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I drafted this myself in Blender. Dado joints are depicted, mortise/tenon joints and bolted joints are not. This is to-scale, planned with douglas fir 2x4s and jute webbing/coil spring/4 way tie construction/ and then tie on cushions. Dimensions are about 31" x 7.5' x 36".

I have never built a couch (or any furniture) before, but have been studying woodworking a lot, so I am looking mostly for tips on how I might improve this, or especially, if it's structurally weak in any way.

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  • The rigidity of furniture is all in the joints. So if they are good, solid, glued joints, you should be OK generally speaking. As for this particular design, I don't see much in the way of lateral stiffness front-to-back. How are you planning on upholstering this? – DA. Apr 28 '15 at 7:25
  • IMHO: Depending on thickness of coils and upholstering you might be able to feel the 2x4 below you knees. – LosManos Apr 28 '15 at 10:26
  • Are you going to put anything in front of the two legs in the middle on the front? Right now, it looks like a heel and toe buster to me... – Maxime Morin Apr 28 '15 at 13:45
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    Are you sure about the depth? 36" is a lot of sitting surface. (Most people don't measure 3' between the knee and the butt.) You could obviously pad this out with a very thick back cushion, but if you've ever priced HD foam, you'll consider changing the design. For what it's worth, I've got a ~1918 stickley prairie settle replica that's about 21" from the cushion to the front edge. Maybe measure couches for the next few days and make up your own mind. One last thing... personally, I love the rectilinear of that/your couch, but others don't. You might consider angling the back cushion slightly. – Aloysius Defenestrate Apr 28 '15 at 14:08
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    @djvs this question may be too localized (likely to be only useful to you) but if you reframe it as a more general question about how to design a couch, I think it would be on-topic. For example: "How do I go about designing a couch? Here's what I have so far, and here's my logic for the design. What other issues have I not considered?" If you also include the type of joinery you're considering for each load-bearing joint, that would also be very helpful. – rob Apr 28 '15 at 16:18
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I would build the bottom grid where the cushions sit with mortise and tenon joints. Then attach it to the back legs and side legs using a dado. The two short legs shouldn't receive much lateral stress - You can probably get away with just a mortise and tenon joint there. If you change the orientation of the long horizontal boards, you could eliminate the middle legs all together (a 2x4 will only sag about 0.1 inches with 250 pounds directly in the middle). I put a 10 degree angle on the back, which is probably too much, but it gives you an idea.

If you attach the back to the base using a glued dado, it isn't going to rack at all.

Bench Image

  • +1 for elegant design, but strongly suggest triangle bracing at the angled back legs... imho, those look like looming disaster if left as drawn – Aloysius Defenestrate Apr 28 '15 at 16:30
  • That is not a complete drawing, I was just illustrating the changes I was proposing to the structure. There will be an additional horizontal brace between the top of the front leg and the middle of the back leg. – LeeG Apr 28 '15 at 16:32
  • @LeeG Sorry, but I must contradict you on the deflection of the long seat members. If the intermediate legs are removed and the stringer is turned 90 degrees, the sag with a 250 load at the middle is more like a half inch, so much that it would be springy. (checked this theoretically and experimentally.) – Ast Pace Apr 28 '15 at 20:06
  • It is possible. I stood on the middle of a 2x4 laid on edge and it didn't seem to deflect at all. In this setup, the load would be distributed (not necessarily evenly) across both of the long seat members. I used the Sagulator to check, and to get the .25" of sag, it needed 200 pounds per foot. My numbers were for a single 250 pound person in the middle. If we assume four 250 pound people, that is about 1/4" of sag, assuming all the weight is on the front piece. If we assume some of the weight to be on the back piece, it reduces it to about .1" – LeeG Apr 28 '15 at 20:17
  • This is a really interesting approach (and I think it would look great if the mortise/tenon joints were visible in the exterior and planed/stained nicely). I love the idea of using the mortise and tenon joints for the connectors in the middle, I suppose all the coil springs could just be placed on the superior surface of the horizontal seat plane. My point of confusion is with the fact that the whole back plane is angled in this design - I am pondering how I could have it constructed out of two pieces, so that the upper back could be angled, but the leg of the back would be vertical. – djvs May 4 '15 at 5:37
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I would say you don't have enough triangles.

