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When I found out how much a pair of prehung French doors cost–and that I couldn’t install them so that the doors open to the outside without voiding the warranty–I decided to make my own. Years ago, Fine Woodworking ran an article on joinery for doors that found that biscuits were even stronger than the traditional mortise and tenon joint. They glued the doors up with yellow glue, and the authors thought that the edge-grain to long-grain butt joint would not survive freeze-thaw cycles thus compromising the long-term strength of the door. I wondered if I could avoid this potential problem by using West System waterproof epoxy to glue up the doors. On the other hand, with epoxy there’s no moisture to swell the biscuit (or tenon or dowel), so would this just weaken the joint in a different way? Whaddya think?
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Laurence,
Built a green house door out of redwood with doubled biscuits for joints about 5 years ago. That's about how long it lasted. Water will migrate into the joint and cause the joint to open and allow more water...so on and on the joint fails completely. Go with M&T.
*Fine Wood Working did a test on various joints not to long ago. If I remember correctly biscuits failed much earlier than mortise and tennon and loose tennons. You might want to do a search on thier site for that article. Good stuff.Dave
*Use West System or MAS 2-part epoxy. Use loose tenons, 1/2" thick, 2" less wide than the rail. Make the tenons a slip fit in the mortises. Make the mortises as deep as possible ( 3" in my doors, using a 4" long 1/2" 2 flute router bit at 22K rpm in a P-C 3 hp router). I use the same wood for tenons as the door is made out of. If the mortise is 4" wide, the tenon is 3-7/8" wide, to give some adjustment during glue up. Bevel ALL edges and ends of the tenons a little to help in assembly. If the total mortise depth (both sides) is X, make the tenon X minus 1/8" long.Dry fit the door with all the pieces first. Once it's clamped square and no gaps, use pencil marks so the pieces can be realigned w/o remeasuring. Then glue and clamp, using the pencil marks between the pieces to line things up.This is the voice of much experience and many exterior doors speaking.
*You could also use 4" long dowls 3 per joint.
*I read that article. I was amazed that codswallop of that nature was given credence by being published. Anyone with half a brain and just a smidgeon of knowledge regarding joinery for external locations could see that the author simply opened his mouth and let his belly rumble. I concluded FW could safely be relied upon to print rubbish as well as useful nuggets. Mortice and tenons are a time proven, and reliable way to join doors. Dowels, biscuits, and any other quick fix is just that, quick in every sense- quick to assemble, and quick to fail. ;-) Slainte, RJ.
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When I found out how much a pair of prehung French doors cost--and that I couldn't install them so that the doors open to the outside without voiding the warranty--I decided to make my own. Years ago, Fine Woodworking ran an article on joinery for doors that found that biscuits were even stronger than the traditional mortise and tenon joint. They glued the doors up with yellow glue, and the authors thought that the edge-grain to long-grain butt joint would not survive freeze-thaw cycles thus compromising the long-term strength of the door. I wondered if I could avoid this potential problem by using West System waterproof epoxy to glue up the doors. On the other hand, with epoxy there's no moisture to swell the biscuit (or tenon or dowel), so would this just weaken the joint in a different way? Whaddya think?