Seeking advice on laying concrete base for hot-tub

14 Jul 2008
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United Kingdom

I need to make a base for a fairly large hot tub. The size is 2.4M x 2.4M and I'm aiming for a 100mm base thickness.

Could someone please advise what the best concrete mix would be and how to calculate the quantity of materials required.

Any particular ground preparation needed? The area it is being installed in has had paving slabs down for many years - these have since been lifted and whatever is underneath seems dense and well compacted.


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Whack the existing material, backfill any soft spots with type 2 or other inert hoggin, cast your 1:5 self-mixed concrete on top 100 deep, stick some A142 mesh in the mid-depth, and tamp. Job's a goodun.

4 bags OPC and 3/4tonne all-in ballast, I reckon.
Shy is not far off but you will need a tonne of ballast and 6-7 bags of dust.

5:1 is a bit rich but it won't shrink too much at that thickness. ;)
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Shy is not far off but you will need a tonne of ballast and 6-7 bags of dust.

5:1 is a bit rich but it won't shrink too much at that thickness. ;)
I bow to you on this. Was I really a bit light with cement though? Tis a long time since I did mix quants at skule....
What was down for paving slabs is going to be totally inadequate to support the weight of a 2.4m x 2.4m (8ft x 8ft) hot tub. Your best bet would be to hire a mini excavator from a local hire company not forgettng the skips (say around 8m3 of spoil) and start from scratch. Your going to need minimum 100mm depth of clean hardcore, say 50mm sand blinding which you will then have to compact down using another piece of kit from your local hire company (compactor plate), then you will be in a position to lay a 150mm reinforced concrete base. Please note: the reinforcing mesh needs to be no more than 25mm above the sand blinding - as concrete is fantastic in compression but pretty hopeless in tension, and steel is excellent in tension and is a cheaper option than having to increase the overall thickness of the concrete slab to otherwise compensate for any possible ground movement.

May be worth considering the options when it comes to the concrete - There used to be a company called mix-a-mate (not sure if they are still about?) but basically they provide the concrete mix and you barrow and lay - takes a lot of the hassle out of the work, or you have ready mix lorries, which can be slightly expensive but if their shutes cannot reach you could always get the concrete pumped-in to your oversite and all you have to do is level and tamp the concrete down.

That seems excessive considering the manufacturer is happy for it to be installed on top of decking? (they specify 6" joists at 450mm centres)

I've since removed the paving slabs and a couple of inches of gunk and found concrete underneath. I've drilled some exploratory holes and it seems to be only a couple of inches thick.

Do I lay on top of this or do I need to break it up first?
I started attacking it with an SDS chisel and very rapidly gave up when I realised it would take about a week!

I'll borrow a sledge hammer from a neighbour tomorrow! It's only a couple of inches thick so shouldn't be too much trouble!

Thanks for your help.

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