When mating wide boards with mortise & tenon joints, what is a good rule of thumb to use for the maximum width of the tenon, or at least, the maximum width to which glue should be applied?
Michael Dresdner: Solid wood tenons should really not exceed two inches, or you may get problems with wood movement. Much more than that and you run the risk of blowing out the top of the mortise. This is not a hard and fast rule, because it depends in part on the type of wood you are using – some woods move more than others. You can get away with a slightly larger tenon if you’re using more stable wood. For very wide boards, use two tenons.”
Rob Johnstone: “Although it is a bit more work, splitting the tenons into multiples not only helps deal with the challenges of seasonal wood movement, but adds glue surface. So, when faced with a wide tenon … over two inches … I like to make it into a dual tenon. Not only is it a good technical choice, but it looks so cool, you feel like a better woodworker!