Seat rail to chair leg joint

[MikeG.]

I'm already planning next year's furniture making, and the biggest single item is a dining set (table and chairs). I'm thinking about the joints, and wondered if anyone has ever seen a wedged through-tenon where the seat rail joins the leg? It seems to me that this would be the strongest joint for the most vulnerable part of a chair, so why wouldn't this joint be used?

 

[Jacob]

I'm already planning next year's furniture making, and the biggest single item is a dining set (table and chairs). I'm thinking about the joints, and wondered if anyone has ever seen a wedged through-tenon where the seat rail joins the leg? It seems to me that this would be the strongest joint for the most vulnerable part of a chair, so why wouldn't this joint be used?

Because it would weaken the back leg at its most stressed point. I've had a few very slim chapel chairs to mend and that is where they break, even without a through mortice.
Wouldn't matter if the construction was fairly chunky, but the thinner you go the more you have to think about bracing, with horizontal rails, or better still diagonals, carefully optimised joints, or as a last resort - metal brackets.
Safest to copy known solutions.
PS rails often joined with a corner block under the seat which braces them, and also their stub tenons are tied/work together with minimal mortice in the leg

 

[MikeG.]

This will be a chunky mediaeval style chair, probably at least 45 x 45 at that point so loads of room for a good mortise. There will be another substantial rail lower down and the "carvers" will have the bracing effect of the arm.

 

[custard]

I'm already planning next year's furniture making, and the biggest single item is a dining set (table and chairs). I'm thinking about the joints, and wondered if anyone has ever seen a wedged through-tenon where the seat rail joins the leg? It seems to me that this would be the strongest joint for the most vulnerable part of a chair, so why wouldn't this joint be used?

That's a very interesting question. You often find through tenons and wedged through tenons for the side rail to back leg joinery on "country" chippendales; but I've never seen it on a finer quality "city" version of the same chair.

You're right that this is the most stressed joint in any chair, and if this joint starts to fail then all the other joinery in a chair will soon follow as the increased racking stresses are relayed throughout the chair. Furthermore, it's a bit of a pig to plan because you're trying to balance the structural needs of both the side rail and the back rail, and there's not enough meat on the bone in the back leg to allow the joinery you'd really like. The traditional compromise is to prioritise the side rail to back leg and to joint the back rail to back leg with a very shallow stub tenon and a couple of dowels that are around about 3/8". This use of dowels was common even in the 18th century, and properly executed has proven capable of withstanding two hundred years hard use.

Here's a chair I've made showing the side rail to back leg mortice being cut at the appropriate angle,

Here you can see the mortice for the stub tenon and dowels,

And here's the jig I make to drill the dowel mortices. It's a simple thing with the tenon on one side and the corresponding mortice on the other, you need to make it precisely, but once done it'll last for about twenty or thirty chairs before the holes get too enlarged by the drill to remain accurate.

Personally I don't like the use of through tenons because I believe it weakens the overall joint in this particular context, but I'm full of admiration for the craftsmen who did it because the compound angles make it extremely difficult to lay out and execute well. I've seen 18th century country chippendales where the through tenon has been cut with unbelievable precision. Another reason I don't like through tenons is I generally put a "D" profile on the back face to the back leg and a through tenon looks clumsy when it emerges through a profile.

As well as the actual mortice and tenons I generally include corner blocks around the seat. These are glued and screwed and I like to plug the screw holes for a neat finish, not that anyone will look under the seat but if they do I prefer it to be tidy!

 

[custard]

probably at least 45 x 45 at that point so loads of room for a good mortise.

Pretty much all chair legs are that dimension or larger at the area around the side rail joinery, they may well taper above and below but that's a minimum cross section in that particular area. It's still not enough to allow you to relax with the joinery!

 

[custard]

This will be a chunky mediaeval style chair

Be careful, no other piece of furniture gets moved around like a chair. Every time you sit or rise you move the chair back and forth, and if it's too heavy it starts to become uncomfortable to use.

 

[MikeG.]

That's a very interesting question. You often find through tenons and wedged through tenons for the side rail to back leg joinery on "country" chippendales; but I've never seen it on a finer quality "city" version of the same chair.

Yes, I've seen that, which was one of the reasons for asking.

it's a bit of a pig to plan because you're trying to balance the structural needs of both the side rail and the back rail, and there's not enough meat on the bone in the back leg to allow the joinery you'd really like. The traditional compromise is to prioritise the side rail to back leg and to joint the back rail to back leg with a very shallow stub tenon and a couple of dowels

That's all helped by the fact that there is a back rail just 4 inches up from the seat rail, plus another towards the top and bottom of the legs.

Here's a chair I've made showing the side rail to back leg mortice being cut at the appropriate angle,

There'd be no angle on mine. Well, you know what I mean: it'd be 90 degrees. These are the sort of chairs I have in mind:

https://www.earlyoakreproductions.c...-dining-chairs-berwick-range/product-3883.php.

Personally I don't like the use of through tenons because I believe it weakens the overall joint in this particular context, but I'm full of admiration for the craftsmen who did it because the compound angles make it extremely difficult to lay out and execute well.

As I say, there are no compound angles on the particular chair I'm planning .....well, other than for the tenon in relation to its rail, which is straightforward (and it's not compound, just being on plan-view only).

Another reason I don't like through tenons is I generally put a "D" profile on the back face to the back leg and a through tenon looks clumsy when it emerges through a profile.

I agree absolutely, but again, this wouldn't be an issue on these chairs.

Thanks for a nice fulsome answer.

 

[MikeG.]

probably at least 45 x 45 at that point so loads of room for a good mortise.

Pretty much all chair legs are that dimension or larger at the area around the side rail joinery, they may well taper above and below but that's a minimum cross section in that particular area. It's still not enough to allow you to relax with the joinery!

The one I'm sitting on is 28 x 30!! No joints, just dowels, and I've re-glued it twice already. It's less than 10 years old........and no, I didn't make it!

 

[custard]

You see that all the time, here's another shocker,

These were actually a very nice design, but the construction was horrible on at least two different levels. Not only were the back legs spliced together from bits, but there was no care in trying to make the bits remotely match. A client had twelve of these and three or four had failed within a few years.

I've checked on the chair designs that I make and the smallest back leg cross section at the side rail/back leg joint is 38mm x 49mm, and that's where I've carefully selected the timber so there's no short grain anywhere in the vicinity.

 

[MikeG.]

Yeah, that's really, really ugly.

 

[Racers]

This isn't a good way.

Chair leg joint by Pete Maddex, on Flickr

Pete