Pole Barn Concrete

foggy1111 · Apr 25, 2009

I have purchased a new 30x40 pole shed and found a guy to do the site fill and compaction and do the concrete work after the pole shed is built (I like that one guy is responsible for this work...no excuses for not doing it right). The way the building is put up....he will use the lower boards for the concrete forms....which is a common practice around here. He is putting in 1/2" rebar on a 3' grid....and a six-bag mix. But....I forgot to talk with him about putting down a vapor barrier under the concrete slab. I spent considerable time getting a good tight building with decent sidewall soffits and ridge venting to reduce problems with condensation.....but I forgot all about the under floor barrier.

What are some thoughts? Is a vapor barrier needed under the concrete or not? How much extra should I pay to have my concrete & fill guy install the vapor barrier?

radman1 · Apr 25, 2009

On my shed (47x72'), I used a single large, thick sheet of plastic. It should not cost much for the installation and the plastic is relatively cheap. A rough guesstimate for installation is about 1-1.5 times the cost of the plastic. The downside to the vapor barrier/plastic is the cement curing time goes up significantly and needs to wait longer before the cement hardens to work the cement such as power troweling. I poured mine in AM and couldn't walk on it until about midnight to power trowel it. (It was the cool time of year when I poured my cement.) Make sure the cement is not too wet. With the vapor barrier, the excess moisture has no place to go but evaporation.

Whether you need it or not is another question. If you have lots of rain or questionable drainage, then yes. If you are worried about radon, plastic is probably the cheapest barrier available. The plastic is cheap. Installation is cheap. Ask your installer what it will cost.

Cat_Driver · Apr 25, 2009

Ya why not add a thick platic barrier, it's cheap enough and may keep ants and termites from popping out where ever they feel like it.

In fact I think you can also get a bug guy out and spray the foundation with some sort of life time insect killer that would help.

biggerten · Apr 25, 2009

Get it down, it will help keep the concrete from getting slippery. He should also provide you with a choice of finishing, don't go for mirror smooth. It's tempting, I know, but you want *some* texture.

Especially since you took pains to ventilate to prevent condensation, and the water vapor will just come up through the concrete like it's not even there.

As for the concrete taking longer to cure, that's a good thing.

The moisture barrier is cheap (check out Menard's) and will take him very little time to install.

Edit: By getting slippery, I mean after the fact, like some spring day when you're trying to lift something heavy and a foot goes out from underneath you. This is a safety thing.

LD1 · Apr 25, 2009

biggerten said:
Get it down, it will help keep the concrete from getting slippery. He should also provide you with a choice of finishing, don't go for mirror smooth. It's tempting, I know, but you want *some* texture.

Especially since you took pains to ventilate to prevent condensation, and the water vapor will just come up through the concrete like it's not even there.

As for the concrete taking longer to cure, that's a good thing.

The moisture barrier is cheap (check out Menard's) and will take him very little time to install.

I agree 100%. Without the barrier, humid times of the year, it will be like some one hosed it down...all the time.

Longer to cure is a very good thing. The longer it takes to cure, the stronger it is and that means less surface cracks. Also, get it as thick as you can. The less water they use in the mix, the stronger it'll be. Once you have it finished, it's a good idea to keep the top wet, to make it take longer to cure. Hose it down with a fine mist a every few hours for about 7 days.

The reason for all of this is becase concrete doesn't "dry" it "cures". There is a chemical reaction that takes place(thats why it gets hot) and it needs water for this to happen. If all the water evaporates before the cement can cure, it isn't as strong.

/pine · Apr 25, 2009

The plastic also tends to support the chairs that hold up the steel (in sandy soil)...another benefit is the concrete will retain the water longer for a harder cure....keeping it wet for the first 3 days is optimal

6sunset6 · Apr 25, 2009

One other thing you might want to think about or 2 actually if you are in a winter climate . Insulation board on top of the vapor barrier and tubing in the floor for heat.
I did a floor with vapor barrier and no insulation right next to a floor with both.
The floor with out insulation would act like a condensate sink on certain days and be very slippery.
On the tubes. i. e. radient heat in the floor. I heat my house like that but I never thought about it for my barn/shop. Hot water panels on the roof with some photo voltaic to run a pump and the floor can be 40d in the winter for free. If you do the tubes you need the insulation.
If you do the tubes now you can add the rest anytime. I kick myself everytime I think about it, which is often Nov to March.

Willl · Apr 25, 2009

I know it rains alot here but I'm on good drainage and without any plastic barrier under my slab.

Going on 8 years with no humidity/condensation problems.

My PB vents up through the side panel ridges and out the roof ridge vent. Doesn't everybody's ?

foggy1111 · Apr 25, 2009

I am putting up an unheated and uninsulated building. I am not too sure about how every other building vents.....but I wanted a vermin and bug proof building....so I have sealed up all those siding vents and have put venting in via the soffits and at the roof peak. I think people do it differently for different purposes. I want to keep out the critters and still vent....thus my venting solution.....sorta like a house vents.

Thanks for all the responses....I think I will talk to my masonry guy about adding the plastic vapor barrier.

foggy1111 · Apr 25, 2009

6sunset6 said:
One other thing you might want to think about or 2 actually if you are in a winter climate . Insulation board on top of the vapor barrier and tubing in the floor for heat.
I did a floor with vapor barrier and no insulation right next to a floor with both.
The floor with out insulation would act like a condensate sink on certain days and be very slippery.
On the tubes. i. e. radient heat in the floor. I heat my house like that but I never thought about it for my barn/shop. Hot water panels on the roof with some photo voltaic to run a pump and the floor can be 40d in the winter for free. If you do the tubes you need the insulation.
If you do the tubes now you can add the rest anytime. I kick myself everytime I think about it, which is often Nov to March.

I also have this type of heat in my home garage and in my basement. I think its the greatest. Many of the heated pole buildings here have that type of heat too....and most really like it. THis building is not heated however....I am not even putting electric in....just some ridge lites.....very basic storage.....but secure and vermin and bug resisitant - I hope.