Layering vapor barriers?

[Rustang]

My concrete contractor initially put down a 6 mil vapor barrier until I had requested a thicker one. My architect's plans spec'd 20 mil, so they triple layered the vapor barrier with 6 mil, then 10 mil, and then another 10mil sheet.

The crushed stone went on top today and the pour is slated to be next week for a 7", 4000 psi slab. The slab is only 21' by 21', and we live in Southern California.

So, is this an acceptable practice? We don't see frost heaves or much water coming up from the soil 'round these parts. Am I worrying too much about this vapor barrier thing? Or is this extra insurance so that my epoxy floor will stay intact?

-Jimmy

 

[bmwpower]

People have done it before, but I did not - based on some research. Water gets between the sheets and will get stuck between the layers. Kind of the same reason you don't want to coat your interior basement walls with a water proofing material if you've already coated the outside of the walls.

Not sure how critical it is as I only found a few bits of info on the web some time ago. I'll see if I can dig up those links again...

 

[Rustang]

Thanks for doing the research, bmwpower! I think I may be a bit meticulous but I just wanted to make sure. I'm well away from any groundwater sources and the slab is pretty much above grade.

I was a little unimpressed that my contractor just put down the standard 6mil instead of what was called for in the plans.

-Jimmy

 

[Steve in Mi]

Maybe I don't understand the purpose of the crushed stone, thought it was to effect drainage under the floor. Now if I read right you put the moisture below the stone? I don't understand, what is the purpose of the stone?

 

[bmwpower]

Maybe I don't understand the purpose of the crushed stone, thought it was to effect drainage under the floor. Now if I read right you put the moisture below the stone? I don't understand, what is the purpose of the stone?

Wait a minute....I just reread that. You're right, it should be on the bottom. I hope the stone was on the bottom, top on top of the barrier. Jimmy?

 

[bmwpower]

Thanks for doing the research, bmwpower! I think I may be a bit meticulous but I just wanted to make sure. I'm well away from any groundwater sources and the slab is pretty much above grade.

I was a little unimpressed that my contractor just put down the standard 6mil instead of what was called for in the plans.

-Jimmy

Not sure I've ever seen a 20 mil barrier... not saying it doesn't exist. One thing is for sure, the higher mil barriers are CONSIDERABLY more expensive per sqft than the lower mil barriers. Probably the reason the contractor just double/tripled it up.

 

[Steve in Mi]

I see I left a word out.

Now if I read right you put the moisture below the stone?

Should have been;
Now if I read right you put the moisture barrier below the stone?
Or could have been;
Now if I read right you put the poly below the stone?

 

[Hurricane]

Maybe I don't understand the purpose of the crushed stone, thought it was to effect drainage under the floor. Now if I read right you put the moisture below the stone? I don't understand, what is the purpose of the stone?

ive seen this several times on here. crushed stone is NOT for drainage. in fact if its compacted "minus", water will have a hell of a time penetrating it. it would need to be clean stone for drainage. you use a rockbase because it is much harder than dirt and doesnt give like dirt will.
________
digital vaporizer

 

[Rustang]

You guys are absolutley correct! I just emailed my contractor (I'm out of town at the moment) and asked why the crushed stone is ON TOP of the vapor barrier.

I owe you guys a big keg of Sierra Nevada.

Keep you posted...

-Jimmy

 

[bmwpower]

ive seen this several times on here. crushed stone is NOT for drainage. in fact if its compacted "minus", water will have a hell of a time penetrating it. it would need to be clean stone for drainage. you use a rockbase because it is much harder than dirt and doesnt give like dirt will.

Actually, it provides both. Stone, even fines, drains better than any kind of dirt. I used 3/4 stone

 

[bmwpower]

You guys are absolutley correct! I just emailed my contractor (I'm out of town at the moment) and asked why the crushed stone is ON TOP of the vapor barrier.

I owe you guys a big keg of Sierra Nevada.

Keep you posted...

-Jimmy

The barier is probably compromised at this point. i would definitely put another layer on top of the stone.

 

[Rustang]

Update: Contractor will be talking to engineer on Monday to see what the fuss is all about (MY fuss). I actually found a great .pdf file I sent to both the architect and the contractor to read and discuss. Here it is for reference:

Omitting "blotter layer of crushed stone"

From a discussion I had with our architect on Friday, she told me that placing the blotter layer of crushed, compacted stone was standard practice for residential slabs in this area as specified by her soils engineer. In any case, the matter is under review and I'm gettting my moisture free slab somehow...even though we live in drought-stricken LA basin!

