Our Spenard's have no butt boards, butt (ha ha) they have strips of cardboard that you can staple to plywood and make your own. I made butt boards for my shop out of plywood and strips of felt paper, but slicing all that felt got old. The cardboard strips are cheap and will be fast. Hope that helps.
i've only used 'em once, so i am no expert. a guy here posted a pic of "real" ones, and they were about 7" wide w/ 1" wide strips near the edges. that leaves a gap between the two strips of about 5 1/2". the strips need to be about 1/8" thick, though mine were probably mostly less than that. i just hung a sheet, insert the butt board, and screwed it in, ensuring that i was not so close to the end of the 'rock that it broke out. when you install the next sheet, you'll see both 'rocks bow in a hair. that's all they need to do. mud right over the valley formed by the bowed 'rock. done. that's my take on the muthas. pretty simple, pretty slick.
make your own, very easy. DON'T use 1/8th, way to thick. you will be damaging the DW when screwing. you don't need, nor want a 1/8 deep joint. thick non corrigated cardboard is what i used, caulked down to 1/2 osb. and make the osb as wide as the cavity they are going in. and screw at least 1" away from the end of the DW. do one side completely. then do the other side, working from one end.
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