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My cousin's fiancee is looking to build a bench for their kitchen. From his description, it sounds like what I would call a "slab bench." Just three thick slabs of wood, two for legs and one long one (about 60 inches he said). He hasn't decided on a wood. I recommended hardwood, but in retrospect given the thickness of the slabs, I think it might actually be worth trying in pine.

I told him I would typically use a frame or angle braces to prevent the seat from bowing and to make a rigid connection from the legs to the seat, but if it's a thick enough slab maybe that wouldn't be an issue: Is a 3-4 inch thick slab enough that it won't noticeably bow?

If they were to not use angle braces or a frame (i.e. there are only 3 members, no supporting structure), how should I connect the legs? I thought maybe just a really big mortise and tenon for each leg would do it, but I'm still concerned about it being weak. Should I just tell them to suck it up and reinforce the connections?

edit I've been provided with a reference picture of what he's looking at. It looks like it's just reclaimed 4x4 lumber with a miter, ... maybe biscuits or something as well, but no braces or anything.

enter image description here

Daniel B.
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  • I don't have time to officially run the numbers, but that sounds sufficiently strong enough that it should be able to support any number of average density humans you can pack onto it. And giant mortise and tenons were my first thought for joining them too. – Doresoom May 11 '15 at 04:08
  • Should be overkill, if anything. I saw pics of a similar bench a few weeks ago but I doubt I can find 'em now; I believe they went with wedged through-tenions. – keshlam May 11 '15 at 04:17
  • Why @keshlam, that sounds like another question to ask! :D – Daniel B. May 11 '15 at 05:16
  • @DanielB. I think I know the design you're talking about, but there are a couple variations I can think of. It would be helpful if you included a diagram or photo. – rob May 11 '15 at 15:16
  • Unfortunately this is third-hand information and the fellow describing it to me has never woodworked in his life so I can only infer. I will ask him, he said he got the idea from a commercial product maybe he can give me a reference. – Daniel B. May 11 '15 at 16:16
  • @rob I've added an image, which has led to another question. – Daniel B. May 12 '15 at 22:06

2 Answers2

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Is a 3-4 inch thick slab enough that it won't noticeably bow?

Running the numbers through the Sagulator, a 3" slab of pine (eastern white) 12" deep with a full 60" span (note this would be only if the legs were right at the ends) and an imagined load of 300lb in the centre has an expected total sag of 0.01", which I think most people will agree is acceptable!

how should I connect the legs?

Many viable methods. The simplest would be butt joints obviously, which would actually be surprisingly strong with thick stock and proper grain orientation. Using epoxy, holes could be drilled at various points along both faces of the joint to provide 'keying' and extra glue surface area, adding greatly to an already strong joint (see Notes bottom).

Next up with be a spline joint, the spline either ply or hardwood (with the grain oriented to go across the joint, not along it). The spline should be glued at centre only or entirely floating to allow for movement.

Possibly the best solution would be one of various kinds of housing joint, from through to blind, plain or dovetailed.

Plain housing joints, through and blind

Should I just tell them to suck it up and reinforce the connections?

Reinforcements don't have to be ugly (although many manufactured ones are of course) but perhaps most important: does it matter if they're invisible in day-to-day use? Some people will be bothered knowing they're there, others are more pragmatics and won't care because it doesn't spoil the aesthetics.

Notes
If using epoxy joint faces should not be smooth like with conventional wood glues, rough is good (approximately the level of scratch pattern left by 60 grit paper is fine). Also do not clamp tightly as should be done with other glues, a tight fit and gravity are sufficient for epoxy bonds to end up strong.

Graphus
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    The Sagulator is awesome! A great name and a great tool, thanks for sharing that! – FreeMan May 11 '15 at 12:16
  • Definitely, good resource. – Daniel B. May 11 '15 at 14:22
  • OK, I would like to add this as a comment to graphus' answer above, since this doesn't address the sagging issue, but I don't have enough reputation to comment. I want to suggest a blind dovetailed joint, which looks like a mitered joint from the side, but is stronger. You could also use a half-blind dovetail if your cousin's fiancee prefers the appearance and if you can cut large dovetails precisely enough so they look good. – Mr. Kevin May 13 '15 at 20:37
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The biggest problem you are going to have is racking. With only 3 pieces you will have to joints and any side to side motion will put a lot of stress on those joints.

Putting a back on it to absorb side to side motion would likely be enough.

There is another joint the through mortise and tenon. You put the tenon all the way through the side, (in a wider bench you could have two of them with the first picture on each side. the pin in the picture can also act as a vertical brace and an adornment. The good news here is as it 'loosens' up with use you just take a rubber mallet and pound the shims back in.

enter image description here

3" board would be plenty of almost any type of wood, hardwoods like white oak or hickory would could probably get away with 1 1/2" - 2" thick. which you could make your own by gluing thinner boards together.

EDT: the picture was added after this answer was given.

bowlturner
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  • Racking is the weakness I was concerned about, do you think the wedge would be enough t prevent it or should I still recommend that he put a brace or back on it? – Daniel B. May 11 '15 at 14:25
  • @DanielB. I think there would be some movement but as I pointed out you can pound the wedges back in. I would tend to have more bracing but the good news is that you can always add that after if you find it is loosening to frequently. – bowlturner May 11 '15 at 14:33
  • @DanielB. Racking is obviously an inherent concern if it's just two boards vertically underneath the slab. But don't overlook if it's 3" material that's a very large amount of glue surface area. I think you'd need to bash it from side to side with a sledgehammer to break those joints! Using housing joints for extra security it might be impossible to break a 'leg' free without splitting the wood apart. – Graphus May 11 '15 at 14:49
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    Depending on the design of the bench, tusk tenons may not be appropriate. I've added a comment on the question asking for clarification. – rob May 11 '15 at 15:20