timber choice for deck with a spa

[georgeandmary]

Hi,
Just wondering if anyone can help with timber size choice or advice for building a deck with a spa on top. We currently have 1200mm joist span and 1200mm bearer centres. Joist spacings are 450mm. Our foundations are 350mm width and 600mm depth. Wanting to have information to confirm whether this is sufficient for a 6 person spa load. Timber is OB hardwood, 100mm x 75mm, is this sufficient? Could i double these underneath the spa area? thanks from george and mary

[brynk]

gday george and mary

can you provide some further info - what is the height of the deck? what is the volume of water that the spa will hold during its use?

r's brynk

[georgeandmary]

Hi brynk top of deck is (is planned to be.) 500 off the ground with total weight approx 1500-1800litres / kgs so i would make the calculation 2000 kgs. The spa will not be moved from one place to another.
Thanks

[brynk]

gday again

what are the outside dimensions of the spa? this way i can work out what the load is over its footprint

what type of timber have you chosen? seasoned? unseasoned? hardwood? which species?

r's brynk

[georgeandmary]

Hi Brynk The spa dimentions are 2.1 m x 2.0 m. The 100 x 75 is OBhard wood and is not kiln dried. I think it is rated at f 11.

Happy new year to you also.

[brynk]

gday - the new year remains to be seen as happy... maybe i didnt drink enough vino

your spa with 2000kg of water and another 600kg of people is putting down around 6kPa over the 4.4sq.m footprint. (2600kg * 10 m/s/s / (2*2.1)). a residential deck built from the timber framing code (as 1684.2006) is rated to 2kPa; it will be more economical for you if you can put the spa onto the ground below the deck - is this possible? if not, lets look at the worst case of your design - doubling the frame under the spa. (btw, for the deck only, your design is more than sufficient! incidently, what are your deck dimensions? how high off the ground is it?)

so, assuming unseasoned f11 - 100x75 bearers - 1900 continuous span and 500 canti; your span 600; around 3 times greater in load carrying capacity (it will actually be slightly more because the relationship is not linear) ~ 6kPa

joists: 100x50 at 450 centres? 2400 continuous span and 650 canti; your span 600; around 4 times the carrying capacity (at least, but slightly more); ~ 8kPa

now your footings - 9 of them - (there will be 4 posts in each axis under the spa) a bearing load of 26kN / 16 = 2.8 kN each. your post might be 100x100 ~ 280kPa at the bottom of the posts; 25MPa concrete (general purpose) can resist just under 100 times the force you are applying - more than sufficient - the main concern is - what type of soil are you putting the footings into??

under the spa you want around 6kPa load carrying since the spacings of everything have been closed up then your plan should be sufficient <looks for="" the="" touch="" wood="" smiley="">... note though, the code does not allow for such extrapolation to loads greater than design loads in the resi code. however, common sense does if you are being pessimistic - anyway, when you get approval from your local council you will have your engineer's certification of design attached anyway

this reasoning will cave if your height goes above around 1.5 m under the spa - you will need some bracing or go down a little deeper with the footings.

r's brynk
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[Loki429]

Is it possible to place/mount this spa on the ground and build the deck around it? Would save a lot of hassle with structural requirements for the deck.
Even if you had to build up the ground slightly and put a slab on it - it would be easier than strengthening the deck.
You could mount the spa so the top is flush with the deck, or so the spa is ~400mm above deck level. This height above the deck makes it very easy to get in and out of the spa as you can simply step over the edge of it and not have to climb down or up to get into it.

Also think about what happens if one or two people jump in the spa at the same time and create a big splash/movement of water. You're going to have about 2 tonne of spa wanting to wiggle around.

Just a thought...

[georgeandmary]

Hi To all that have passed on information deck is well under way.(Taking longer than anticipated) Will have photo's up when complete.

Thanks again.

[brynk]

did you opt to put the spa on-top or cut it in like <a href="https://www.woodworkforums.com/showpost.php?p=662609&postcount=40">loki's deck</a> ??

good luck with the build

r's brynk