Base for a hot tub on a terrace which is asphalted - Recommendations on best type of base?

[wondergirl29]

I have a terrace (forming the roof of the flat below) onto which I am putting a hot tub
(a structural survey has been done and approved)

The hot tub manufacturers have indicated that the hot tub need to be placed onto a solid, flat base.

Various builders have proposed either:
a) pouring an approx. 40-50mm of concrete the same size as the tub (approx. 180 mm in diameter)
b) bedding in concrete paving stones with a cement mortar mix (also to ensure the stones are level)
c) following an earlier post I have proposed if they use paving stones they should use promenade tiles as they are lighter

NOTE:
The terrace is formed of a concrete base with a layer of hi-density foam and then asphalt on top.
- It is very exposed and often gets a frost in the winter
- The hot tub weighs about 200kg empty but with water and the addition of up to 6 people I have been told I should allow for as much weight as 1450kg!

Please can anyone advise what they think would be the best method and what the pros and cons are of each.
- Or if you think there is a better method?

Any advice, suggestions would be welcome.
Thank you
Ann

[lutonlagerlout]

do your neighbours mind this?
I have a feeling that they may have input
regarding hot tubs,I know a few that have them ,most get used once or twice a year max
its a lot of money for something so little used
i cannot see how the weight C 1.5 tonnes isnt going to compress the foam,and maybe split the asphalt
If i lived downstairs I would not be happy with a ton and a half of water sloshing around above me
not trying to be negative Ann, but for me it seems mad
LLL

[wondergirl29]

Thanks for the thoughts but actually my neighbour doesn't mind and it's all approved.

- so do you have actually have any advice on the base?

Thanks
Ann

[GB_Groundworks]

This needs running by an engineer that eight is far to much for an asphalt and insulation roof

We did one last year for a bar terrace above new change rooms sloped it with the foam then asphalt then the light weight promenade tiles but some of these have cracked under just foot load.

You are gonna need to spread the weight over the entire area to stop damaging the asphalt and leaking water onto your neighbours

I want a hot tub for my garden pimpin :p

[dig dug dan]

Surely the base will add even more weight on top of the roof?
I would opt for concrete, min 100mm

[wondergirl29]

Thanks Dig dug dan and GB Groundworks.
I guess that yes, just concrete will provide the lowest additional weight and be strong.

For info:
The hot tub was actually put in 10 years ago with the incorrect foam insulation used under the asphalt (it was load bearing at all) - thought it was too complicated to explain this in my original post!

- Interestingly it has been removed and new high-density foam and asphalt put back due to an insurance claim for a different part of the terrace around the corner of the building!

- The guy below had leaks from my terrace from the decked area around the corner where foot traffic over time had compressed the foam but absolutely NONE anywhere near the hot tub!

Previously it was supported on large, heavy concrete paving stones, 50mm thick supported by a concrete mortar of about the same thickness on top of the asphalt!

We're just trying to put it back with the best methods possible used as I never want to go through the mess of having it taken out again!!
Ann

[lutonlagerlout]

aha!
bearing in mind the weight could it not be put onto a piece of celotex or similar cut to size
lightweight but spreads the load?
even 1 cubic metre of concrete as a base is likely to weigh in excess of 2.5 tonnes additional weight
its difficult to asses without seeing the site but I would have thought something to cushion the weight rather than reinforce the slab was the answer
cheers LLL

[Dave_L]

Structural Engineer required, calculations etc etc. End of, sorry.

[GB_Groundworks]

Yep structural engineer something like 30mm 8x4 plastic sheets to spread the load but expensive