finehomebuilding article called "Open-Joint Siding" in which they use a fancier housewrap which is UV stabilized, you may want to check it out.

  3. slugboy6000 | | #3

    I'm going to post a new question on how best to install unfinished cedar.

  4. slugboy6000 | | #4

    What about mitering (is that the right term?) horizontal board edges to allow for better drainage?

  5. user-1045979 | | #5


    I understand that lap siding has advantages but it's a very distinct look that is not in line with the house in question. Horizontal board siding, despite it's problems, is a commonly seen siding and I want to make sure that, to the extent possible, I'm detailing it correctly. And to answer your question, I'm concerned about the gap between courses.

    I've come across UV stable building wraps before, but they are pricier than I'd prefer if that can be avoided. Those seem to be used when there are very visible gaps between boards -- highlighting the gaps. I was wondering if a 1/8" gap (less? More?) would be enough to allow expansion while being narrow enough to not really allow light behind. 1/8" high x 1/2" deep shouldn't allow much light -- will it still be a problem, though?
    For the water and the tannins, wouldn't the 3/4" furring gap mitigate those concerns?

  6. davidmeiland | | #6

    Daniel, why not mill the boards for shiplap installation? It will still look like plain boards, but you'll have much better weather performance. The easiest way to do this would be to buy square channel siding and rip a bit off the top to eliminate most of the channel. I bet you could order it from your lumberyard with that done.

    To install something like that, you need to measure the MC of the boards and space them accordingly. If they are very dry and installed in summer, they need a gap. If they are wet and installed in winter, they do not, but one will appear as they dry. There's no way around it.

  7. user-1045979 | | #7

    David, thanks -- that seems to be a great solution. Could you just reverse the channel siding since the back doesn't seem to have a gap anyways?

  8. davidmeiland | | #8

    Daniel, seems like you could, but I would caution you strongly against trying to get a "gap-less" look. I've tried that sort of thing a few times in my ~30 years of making stuff out of wood, and lost every time. Failure to allow a bit of expansion room will probably mean a failure as the board swell and try to crush each other. If you're installing 1x6 I would allow about 1/8", assuming you are installing dry material right now (now being summer).

  9. user-1045979 | | #9

    I was intending to leave a 1/8" gap, just looking for a solution other than rip the top of each piece.

    Thanks for your help.

  10. slugboy6000 | | #10

    What's the advantage of profiles like channel rustic over shiplap. Is the more open joint considered more durable, or do they just do that for looks?

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