Bench Design and Wood Movement

[Tom Hyde]

I'm designing and building a sitting bench out of yellow cedar (Alaskan Yellow Cedar, Nootka Cypress, Cupressus nootkatensis). This will be used outdoors so I'm debating design choices while considering wood movement. Top dimension is 14" x 44". Legs are inset 6" from the ends and 1" from the faces.

The wood is quarter sawn and 2.25" thick. The top is three pieces. The legs will be joined directly to the top with double wedged through-tenons. I've cut the mortises and tenons and dry fit them. I have not joined the three top pieces together yet, just clamped everything together for now. The legs are joined in the outer two top pieces with another piece in between.

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The inside distance between the tenons is a little over 7". Using an online calculator, I estimate potential wood expansion and contraction of the top at 1/16" between the tenons (going from current 8% moisture content to 15%).

Question: Can I get away with edge gluing (and likely throwing in some 14mm dominos double pegged from underneath for piece of mind) the top into a solid piece AND putting stretchers between the legs front to back?

Options I'm considering:
a. Edge gluing top and adding stretchers between the legs near the bottom of the legs.
b. Edge gluing top with no stretchers between the legs (which eliminates any wood movement problems but sacrifices strength)
c. Splitting the top into two pieces with a curved cut down the middle and adding stretchers between the legs top and near the bottom. Also, adding a slotted screw up through the top stretchers into the top near the center cut.

I hope that's clear. I might be overthinking this!

Appreciate any thoughts on this.

[Bob Lang]

I think you'll be OK with just edge gluing the top pieces together. Structurally I don't think you need any stretchers front to back. The legs are pretty beefy and solidly connected to the top. If the top moves the legs will stay with it. If you add a stretcher into the mix at the top it won't play nice with the other elements. Stretcher at the bottom might be all right, but the legs would need to bend a little as the top expands and contracts, or the joints between the stretcher and legs would give up. I think the risk outweighs any benefit of adding the stretchers. Looks solid as it is.

Bob Lang

[Robert Engel]

If there is no stretchers why are you worrying about it? The legs will move with the top, right?

A trestle type base will accomodate movement.

[Wayne Lomman]

Glue the top into one piece. If you came across a slab the right size you would not hesitate to work with a single piece of timber. Butt joining will be satisfactory provided your glue will withstand the weather.

A stretcher near the bottom of the legs is your best option. The movement in the seat won't be enough to bother the joints down low. Also consider a length ways stretcher between the end ones. The purpose of all these stretchers is to extend the life of your tenons. The tenons are fine if you never move it or kick the legs but that is never going to happen. Single tenons tapered all sides are best if you don't want stretchers. The more you sit on the bench, the tighter they get. Cheers

[Tom Hyde]

Thank you all for the feedback. Very helpful! I’ve been considering the lengthways stretcher as well. I was thinking it wasn’t necessary given the 2.25” long doubled up wedged through tenons ... but I see your point as I picture the bench getting dragged, legs kicked, etc.

[Tom Hyde]

Thanks again for the feedback. I am going to build a "simple" bench for the price point but for this one, I went all out. This build gave me a good idea on time versus feature so I can build a series of benches at different price points. First time I've worked with yellow cedar, it's really nice and doesn't splinter. Does seem to swell quite a bit with glue, though. The through tenons one the top were a nightmare to get fully seated, even though I did a prior dry fit. Overall, it's not perfect but for my first bench, very happy.

Yellow Cedar Bench. 44"L x 14"W x18"H. Finished with OSMO Exterior UV Protection Oil, Clear, Satin.

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[Andrew Hughes]

That’s a very good looking bench. I would like to suggest if your benches are going to live outdoors consider not exposing your jointery.
Your work will last much longer. The exposed ends suck up water then dry cracks open then it just get worse.
Anything we can do to keep water out of joinery.
I’m also a fan of AYC
Good luck

[Tom Hyde]

That’s a very good looking bench. I would like to suggest if your benches are going to live outdoors consider not exposing your jointery.
Your work will last much longer. The exposed ends suck up water then dry cracks open then it just get worse.
Anything we can do to keep water out of joinery.
I’m also a fan of AYC
Good luck

Excellent point! I’m thinking of using a sliding dovetail joint (on a front to back hardwood top stretcher between the legs) to hold the top down which will also allow for wood movement with a solid top while negating the need for an apron