# Forum Home Renovation Concreting  Concrete slab for water tank

## Programmer

Hiyas,  
Installing a small slimline water tank. Slab size about 1500 x 1000. Tank size 1000L. Since water is about 1kg to the litre, would a crushed rock base of 50mm, and 50mm concrete do? Or should I increase concrete to 100mm? Area can be ocassionally wet (under eaves but receives a hammer of rain now and then). 
Lastly, what point/size/thickness (forgotton the term) reo should I be using?  
Thanks all. Cheers  :Biggrin:

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## Gumby

You don't need concrete at all. I have a 2200L tank sitting on some fine sand which is boxed in with some treated pine sleepers. It wont go anywhere. Just get some at the local sand/soil supplier.

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## DJ's Timber

:What he said:

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## Programmer

Thanks Gum, I didn't state before, but there's a brick verandah pillar close by ('round the corner) and its 'leant' [and a bit sunk] from previous moisture/movement. OK, so it's not the preverbial 'leaning tower', but I'm worried about the damn tank moving/sinking. We've had the area checked and there are no pipe leaks/excess ground water. It's fairly sandy soil on a clay base. 
That said, I've seen other tanks installed on blue metal, pavers, bricks etc. I just can't get around the thought that the tank might move. Any further comments anyone can make to convince me? I'm HAPPY to be convinced!!!! 
Cheers again,

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## SilentButDeadly

The other lads are right.....in fact if the soil doesn't have any stone or rock in it then (as long as the site is level) the tank can sit straight on the ground without much drama.  But you should then surround the tank with a couple of inches of gravel/crusher dust so any water coming off the tank doesn't erode out around the edge of the tank.....

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## Gumby

Don't forget, the concrete will be sitting on the ground as well.  :Wink:   
The tank PLUS the concrete is more weight than just the tank which isn't a big one. You'll be surprised how fast you go through 1000L. 
If you are down to the clay, it will be fine. The weight is srtaight down so there's no real shear force.

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## DJ's Timber

There is really no differnence from having it on concrete or sand. 
If the concrete is sitting on top of the ground where you propose to lay it and the ground moves or sinks the concrete will still move unless you plan on excavating the site and putting down foundations for concrete slab that the tank will sit on, which is alot of money and overkill for a 1000L tank.

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## Programmer

Thanks all, yes, the tank is a small one. Just enough to keep the garden green(er). :Cry:   
I'm not plumbing to the toilet etc. There's also likely to be changes to the house +-2 yrs so I'm happy to simply place a base of crushed rock and secure it. 
Gum, did you further dress your treated pine sleepers? I'm not happy placing them right up against the house with soil that can stay moist for months... (on the shade side). There's been evidence of termite history here in the past. Obviously, I get it checked, and allow zip wood direct soil contact unless treated and absolutely required - but it still concerns me.

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## Gumby

Treated pine is fine just sitting on the ground. You could give them a dose of decking oil if you like and I'd leave a small gap between them and the house . In fact my tank is around 250mm from the house. It will eventually rot but we're talking many years and those boards are easy to replace. It's not as if they are in a retaining wall or anything drastic. 
You'd need a lot more than 1000L to even think about connecting to the plumbing so you are on the right track as far as it being only for the garden. Its also good to use for washing the car. 
I just put a depth gauge on mine over the weekend. Cost me $25 at Bunnings for 2 connectors and some clear plastic tube to run up the side of the tank. Now I can tell the level in the tank just by looking at the tube.  :Smilie:  
(as I mentioned before, use sand in the pit, not soil)

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## Grunt

I've got a 23,000l tank on a sand base with treated pine edge bits. It ain't going anywhere. 
Chris

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## schwerdty

The pro's use blue metal which they screed flat with a spirit level.  It needs to be only 20-30 ml thick which sounds like only a couple of barrow loads.  Given that you are on clay soil, blue metal may be the best idea (always follow the pro's I say) and the tank will go beaut! 
Plus, don't feel like you need to 'hold the tank in' it can sit completely un-bordered if you like.

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## KevM

If it is a a GI or colourbond tank do not sit it on a surface which will retain water, that is a recipe for disaster as the surface will retain moisture causing the bottom of the tank to corrode.  I used to do a concrete base with timber slats, but later changed to 12mm aggregate as this provides plenty of support but also allows water to drain away and air to circulate, keeping the base dry. 
Don't use material which does not allow the water to drain and keep the base dry.  Definite no-nos for GI tanks are FCR, fat sand or sitting directly on concrete.

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