The short front legs are the most obvious place, they will tend to be a weak spot. They only have one point of attachment and that is on the end, the weakest joint.

I would put triangle pieces on all four of the legs in the middle and attach to the cross piece under the seat.

There is also a chance of racking the whole thing from side to side (the long way). For that I would put a board (an other triangle) from the bottom of the feet on the outside (in the back) to the next upper corner.

enter image description here

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    What drafting program did you use for your markup? ;) – Matt Apr 28 '15 at 13:46
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    The only thing I would add is that the back support should have a slight...maybe 10% angle away from the bench. If you were to use 4x4's you could just cut the angle into the board, would add a little extra stability to the back as well. To be clear, where the seat meets the back support the 4x4 would be uncut and at the top it would be about 2" thick. I can draw something up if you cant picture it. – James Apr 28 '15 at 13:47
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    @Matt the windows snipping tool! :) That's professional quality there! – bowlturner Apr 28 '15 at 13:47
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    If the joints are done with a half-lap, you don't need so many triangles. The one that I would definitely agree with is the front stub legs. Also, you could use a sheet of 1/4" plywood to prevent racking (much like the back on a bookcase or entertainment center does. – TX Turner Apr 28 '15 at 13:48
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    @bowlturner I don't think strands of red yarn will add much reinforcement. Brown yarn would work much better. :P – rob Apr 28 '15 at 16:21
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I'd say you probably want some triangular braces under where the legs meet the lower (front to back) supports. They probably only need to be a couple of inches deep/tall, glued/pinned/screwed into the legs and underside of the supports.

It may not seem like they'd do much, but you should find they'll add a lot of rigidity. Anywhere you can add in triangular bracing without spoiling the look is going to help against the kind of lateral racking forces you're going to get when people stand up and sit down, and are moving around on the couch.

  • Is that necessary even with large mortise and tenons around the structure? – djvs May 4 '15 at 5:43
  • You might get away without them, but if I were designing it I'd put them in. You have to bear in mind that the seat will undergo "live loading" which means that the load on the joints and directions of forces will be changing all the time as people sit down and stand up, and shift around. Imagine grabbing hold of a mortice and tenon joint and trying to wrench it back and forth as hard as you can - this is pretty much what the joints will be going through. – WhatEvil May 5 '15 at 11:23
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Your design needs a wee bit of work before it becomes a complete or workable design. Design is an iterative process of which you have made the first iteration. Two or three more cycles and you should be ready to begin construction. Some things that I think you should consider:

  • Others have addressed the fact that the long horizontals in the seat structure will deflect vertically; they will also be have large horizontal loads induced by the jute webbing.

  • Well constructed lap joints as well as mortise and tenons can be considered as rigid joints lessening or eliminating the need for triangular bracing (think Danish Modern furniture from the mid 20th century.)

  • In your next iteration make the 2x4's their correct size (1 1/5 x 3 1/2) and consider the orientation of your mortice and tenon joints. Also will they be blind or through tenons?

  • Although many woodworkers eschew mechanical fasteners, you might consider the concept of "screw and glue" making sure not to screw into end grain.

  • Even if the structure for the back remains vertical, you still need to consider the cushioning to be inclined. With your 36" depth for the seat, your cushioning should be 14" thick at its base. This suggests that the seat cushion is going to thicker and that the seat structure will have to be lowered considerably.

Good luck on your next iteration.

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I really think you need an angle in the back leg. You're going to have your guests sitting pretty upright if you don't and it won't be very comfortable.

  • 36 inches seems deep enough to allow cushions allow the angle does it not? – Matt Apr 28 '15 at 13:08
  • @Matt--I'm not sure what you're saying. Are you saying that the cushions will provide the angles? If so, perhaps. I've never made a couch (though I have made chairs and a garden bench). – dfife Apr 29 '15 at 13:58
  • I have never made a couch either. The couple I have at home the rear frame is completely vertical. Like you understood, I was suggesting that the cushions allow the user to lean back – Matt Apr 29 '15 at 14:54
  • I apologize, I thought I wrote this in the original post - the point of the long, horizontal piece in the back is to serve as the tie-on point for jute webbing material. on top of which cushions would be placed. That gives it about a 8-15 degree angle (not doing the math atm). – djvs May 4 '15 at 5:34

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