Updates to follow...

-Jimmy

 

[Rustang]

Update: Contractor and conrete contractor were just following architect's plans that had been approved for code. Evidently, its a relatively common practice out here to place a 2 inch "blotter layer" of sand over the vapor barrier. Plans called for this atop a 20 mil vapor barrier and a 4" slab over it.

A little miscommunication early on led to this and when the dig started I specified something different. I wanted at least a 5" slab to be poured, so the concrete contractor is going to raise the level of the slab and the forms and rebar. We just had the city inspector come out yesterday to sign off on the demolition (yes, the insepct this after everything has been hauled away...go figure), addition to the house, and the new garage. So everything was cool to the inspectors eyes...except for the fact that the south wall of the new garage was sharing the property line with the neighbors. Long story short, it was eventually deemed okay.

So the sand was cleared away today so that they pour directly over the vapor barrier (3 layers: 6 mil, 10mil and another 10 mil). Rebar was double checked to make sure it'll sit about 2" below the surface of the new slab, and I get a resultant 7-9" slab as a result of clearing away the 2" of sand blotter layer!!

The slab is now extra thick (I guess extra insurance for earthquakes), has #4 rebar 16" on center placed 2" below the surface, and will be a 4000 psi mix. The slab is monolithic with 2 foot-deep footers being integral with the slab. Pour is on Friday when I'm out of town.

Wish me luck!

-Jimmy

 

[bmwpower]

Update: Contractor and conrete contractor were just following architect's plans that had been approved for code. Evidently, its a relatively common practice out here to place a 2 inch "blotter layer" of sand over the vapor barrier. Plans called for this atop a 20 mil vapor barrier and a 4" slab over it.

A little miscommunication early on led to this and when the dig started I specified something different. I wanted at least a 5" slab to be poured, so the concrete contractor is going to raise the level of the slab and the forms and rebar. We just had the city inspector come out yesterday to sign off on the demolition (yes, the insepct this after everything has been hauled away...go figure), addition to the house, and the new garage. So everything was cool to the inspectors eyes...except for the fact that the south wall of the new garage was sharing the property line with the neighbors. Long story short, it was eventually deemed okay.

So the sand was cleared away today so that they pour directly over the vapor barrier (3 layers: 6 mil, 10mil and another 10 mil). Rebar was double checked to make sure it'll sit about 2" below the surface of the new slab, and I get a resultant 7-9" slab as a result of clearing away the 2" of sand blotter layer!!

The slab is now extra thick (I guess extra insurance for earthquakes), has #4 rebar 16" on center placed 2" below the surface, and will be a 4000 psi mix. The slab is monolithic with 2 foot-deep footers being integral with the slab. Pour is on Friday when I'm out of town.

Wish me luck!

-Jimmy

Sounds good. I would see if you can get someone there to watch the pour. Do not let them add water before the pour. Make sure the finish is good. Make sure it's covered from the weather. Keep the slab moist, etc.

 

[bigdav160]

Does anyone see the lesson here? Building practices very widely across the country. What's acceptable "up North" or "out West" may not be acceptable in YOUR area. A lot of people like to quote IBC or UBC or just watch TV but those practices may not be what's used in your area.

Jimmy, they put poly down so the ground does not **** the water out of your slab while curing.That would weaken it.

 

[Wardrum]

Sounds good. I would see if you can get someone there to watch the pour. Do not let them add water before the pour. Make sure the finish is good. Make sure it's covered from the weather. Keep the slab moist, etc.

Good advice.

Also, you said you were going to put epoxy on your floor. If that is still your intention, DO NOT let your contractor put sealer on the new concrete. Epoxy should go on unsealed concrete.

 

[Rustang]

I'll have my buddy drop on by during the pour to call me when the fiasco happens.

Everyone is on board now with the plans. Both contractors know what psi to use, no added water, light broom finish (as per the epoxy flooring guys), no slope, keeping slab moist for the two days I'm out of town, etc.

Thanks for all of the enlightenment. Indeed, it was a good lesson in regional differences and coding.

Pics after the pour in a new thread. I guess.

-Jimmy