Need Advice on Chair Design

[Jan Bianchi]

I'm trying to make a chair from one I've seen, but I don't have the chair and from the photograph I can't really see how the seat is attached. I am attaching three photographs, a view of the seat from the side, bottom and top. (or at least I hope I am, as I haven't done this before)

My question is whether you think the seat slats are tendoned into both the front and the back seat rails or only the front. As the bottom view shows, the seat slats at the rear rest on and are secured to either a rabbited back rail or a board that functions like a rabbit. From the top view it doesn't look like the seat slats could be mortised and tendoned into the back seat rail because if they were it seems like they would cut into the back slat tenons. But is that rabbit or board really enough to hold the slats? They seem too tight up against back slats to just be abutting the rail and resting on a rabbit.

[Jamie Buxton]

You're right, it looks like a tenon on the rear end of the seat slat would collide with a tenon on the bottom of the back slat. You could avoid that by splitting the seat tenon in two, with a gap between those two tenons, and having the one tenon from the back slat going through that gap. All the tenons could be long, but not interfere with each other.

[Jan Bianchi]

Or maybe use two dowels rather than split tenons on the rear seat slat?

[Sam Yerardi]

Jan,

It's hard to tell but in the third picture it almost looks like perhaps they might be using tenons but in a unique way. It might be possible to have one large blind tenon at the end of each seat rung, and then blind-straddle it with two smaller tenons on the end of each of the back pieces. If that's how they are doing it, which is just a wild guess, the smaller tenons would have to be at least 1/2 to 2/3 the depth of the rear rail to try to give strength to the joint. IMHO, it is worth noting, though, the joinery such as this might not be too critical an issue as the strength of the chair will come in major part from the outer framing. The seat and back will add to the strength but I don't believe the chair is depending on it entirely. The seat only needs to capture and support weight in a downward direction. Structurally as far as weight, the joinery looks like it would work. The sizing of the tenons would work for the heavier loading of the seat where it is needed than at the back where it wouldn't be needed as much. I would have concerns about how well that type of joint would hold up, but it obviously (by the picture) looks like several chairs are made this way.

[Jan Bianchi]

Both you and Jamie seem to agree that the seat slats are secured by tenons and not just the cleat underneath. Whatever it is, the joinery does seem to be holding up. All those chairs are pretty sturdy still after sitting out in Seattle rain for several years. Maybe it's the combination of the tenons and the cleat.