# Framing a floor over concrete



## Brian Peters

I've got a project coming up and have some questions. These folks have a sunroom which is built with concrete blocks, concrete floor and old aluminum storm windows. The floor is 4 inches lower than the adjacent kitchen floor. Plan is to open things up between the two, knock out the old windows and the block wall down to the level of the concrete floor, frame in a new wood framed wall to accommodate new windows and insulation , outside will get vinyl siding.

I want to raise the floor to the same level and they are wanting a hardwood floor so seems like a wood subfloor would be best, I'm just wondering about possible moisture issues. The concrete is probably twenty years old if that makes any difference. 
I'm sure someone of you has run into something similar, just looking for some advice on this.


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## griz

You could always test the concrete for moisture content.

There are products that are an applied coating to the concrete to seal from moisture. They are usually expensive & labor intensive.

Another option is to put down a roll out vapor barrier, not a retarder, but a listed vapor barrier.

They work quite well, easy to install and price isn't bad.


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## scottktmrider

You might want to lay some polly or styphrom before you build the floor.And treated lumber if its going to be in contact with the concrete.


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## ubcguy89

I would put down a vapor barrier and frame sleepers


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## EricBrancard

I've done the double layer of 1/2" plywood over EPS foam as detailed in the Fine Homebuilding finished basement article from several years ago. That made for a pretty nice floor system.


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## asgoodasdead

like someone else said, vapor barrier and green lumber


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## C2projects

As it's been said vapor barrier and PT lumber, I'm not sure how large the span is but I have ran into several issues of either having to shim under my joists or cut them at an angle to account for the slope. Double check the level of the concrete it could vary depending on if there are a bunch of dips in the finish. Be prepared for every joist to be custom cut.


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## Brian Peters

C2projects said:


> Be prepared for every joist to be custom cut.


I am expecting that.


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## hboogz

Wanted to get something clear on this build. In the OP's post he mentioned that this sunroom is 4 inches lower than the adjacent kitchen. So, to build up to level from the poured concrete, you're suggesting something like this:

vapor barrier sheet across concrete.
mechanically fastening PT 2x4's across concrete acting as "joists"
lay osb/plywood on top of PT joists to prep for finish floor which will be level with adjacent kitchen floor.
start framing.

is that about right? Also, what are the choices for fastening the PT to the concrete? Ramset's? Is it standard practice to silicone the screwhead, since it's punctured through the vapor barrier ?

Thanks,

Harry.


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## greg24k

Knock it down what needs to be knocked down, have a step down finished nice in wood and use engineered glued hardwood floor or insulated floating floor. You will save so much work and you will make same money if not better and it will look real nice.

Also Depends on the budget, see if you can upgrade the job to radiant floor heat, nice trim package, etc. Jobs like this are good money makers if people are willing to spend.

Good luck:thumbsup:


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## Bearded Wonder

Last 2-3 jobs like that I just had my concrete guy come in and pour a cap up to the level I need. Works like a champ, and you aren't custom cutting joists or waterproofing or plywood or deflection etc. 

Last couple cost me maybe $1000-$1500, but saved me more than that in labor and materials.


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## hboogz

ClaytonR said:


> Last 2-3 jobs like that I just had my concrete guy come in and pour a cap up to the level I need. Works like a champ, and you aren't custom cutting joists or waterproofing or plywood or deflection etc.
> 
> Last couple cost me maybe $1000-$1500, but saved me more than that in labor and materials.


That's cool. I hear a lot of the curing time in concrete ( 30 days, etc), I assume you laid an engineered or tiled above the concrete for the finish to account for expansion ? I'm guessing with the engineered you would still need a thin vapor barrier ?


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## Bearded Wonder

hboogz said:


> That's cool. I hear a lot of the curing time in concrete ( 30 days, etc), I assume you laid an engineered or tiled above the concrete for the finish to account for expansion ? I'm guessing with the engineered you would still need a thin vapor barrier ?


Yes. I did the concrete at the start of the projects and by the time it was ready to install flooring it had cured for almost 6 weeks. I'm not sure if there's a technical specified time to wait. 

