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When you install plywood board and batten (or in my case, reverse board and batten) siding, how do you seal the vertically-running seam of two sheets?

For example, I know the horizontally-running seam you use z flashing, so that any water running down the upper sheet will fall onto the flashing and in front of (not behind) the lower sheet.

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  • You don't. That's the purpose of the "board".
    – isherwood
    Jul 3, 2017 at 4:15
  • so youre saying the tightness between the board and batten (or the two plywood sheets of board and batten) due to the fastener (eg, nail) is sufficient?
    – tau
    Jul 3, 2017 at 4:17
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    Yes, that is all that has ever been done. Caulking will hinder the expansion and contraction of the wood and rip the caulk anyway. Keep the coverage better than 1", 1 1/4" is great, though I have seen as little as 3/4". Do not nail through both boards, the expansion and contraction will crack the wood.
    – Jack
    Jul 3, 2017 at 4:49

2 Answers 2

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To me, board and batt means the larger board goes on first and the smaller board (batt) goes on second and laps the edges of each board. Likewise, reverse board and batt means the small board (batt) goes on first and the larger board goes on second and covers the spaces between the batts.

Over the years, we've learned that using plywood for the "board" and just installing batts at regular spacing on the plywood will give the same affect as "board and batt" siding...for less cost and fewer problems because the "boards" are wide and tend to split (check).

I'm not sure I've ever seen (or understand) how reverse board and batt works with plywood. Seems like installing large boards on the plywood with small spaces between them would defeat the purpose (and cost effectiveness) of the plywood.

In any event, both systems are installed on a moisture barrier, like building paper, etc. This is essentially your moisture covering over the walls and the wood board and batt (or reverse board and batt) is just to keep the moisture barrier protected. Protected from wind blowing off the paper, UV protection, etc...

So, just as @isherwood says, you don't need to "seal" the vertical joint. It will be covered by the batten, or in your case, covered by a board.

By the way, we've learned to extend the moisture barrier about 1" past the bottom plate so water doesn't run out the bottom and blow back up under the bottom plate. If you live in a high wind area you'll want to put a felt pad between the plate and the top of the concrete stem wall to make a better seal against wind blown rain.

Also, we've learned NOT to caulk the bottom of the board and batt to the horizontal Z metal. Rather, we want that moisture to run out. After all, we know the board and batt (or whatever is above the metal) will leak. That's why we install a moisture barrier under the siding. If you caulk...you'll get dryrot on the bottom of the siding that sits on the Z metal. (We use a peel and stick membrane to seal the metal Z flashing to the upper moisture barrier.)

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  • this looks like a fine answer, but im curious why it is currently at -1... is there something inaccurate in here?
    – tau
    Jul 3, 2017 at 16:24
  • That's one flaw of the site. If someone doesn't like what you say, they can "down vote" you without saying why. So there's no discussion as to why one thought process (opinion) might be better than another.
    – Lee Sam
    Jul 3, 2017 at 18:51
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Isn't the OP asking about plywood RB&B, a Douglas Fir plywood product that is textured to look like Reverse Board and Batten?

As such, the vertical seam between two sheets is not covered by a board or batten, unless battens are added over the indented "reverse battens". A batten or board over the seam defeats the aesthetics of the RB&B.

So, the question remains, of what to do with the vertical seam. Is the house wrap under the plywood enough to limit moisture infiltration, dry rot around the edges? It's probably a good idea to "pre-stain" (weather-proofing stain) the siding on both sides, or at least around the edges, before installing it.

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