Patio sub base for hot tub

[Chrispy]

Hi all, firstly thank you for all the posts and information on this site. Has been so useful in planning my project. So the bulk bags arrived today and the compacting plate arrives tm and I've suddenly begun to doubt myself!

We have excavated an area 6.67M long x 3.57M wide for a new patio to a depth of 170mm in addition to the existing patio we have by the house, which is 3.31M long by 5.54M x wide, where we have removed existing slabs and the rubbish mix of sharp sand with the odd spot of cement that they were laid on. So we will end up with an L shape patio.

Underneath the existing patio is a 100mm deep MOT Type 1 base, which we are reusing, but we will need to rough up the surface and compact / level off again as there wasn't really a drainage slope before and there were dips in the base height, not where it has sunk, but where more sharp sand was used to level it out instead. The 50mm concrete slabs hadn't shifted in 16 years so am reasonably happy with it.

On the new "second patio" I was going to whacker the soil, lay membrane, lay 100mm compacted MOT Type 1, then a 35mm full mortar bed and 35mm slabs (450mm x 450mm - Marshalls Perfecta). I was pretty confident this would be good enough for taking the weight of a hot tub we hope to add in future as it would be a static rather than dynamic load. Full, it would weigh about 2 - 2.5T. I've seen a 150mm sub base can support up to a 7.5T vehicle, so had thought I'd be fine with 100mm, but we do have some clay in the soil and are built on an old quarry site, so I'm having a last minute panic! Do I need a deeper sub base or is 100mm OK do you think?

Other than that, aside from where the Hot Tub will live, I'm planning on adding a 1 in 60 drop away from the house, although because our lawn is higher than the existing patio the first patio will end lower, than the second begins (I can't start higher at the house end as would not have the 150mm minimum before the DPC). So rather than joining the two, I'm going to run a 225mm gravel channel between the two, connected by one slab (either ramped up slightly, or built in as the smallest of steps). I've also got a 100mm gravel channel between the house and first patio and a 350mm channel between both patios and the wall/fence to the side, the thinking being that this will aide drainage to be safe (and if I totally cock it up I could always get a drain added, but I don't believe drainage will be an issue as isn't currently and the first patio is very sheltered).

If I do need a deeper base, my likely option would be to go for a 30mm higher second patio (so it would be the patio slab thickness higher than the adjoining grass), as digging the ground at this point will be very hard given the dry spell we have had - would anyone see a problem with that, other than mowing the edge of the lawn?! That would give me about a 130mm sub base, maybe 140mm, once I've whacked the soil.

Thanks a lot in advance for any advice.

[Tony McC]

It's that mention of "soil" that worries me. Is this really a soil, or is it a sub-soil? Soil contains organic material and is BAD NEWS beneath any paved area. Sub-soil is organic-free (or very largely so) and therefore less at risk.

Other than that, the rule of thumb is that 150mm sub-base is always better than 100mm, but for a fairly regular-issue Hot Tub/Germ Bath, 100mm of *thoroughly compacted* Type 1 aggregate would most likely be adequate.

[Chrispy]

Thanks Tony. My bad on the soil comment. I'm pretty sure we are at the subsoil/substrate layer. We've removed the top 160-170mm odd of turf and soil, making our way through old tree routes (removed), clay lumps and some massive hunks of limestone, in fact one could almost have been a patio slab . I'd describe the place we are at as light in colour and very sticky when wet - boots ended up five times their weight when I was walking around on it in the wet a month back (prob the clay content I guess).

Thanks for the answer, gives me some more confidence in what I've taken on. Staked out now and would say we'll have around 100-120mm depth of sub base material across the site when topped up and compacted.

[MikeG]

I would take reassurance from your old patio that has been on 100mm sub base if you haven’t had any issues with that? Given that it hasn’t failed and is in the same ground conditions as your proposed.
I used to put down bases for hot tubs for a local company that supplied them. Spec from manufacturers was for 100 mm concrete base. I wouldn’t of had any issue in using your spec to support one.