I've glued engineered wood flooring to a couple, and I've also done tile or carpet. Once it's poured and cured, I treat it the same as a new construction on a slab. Perhaps if there was a question about the moisture barrier under the original slab you could roll on some kind of waterproofing.


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## SclafaniBuilder

I just did this one. 2x4 joist.


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## hboogz

ClaytonR said:


> Yes. I did the concrete at the start of the projects and by the time it was ready to install flooring it had cured for almost 6 weeks. I'm not sure if there's a technical specified time to wait.
> 
> I've glued engineered wood flooring to a couple, and I've also done tile or carpet. Once it's poured and cured, I treat it the same as a new construction on a slab. Perhaps if there was a question about the moisture barrier under the original slab you could roll on some kind of waterproofing.


Awesome. Thanks for the reply!



SclafaniBuilder said:


> I just did this one. 2x4 joist.


Very cool, this what I had pictured. What are those blocks on the corner of each row? Are those supports for radiant heating ?


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## Brian Peters

hboogz said:


> Awesome. Thanks for the reply!
> 
> Very cool, this what I had pictured. What are those blocks on the corner of each row? Are those supports for radiant heating ?


It looks to me like the blocks are fastened to the concrete and support the joists slightly above the floor , he 's got blocks fastened into the side of the joists at those locations.


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## SclafaniBuilder

Brian Peters said:


> It looks to me like the blocks are fastened to the concrete and support the joists slightly above the floor , he 's got blocks fastened into the side of the joists at those locations.


Correct. The floor is shimmed in spots as well. It's a solid floor. Had the whole apartments cdx sitting on it, never even flinched.


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## hboogz

SclafaniBuilder said:


> Correct. The floor is shimmed in spots as well. It's a solid floor. Had the whole apartments cdx sitting on it, never even flinched.


Thanks! what did you use to fasten into the concrete?


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## SclafaniBuilder

hboogz said:


> Thanks! what did you use to fasten into the concrete?


3" pressure treated ramset with washer


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## greg24k

In any case I would stay away from using plastic over concrete slab.


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## SclafaniBuilder

greg24k said:


> In any case I would stay away from using plastic over concrete slab.


I just do what the plan says, this architect is a goofus.


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## Brian Peters

greg24k said:


> In any case I would stay away from using plastic over concrete slab.


Why? (I'm on here to learn, this is the kind of discussion I'm looking for. )


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## greg24k

Brian Peters said:


> Why? (I'm on here to learn, this is the kind of discussion I'm looking for. )


Because of mold issues... use XPS foam directly on the concrete floor, follow manufacture instructions how to seal properly and that will provide you with a continuous semi permeable vapor barrier... not to mention it will enable water vapor to diffuse harmlessly while providing moisture barrier.

Good luck :thumbsup:


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## MTN REMODEL LLC

greg24k said:


> In any case I would stay away from using plastic over concrete slab.


Greg.... Could you explain further...TIA


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## asgoodasdead

tar paper.


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## Deckhead

In my experience Greg is right, you don't want anything completely impermeable or it will completely trap moisture and not be able to evaporate into the air, thus creating a habitat for mold. You want to slow the moisture to a manageable level for the amount of air underneath the structure so the water vapor doesn't metastasize into liquid which will over time exponentially increase the moisture underneath.

If it can transfer into the air at a slow enough pace it will go bye bye, if not than it just recycles itself along with the added moisture from the concrete. Its the reason tyvek is not impermeable nor is tar paper.


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## MTN REMODEL LLC

Deckhead said:


> In my experience Greg is right, you don't want anything completely impermeable or it will completely trap moisture and not be able to evaporate into the air, thus creating a habitat for mold. You want to slow the moisture to a manageable level for the amount of air underneath the structure so the water vapor doesn't metastasize into liquid which will over time exponentially increase the moisture underneath.
> 
> If it can transfer into the air at a slow enough pace it will go bye bye, if not than it just recycles itself along with the added moisture from the concrete. Its the reason tyvek is not impermeable nor is tar paper.


