I’m replacing the rotting LP OSB siding after 16 years, and at the same time I’ve decided to add a water table board, and skirt board (between first and second storeys), and was wondering what the consensus is about joining boards on a long run; should I join them with a scarf cut (raw ends dipped in primer/sealer) or butt them together and then caulk (and/or flash behind the joint and no caulk)? The boards are WRC.
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Replies
Either joint will expose the same amount of end-grain, which needs protection from water absorption, as you know.
I would use a gapped butt joint. Gap it by using a 16d nail as a spacer--about 3/16". This provides enough of a gap to allow enough caulking to be able to expand and contract as needed, and allows enough space to ensure that you fill the joint for its entire thickness, all the way to the back side. Use a top-quality caulk, like DAP's Dynaflex 230. (Others may post with recommendations for caulk that is even better in their experience.)
A smaller gap that can only be surface-caulked, or a tight joint will end up creating a capillary space that will "suck" water in and hold it there.
Also, be sure to use Z-flashing on the top side of the water table board, so that drainage from the siding above will get kicked out to the surface of the water table board, rather than finding a path behind it.
My general understanding was that it's not wise to fill a joint to its entire depth in such situations. Filling the joint results in less flexibility in the caulk, because it doesn't have room to expand and contract parallel to the joint surface.
Try PVC.
The new breed of exterior trim is PVC such as Azek, Versatex, Kleer and so on. They are impermeable to water and will last longer than you and I. They hold paint excellent and there is no need to caulk joints because the glue bonds the pieces permanently. If however, you choose not to use these boards then use water proof glue like Titebond III or Gorilla Glue with a scarf joint when joining these boards. This along with a high quality primer, paint and cap flashing should do the trick. Hope this helps. Eric Rodabaugh A&E Builders Corp., Long Island, NY
Plus, PVC comes in 18' lenghts...
At a real lumber store that is.
Personally I prefer scarf joints.
Yeah, but that's just because scarves look so suave blowing in the wind.
Thanks for all the advice. I wish stuff like Azek were available here at a non-suicidal price, but I haven't found it. While waiting for your responses I also did a bit more googling and it does seem that scarf joints are preferred by the majority (or at least by the majority of people posting on the net).
The depth of the fill
to the thickness of the board serves to seal the end grain and to give more surface for adhesion.
I don't agree with your thinking. Information I gathered from sealant manufacturers seems to contradict what you write. And, for full sealing of end cuts in siding and usually trim, I would use paint or other sealer which could be absorbed into the material.
Backer rod is recommended to "backfill" an area where caulk is used to bridge a gap. Otherwise, the adhesion to three surfaces has a tendancy to pull the caulk 3 ways which just doesn't work. It's can't expand or be pulled sideways when it's movement is stopped from the backside. In addition, anycaulk gap exceeding 3/8's to 1/2 inch need to be done in additional applications.
Check out this link; after opening, you'll get a list of installation instructions, and the one that's pertinent is the one for Smartside fascia and trim.
http://www.lpcorp.com/resources/literature/
Follow the sealant manufacturer's instructions for application
I skimmed the instructions after I found them.
And above is what they said about caulking. Did you happen to read about filling the gap to seal the board ?
thanks.
Caulk doesn't expand or contract at all -- it's essentially incompressible. What it does is change shape -- fattens out when compressed and slims up when stretched. If installed to too great a depth in the joint there isn't enough flex in the caulk to allow it to change shape, and it separates from the surfaces instead.
Picky, picky, but you're right--you're saying it acts as a liquid--noncompressible? True. To say, as I did, that it expands or contracts is really not correct. It only appears that way.
So, in my 3/16 gap, filled to full thickness of the material, what happens if the gap closes slightly? I visualize a slight fattening of the caulk along its length, front and back, and at each end. If the gap closed by 1/16", that would mean 1/3 of the total volume of caulk needs a place to go. If the depth of caulk is only half the depth of the 3/4" trim board, total "expansion" or displacement would only be half as much as a fully-filled joint, right?
I calculate that a 3/16 x 3/4 x 6 gap would have a volume of 0.8 cu. in, and 1/3 of that would be 0.28 cu. in. Would that really be a problem? Do you think it might be an advantage to have full contact for purposes of sealing? (I am sealing the cut ends with primer, BTW.)