Mitre joints

[gatesy]

Hi all, after a little advice on mitre joints. I'm building an outdoor table and bench seats, and the legs for the benches are going to be s loop of timber at either end as opposed to 4 legs. I'm using 80mm square remilled messmate. Was going to mitre joint the loop but was wondering if I should reinforce the joints the with biscuit/Dowel/spine somehow? The benches will obviously take the load, and there will be 3 legs across 1800 span (would one either end suffice? I can't see just a glued mitre joint being strong enough but happy to be talked down in that! Cheers

[orraloon]

Reinforce the miter joints with hardwood. A contrasting colour would look good too. I would use epoxy as it is an outside table and that will not come apart with moisture. The the flat frame pic is what I am getting at. You can scale it up to suit your frame and use more that one key per joint too if you like.
http://www.woodworkingarchive.biz/machine-cut-joint/reinforced-mitre-joint.html

Regards
John

[joe greiner]

Half-lap joints would be stronger - a lot stronger.

[RoyG]

Hi all, after a little advice on mitre joints. I'm building an outdoor table and bench seats, and the legs for the benches are going to be s loop of timber at either end as opposed to 4 legs. I'm using 80mm square remilled messmate. Was going to mitre joint the loop but was wondering if I should reinforce the joints the with biscuit/Dowel/spine somehow? The benches will obviously take the load, and there will be 3 legs across 1800 span (would one either end suffice? I can't see just a glued mitre joint being strong enough but happy to be talked down in that! Cheers

Gatesy,

I'm not absolutely sure what you mean by "the legs for the benches are going to be s loop of timber at either end as opposed to 4 legs"..... a photo or drawing would help immensely to make sure we are both talking about the same thing. However, I'll have a go at responding, based on the assumption that the design that you envisage will be something like the following photograph.
356840
If your proposed design will be similar to the photograph at left, then there are plenty of problems that will be created if you use use mitre joints on the leg frame.

Structurally, the loads imposed by people sitting on the bench have to be carried through the legs to the ground. If the legs are joined to the bottom cross-piece (that runs between the legs) using mitre joints, then the downward load will cause those mitre joints to fail, probably on first loading. You could reinforce the mitre joints with splines, but I doubt that a even a reinforced mitre joint would be strong enough to carry the weight of four adults (based on a 1.8 meter long bench).

If you want to go with this design, then go for a type of joint that allows the seated load to be transferred down the leg and onto the ground. If it was me, my first choice would be a mortice & tenon joint, with the tenons on the bottom of the legs fitting into blind mortices (not thru-mortices) cut into the horizontal cross-piece that joins the legs. This approach also has the advantage of preventing the end grain of the legs from being in contact with the wet ground seeing it's a bench seat for outdoor use. As you can see the end grain in the cross-piece shown in the adjacent photograph, I think that is most likely how the bench in the photograph was built.

An alternative as suggested by someone else in this thread, would be to use half-lap joints to join the lower cross-piece to the legs. The disadvantage with the half-lap approach is that half of the end grain of the leg will still be in contact with the ground and therefore subject to timber damage by water getting into the end grain.

Having built a chair (many many years ago) that had the front and back legs joined this way, I can also advise you that unless the ground or surface you are placing the bench on is perfectly flat, this design bench can have a tendency to rock back and forth, and will be unstable. Based on my previous experience with that dodgy chair design; one way of minimising the instability might be to put a wooden foot under the cross-piece, under each leg.

Hope that helps,

RoyG