Deck...Thanks.... I sorta undersytand where you are coming from.... but there is a material difference....

when you are roofing with permeable felt or siding with permeable/breathable Tyvek, you are protecting moisture entrapement from your sheathing or decking that can rot...

that is not the case with a concrete slab....

I guess you think that a delayed or slow migration of water vapor from a damp SOG, is somehow better for flooring material than a more rapid migration.

Justaposed to this view is that most (or at least a majority) of underlayments for SOG by manufacturers are a vapor barrier, not a permeable vapor retarder. Although, I admit that some are a retarder.

Not saying you're wrong.... just a discussion.

Best


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## Deckhead

If the finish is going directly on top then I would agree that impermeable is necessary, however this is speaking of an air gap in which case naturally water vapor will be in the space no matter what. Plastic will trap the moisture coming up from the concrete and mold or mildew will develop.

Here in Florida everything is concrete and impermeable is not good with an air space with all the humidity we have. It was explained/proven to me by PE/Archy who is a genius and an a-hole and forced me to eat crow.


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## MTN REMODEL LLC

Deckhead said:


> If the finish is going directly on top then I would agree that impermeable is necessary, however this is speaking of an air gap in which case naturally water vapor will be in the space no matter what. Plastic will trap the moisture coming up from the concrete and mold or mildew will develop.
> 
> Here in Florida everything is concrete and impermeable is not good with an air space with all the humidity we have. It was explained/proven to me by PE/Archy who is a genius and an a-hole and forced me to eat crow.


Good point and consideration:thumbsup:

In fact, there is an underlayment for direct application of floating floor on SOG that has tiny bubbles/protrusions/ribs on the underside so you get a minor air flow/ventilation between the flooring and the slab...... similar to as you are pointing out.... just to a smaller/lesser degree.


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## hboogz

greg24k said:


> Because of mold issues... use XPS foam directly on the concrete floor, follow manufacture instructions how to seal properly and that will provide you with a continuous semi permeable vapor barrier... not to mention it will enable water vapor to diffuse harmlessly while providing moisture barrier.
> 
> Good luck :thumbsup:


Are there any other semi-permeable vapor barriers you would recommend besides XPS foam?

I believe someone mentioned tar paper, which I believe is inherently semi-permeable.

The takeaway being, on slab or poured concrete on top of slab, it's best if you use a semi-permeable barrier that allows for vapor to pass-through in order to prevent any potential moisture trapping issues ?


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## Deckhead

Pouring concrete over a slab you would want plastic unless its suspended over top (which I honestly don't see a reason for)


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## jlsconstruction

asgoodasdead said:


> like someone else said, vapor barrier and green lumber


You do know green lumber isn't the same as pt, right?


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## EricBrancard

hboogz said:


> The takeaway being, on slab or poured concrete on top of slab, it's best if you use a semi-permeable barrier that allows for vapor to pass-through in order to prevent any potential moisture trapping issues ?


Yes, allow the moisture to enter the space and deal with it mechanically. Much better than trapping it and allowing problems to develop unseen.


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## tonylumps

How come no one has mentioned sealing the concrete. I did it for years.


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## MTN REMODEL LLC

tonylumps said:


> How come no one has mentioned sealing the concrete. I did it for years.


Tony.... What have you used/recommend for sealing concrete relative to moisture migration from below.....

I've used sealers to prevent surface staining etc.... are those very effective in blocking moisture migration.

Would UG Water Bloc be effective on SOG?

TIA


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## tonylumps

MTN REMODEL LLC said:


> Tony.... What have you used/recommend for sealing concrete relative to moisture migration from below.....
> 
> I've used sealers to prevent surface staining etc.... are those very effective in blocking moisture migration.
> 
> Would UG Water Bloc be effective on SOG?
> 
> TIA


Every time I have to seal concrete and go to a supply yard they never carry the same brand sealer.I have used it for sub grade and slab.I have layed Vinyl and carpet on it. Never had a problem.What ever you get make sure you read there label. Everyone has there own way of using it.Once sealed do not penatrate. If you are going to put a sub floor over it Lay your joist and PL your scabs down and wait until it sets then nail. The PL adhesive holds better than nails anyway.


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