Deck to hold hot tub

Sep 19, 2006 4 Replies
T

Hi



I've been asked to build a 15' x 12' wooden deck to hold a hot tub. Most of the joists can rest on a small retaining wall, a concrete slab and a wall plate fixed to the garage wall.



The tub holds 1300 + litres of water & up to 6 adults, so adding the (as yet unknown) weight of the tub itself I could be looking at 2 tonnes here.



Would I increase the strength much by spacing the joists at 12" rather than



16" or would extra noggins be a better bet?

Cost isn't the major issue, I just want to make sure it's strong enough.



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S

will the footing for the retaining wall take the extra weight? it would be like parking a couple of cars on it !!

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T

I'd definately over engineer. 8" timbers, 12" centres and lots of intermediate supports along the length. Noggins will just prevent twisting. Running the joists the shortest distance (12') I'd go with a 3' span max. I assume than means the joists underneath the tub would be supported in at least 2 places.

I base this purely on experience with a neighbours tub and no scientific reasoning!

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A

Online calculators should help. A quick look turns up

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- it's american so not ideal, but suggests that 2 x 10 softwood joists at 16" centres can span 11'6" with a 2 tonne dead load. Playing about with the options should give you a good guide.

Andrew

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T

I'll add my 2d worth...strength is less an issue than flexure. Make a truss, or use a really well screwed down stressed skin of marine ply to stop things wobbling. In fact i'd do both. Mock it up small scale with balsa CA glue and so on, and push it every which way to see how it bends. Then add struts and plates till it stops.

Then copy that.

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