# Forum Home Renovation Concreting  Do you need to put plastic under a slab?

## davegol

Putting in a small slab (2.5m x 2.5m) for a spa to sit on.
Do you have to put a plastic membrane under the concrete?
I've seen it done with and without. 
Thanks!

----------


## johnc

Plastic is a moisture barrier generally to stop ingress from beneath the slab. If this is outdoor in an exposed area it probably doesn't matter.

----------


## phild01

I'd probably use plastic if the area is moist with little sun, just to alleviate any algae growth.

----------


## davegol

It's outdoors, north facing, lots of sun. Don't think I'll bother. Thanks for the quick response! 
Sent from my Nexus 5X using Tapatalk

----------


## PlatypusGardens

I thought it was so it goes off slower = Less cracking?

----------


## webtubbs

> I thought it was so it goes off slower = Less cracking?

  I thought the same. If the ground is dry, plastic prevents water from being sucked out of the concrete while curing. You can always wet the ground prior to laying the concrete for a similar effect.

----------


## METRIX

> I thought it was so it goes off slower = Less cracking?

  It's a vapour barrier, stops moisture being sucked out of the ground into the slab for the rest of the slabs life, mandatory for inside or habitable slabs, but for outdoor stuff up to you if you want to use it.

----------


## PlatypusGardens

All this talk about slabs is making me thirsty

----------


## Bloss

> It's a vapour barrier, stops moisture being sucked out of the ground into the slab for the rest of the slabs life, mandatory for inside or habitable slabs, but for outdoor stuff up to you if you want to use it.

   :What he said:  For the small cost of adding a membrane it's good practice outside too. Not an issue re curing - main thing is to keep slab damp for the first few days and ideally for the whole curing period, but since this is not strictly a structural slab not so critical. But a spa of say 2m x 2m and 900mm deep will hold around 1100-1200 litres i.e.: 1100-1200kgs of water plus the weight of the spa itself so requires a reinforced slab well made.

----------


## JB1

Actually a spa of internally 2m x 2m x 0.9 deep will hold 3600L. 
Sent from my SM-G930F using Tapatalk

----------


## davegol

according the brochure it's 2280 x 2280 x 900 weighs 460kg dry and 2110 filled and holds 1650L of water
I'm going with a 2.5 x 2.5 slab, at 150mm thick with lots of reo.

----------


## Bloss

> Actually a spa of internally 2m x 2m x 0.9 deep will hold 3600L. 
> Sent from my SM-G930F using Tapatalk

  Would if they were full - they aren't usually and the measure is not a straight multiple of 2mx2mx.9m because the inside is bolded and a smaller volume. As the OP has said his is a bit larger. Most units weigh between 400-500kgs and then the water depends on how many seats etc. Small ones for 3-4 people at around the 1100-1200L and larger ones more. Anyway the critical factor is that the overall weight will generally be around the 2000kg and more so although it is evenly spread the base needs to be designed and built with that in mind. The OP is probably over-engineering, but the small cost of doing so is probably with the peace of mind to him.

----------

