Air dried oak for a garden bench - what expansion allowance?

[Benpointer]

Hi there,

I am making a garden bench out of air-dried english oak boards. The bench will have interlocking slats for the seat and back (running fore and aft, and up and down, rather than along the bench). I have posted a couple of pics below.

My original plan was to thread the 20mm wide slats tightly together onto a 10mm stainless steel bar running the length of the bench where the slats interlock (at the back of the seat / base of the back). I planned to thread the ends of the bar and secure with s/s nuts and washers at each end.

However, now I am wondering what will happen when the bench gets saturated with rain? The oak will obviously want to expand but would a 10mm s/s bar stop them expanding and just cause them to compress?

Or do I need to leave small gaps between the seat and the back slats - and if so how much gap?

Any ideas / thoughts / experience would be greatly appreciated.

Thanks

 

[MikeG.]

I like that design. It should work perfectly well. How are the back slats fixed to the horizontal member behind them? The only negative about the design is that you've got an awful lot of end grain sitting almost horizontally, and that of course is where the rot will start.

Why don't you just reduce the number of slats by 1, and thus have a 10mm gap either end. That'd work beautifully.

 

[Benpointer]

Thanks. Leaving space for expansion across the 'hinge', if I can call it that, is an interesting idea. Just wonder how I square it with the need to keep the stainless steel bar in place.

"How are the back slats fixed to the horizontal member behind them?"... Stainless pocket screws from behind, underneath the back rail. Probably

 

[Benpointer]

PS I feel pretty relaxed about the end grain of the back slats sitting almost horizontally. They are rounded there's not really a flat surface. I think it will easily see me out :wink:

 

[Benpointer]

Forgot to post a photo of the finished bench - seems to have worked out ok, quite pleased with it...

 

[Benpointer]

 

[Jameshow]

How about fitting some rubber bushes before the nuts to either end these will compress rather than the oak being crushed by the rods.

A motor parts store should have some in stock.

Just a thought

Cheers James

 

[Benpointer]

Hi James I did think about that but in the end I just left a small gap (circa 1mm) between each vertical and horizontal slat, where they threaded onto to s/s bar. Since the slats are quite a tight fit on the bar (10mm holes, 10mm bar) it has all seemed to stay in place and survived the winter outside, uncovered, fine.

 

[Jameshow]

Hi James I did think about that but in the end I just left a small gap (circa 1mm) between each vertical and horizontal slat, where they threaded onto to s/s bar. Since the slats are quite a tight fit on the bar (10mm holes, 10mm bar) it has all seemed to stay in place and survived the winter outside, uncovered, fine.

Sorted - looks fantastic!

Cheers James

 

[Cabinetman]

Also you used air dried oak which has quite a high moisture content to start with I think you’ll be ok.
That is a really stunning looking bench – I really like that. Ian
Ps, my only comment would be that I think the legs are a little on the thin side.

 

[Benpointer]

Thanks

Yes, I agree about the legs. It was all done from left over green oak planks which I had air dried for a couple of years. The boards were originally about 30mm thick but by the time I had planed them enough to clean them up they are down to about 23mm.

I think they are strong enough but I agree thicker legs would look better.