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Why would you have horizontal butt joints? Seems like that's asking for trouble.
Do it right, or do it twice.
If you are painting the siding, it can't hurt to caulk or prime the butt joints. If you are in an area that receives lots of precipitation, a tar paper spline behind each butt joint is not a bad idea either.
I would suggest priming but let the structure breathe.
horizontal cedar siding ....is that the same as clapboards ?
we always use a scarf joint with claps... and we prime both ends before we hang them... so, no caulk for us..
with fibercement , we do a regular butt joint, with a n aluminum spline, and caulk
Mike Smith Rhode Island : Design / Build / Repair / Restore
Another vote for scarf joints. Allows expansion and contraction without a gap. A steep angle is better for more overlap (many tricks to accomplish), but 45 degrees is pretty good. Felt piece to back it up is a good idea, though I just did a couple on a patch job with bits of adhesive membrane, and found this faster and easier (stays put).
Suggest priming or otherwise sealing the raw ends. Looking at any older building, you will see the joints fail first.
You can caulk or even glue a scarf joint if you like. Caulk in a butt joint would be unlikely to hold.
The joint on my butt is vertical, and you keep your caulk away from it hear!!!
Mr T
Do not try this at home!
I am an Experienced Professional!
Ok, looks like my comment was way off base...ignorance I'm sure. Can someone 'splain to me what a horizontal butt joint is...in this context?
Do it right, or do it twice.
If you put up two pieces of horizontal siding end to end, because you don't have one piece that's long enough to fill the position, the joint where the two ends meet is vertical. He never said the joint was horizontal, just the siding.
Oops. I was imagining something like horizontal board & batten...Do it right, or do it twice.
I can see why that troubled your mind. It sounds like a really bad idea.