Through mortises on bench seat

[mr_skittle]

All of our cheap dining room chairs that we've had since college have bit the dust and we are down to two, more recently purchased chairs. Instead of buying more cheap chairs I've decided to build a nice big bench seat. I'm going with trestle-style construction with two big "legs" and a stretcher. I want to join the legs to the seat with mortises but am not sure how I feel about capturing the seat with two through mortises on each leg. The plan is to put one shallow mortise the size of the leg to hold it in place but the real strength will be from two through mortises for each leg on the front and back of the seat.  My concern is that these will lock the seat in and give me trouble with seasonal changes in the wood. The bench is made out of 6/4 pine with a 3 1/2in birch strip down the middle.

[Hank Knight]

Your construction plan shouldn't present cross grain problems since the seasonal expansion and contraction of both the seat and the legs will be in the same direction. I don't know how the expansion coefficient of birch and pine compare, but I wouldn't think they would be dramatically different, at least not enough to cause cracking and splitting problems. If you wedge the through tenons, be sure the wedges run perpendicular to the direction of the grain direction. Otherwise, the force exerted by the wedges will contribute to any splitting forces that may exist and cause problems.

[mr_skittle]

I was worried that my post wasn't going to make sense without a picture but, Hank Knight, you got it and answered it perfectly. The tip about running the wedges perpendicular to the grain is something I may not have thought of when the time came either. So thanks for that tidbit.