Concrete slab

[CurlyDave]

I had a 24'x42' slab poured for my metal garage/shop in Jun05. Here's what it looked like before the concrete pour.
The black plastic is 10-mil Visqueen laid over about 4" thickness of gravel.

The picture shows exactly the right way to do it.

Gravel/Visqueen/Sand/Rebar

Don't forget the sand on top of the barrier, it prevents cracking.

In my part of CA this is the way both the engineer and the bldg department insisted I should do it. I tried to leave the sand out & got a ration of chit from both of them, so I added it.

 

[MrJimi]

When I lived in Maine, we used plastic. We live in Florida and its code here, never heard of sand over it though?
Jim

 

[Humbleto]

Thanks everyone. I'll just go with the plastic and be done with it. The guy didn't have a problem putting it down, he just didn't see the need for it in the area that I am building.

 

[slowzuki]

Concrete contractors like to leave the vapour barrier out because the slap dries out quickers and sets up quicker so they can go home sooner. This is not the best for the concrete though, it makes a weaker floor.

 

[radman1]

The plastic will definitely slow down how quick the cement cures. I did mine on a cool day and couldn't run the power trowel until 12 hours later. Nothing like being by yourself, in a poorly lit shed after midnight smoothing out the cement with a power trowel.
Unless there is a strong reason not to do it, I would use the plastic. Really cheap compared to the overall cost. In my area, the cement will sweat in the spring time as the temperature fluctuates. Also keeps down radon levels!

 

[Farwell]

My attached two car garage does not have a moisture barrier under the slab and when the slightest bit of moisture is in the air it sweats a lot. My soil is composed of 12 inches of top soil on top of sand. My pole barn has a moisture barrier as well as 2 inch foam insulation under the slab and does not sweat. My small red barn does not have any moisture barrier under it but the slab is directly on sand and it does not sweat.
Any concrete work I have done will have a moisture barrier under it and if it is going to be under a structure I will add insulation as well.
I didn't know about the sand on top of the barrier and don't know the purpose of the sand other than to hold the barrier down and protect it from damage while the concrete is being poured. Sounds like a good idea and will probably have it added on any more of my projects.
Farwell

 

[RobJ]

Around the weekend place they all use it, also every home in Houston. So I used it on the house and later on the garage. Just to cheap of stuff not to use.

 

[EddieWalker]

All this talk about a plastic moisture barrier stoping a concrete slab from sweating has me scratching my head in bewilderment. Here in East Texas, 80 percent humidity is pretty common in the summer. The air holds tons and tons of moisture in it. I don't have a plastic barrier under my slab and I've never seen moisture come up from the ground through the concret. My slab is dry at all times.

After a few months, the ground under a slab is so dry that when you go to break out any part of a slab, you normally need a jackhammer just to dig in it. There is none, to very, very little moisture in the soil under a slab.

The faom makes sense to me in Norther and cold climates. The plastic sheet doesn't.

Adding things to a mix just because they don't cost that much is a real good way to run out of money. Some places you use 3/8 rebar on 2 foot centers. Others use wire mesh. But if you really want a strong foundation, you go to half inch. But why stop there? You can put them on 12 inch centers and than realy beef it up with 5/8 rebar. I was told that's code in parts of Florida. 5/8 rebar on 12 inch centers.

So far I haven't read a reason to put the plastic down that makes any sense to me. I'll be happy to start using it if there is a valid reason to do so, but until then, I'll do like the other contractors I know and leave it out.

Eddie

 

[mudcat]

I like it because I believe the plastic allows the conrete to cure slower and more even. If poured directly on top of fill the water will be pulled out causing the slab to dry to fast. I have seen slabs sweat just thought it was because it was cooler than the hot sticky air. Never looked at it as moisture coming up from underneath. Here in Florida it is called a vapor barrier. That leads me to believe that it is more for Radon protection than any thing else.

 

[CTyler]

All this talk about a plastic moisture barrier stoping a concrete slab from sweating has me scratching my head in bewilderment. Here in East Texas, 80 percent humidity is pretty common in the summer. The air holds tons and tons of moisture in it. I don't have a plastic barrier under my slab and I've never seen moisture come up from the ground through the concret. My slab is dry at all times.

After a few months, the ground under a slab is so dry that when you go to break out any part of a slab, you normally need a jackhammer just to dig in it. There is none, to very, very little moisture in the soil under a slab.

The faom makes sense to me in Norther and cold climates. The plastic sheet doesn't.

Adding things to a mix just because they don't cost that much is a real good way to run out of money. Some places you use 3/8 rebar on 2 foot centers. Others use wire mesh. But if you really want a strong foundation, you go to half inch. But why stop there? You can put them on 12 inch centers and than realy beef it up with 5/8 rebar. I was told that's code in parts of Florida. 5/8 rebar on 12 inch centers.

So far I haven't read a reason to put the plastic down that makes any sense to me. I'll be happy to start using it if there is a valid reason to do so, but until then, I'll do like the other contractors I know and leave it out.

Eddie

My experiance has been that its the moisture coming up through the ground not the humidity in the air. Here are just a couple of my personal experiances.

Our last house was built in a subdivision before most of the other houses. They poured the garage floor on ~4" of pea gravel, no plastic. Everytime we had a fair amount of rain or exteneded wet period the garage floor would be damp. This was the same for the other houses as well except for one. The guy across the street put down plastic and his was the only one that would stay dry.

We built a new house in the country and a 50x64 shop several years ago. We did not put down plastic in the garage because it did not need it. The Garage floor was poured on top of ~6' of recycled concrete. Not much chance of moisture wicking its way through that. If we have that much moisture I'll be collecting animals and looking for an Ark.

We did put down plastic in the shop on top of ~8" base of crusher run and it stays dry as well.

So do you need plastic? The answer is...it depends however as a general rule I'd say yes. I've seen the same type of results listed above in Iowa, Nebraska, Montana and California.