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Old 06-15-2012, 08:28 PM   #16
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Hardwood over concrete slab


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Originally Posted by joecaption View Post
By doing it your way your going to throw off every baseboard, door opening, and where it meets any other flooring.

Why in the work would use you 2, Layers of 3/8's. If you insist on doing it your way use 3/4" Advantec sub flooring.
It's not something I have come up with. I found a similar suggestion as one of the options for concrete installation on the vendor's website. I also found similar suggestions on other websites as coming from installers. I am aware that there are different opinions among installers.

Since you are asking, the reasons for using two layers of 3/8" plywood instead of a single 3/4" layer are (1) to avoid nailing the plywood to the concrete, (2) to avoid poking the moisture barrier with the fasteners. Apparently, people found out that the moisture can make its way through the holes in the moisture barrier, so it's a good idea to keep the moisture barrier intact. The two layers screwed together will form a large robust block covering the whole surface.

Today I asked the vendor for their opinion on this procedure and their answer was that it's fine.

Could you explain to me what you mean by, "you are going to throw off every baseboard, door opening, and where it meets any other flooring"?

- Baseboards: I know I have to replace the baseboard. I am going to do it anyway

- Door openings: Today I have carpet, and will be replacing it with hardwood. What would you suggest that I do with regard to the door openings instead of "doing it my way"?

- "Where it meets other flooring": I am going to replace the whole flooring of the house


Last edited by alessandrot; 06-15-2012 at 09:22 PM.
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Old 06-15-2012, 09:32 PM   #17
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Hardwood over concrete slab


Hardwood over concrete can work if all the stars align. If you have any moisture at all, it's toast. The two layers of thin ply makes not sense to me. loose laying the ply makes no sense to me.

I don't care what some other installers say, they're not at your house. I don't care what the vendor says, his job is to encourage you to do it. Plastic over a concrete slab is asking for it. I asked what the manufacturer says, still no reply. OK, so I will check for you just tell us what you've got.

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Old 06-15-2012, 09:42 PM   #18
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Hardwood over concrete slab


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I asked what the manufacturer says, still no reply. OK, so I will check for you just tell us what you've got.

Jaz

It's Bellawood, 3/4" solid American Walnut.
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Old 06-15-2012, 09:58 PM   #19
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Hardwood over concrete slab


Plastic on raised slab is not asking for it.
1 ply is plenty no need for 2 but the op can do as wishes. The op wants hardwood install and has raised slab, there isn't any issues other then 2 layers of ply would make floor level to high.
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Old 06-15-2012, 10:05 PM   #20
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Hardwood over concrete slab


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Plastic on raised slab is not asking for it.
1 ply is plenty no need for 2 but the op can do as wishes. The op wants hardwood install and has raised slab, there isn't any issues other then 2 layers of ply would make floor level to high.
I am using two 3/8" layers screwed together instead of one 3/4" layer, so the total height should be about the same.
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Old 06-15-2012, 10:26 PM   #21
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Hardwood over concrete slab


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I am using two 3/8" layers screwed together instead of one 3/4" layer, so the total height should be about the same.
It's not the same. You are not fastening the ply to the concrete. How are you gonna nail two thin layers together? The screw will only bite the bottom sheet by maybe 1/4". How about the cleats for the hardwood? How short can you get? They're gonna hit the concrete maybe. It sounds to me that although the concrete is higher than another area next to it, it is not suspended, it's still on the ground, on grade. Plastic over concrete can be a nightmare if there's too much moisture. In any case I haven't heard the results of the moisture test yet.

It seems Bella wood doesn't want me to find instructions specifically for their 3/4" hardwood. Instructions a vague, not a good thing.

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Old 06-15-2012, 10:48 PM   #22
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Hardwood over concrete slab


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It's not the same. You are not fastening the ply to the concrete.
So what? Are you saying that the total height of the flooring will be significantly greater just because I am not nailing the plywood to the concrete? How much greater do you expect it to be?

What I have read from other installers is that the two layers of plywood form a very strong and stable block over the whole room--as stable as you would get by nailing thicker sheets individually to the concrete.

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How are you gonna nail two thin layers together? The screw will only bite the bottom sheet by maybe 1/4".
Screws plus staples. I will do some experiments.

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How about the cleats for the hardwood? How short can you get? They're gonna hit the concrete maybe.
1-1/2" at 45 degrees. Starting from the top of the tongue, they won't be long enough to poke the plastic under the plywood.

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It sounds to me that although the concrete is higher than another area next to it, it is not suspended, it's still on the ground, on grade.

The top surface of the concrete is at least one foot higher than the lawn outside. Does that count as "on grade"?


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Plastic over concrete can be a nightmare if there's too much moisture. In any case I haven't heard the results of the moisture test yet.

I doubt I am going to have one done.

I have ripped off the carpet and the underlying padding from one room, and I did not see any sign of moisture. No particular smell either. It's very nice and dry. I don't know what I should look at.

Last edited by alessandrot; 06-15-2012 at 10:55 PM.
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Old 06-15-2012, 11:31 PM   #23
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Hardwood over concrete slab


Your slab is on grade, there's dirt under the concrete which wicks water into the concrete. No one would build a house with the floor at the same level as the grass, I don't think.

You need a moisture test. It's simple and you do it yourself. Read the directions in the box.

Quote:
I have ripped off the carpet and the underlying padding from one room, and I did not see any sign of moisture. No particular smell either. It's very nice and dry. I don't know what I should look at.
Of course you didn't see any moisture, if there was excess moisture it evaporated through the carpet. Now you wanna put a sheet of plastic down, completely different scenario. If there is moisture wicking, you're gonna have problem down the road maybe.

The good old fashioned way to install 3/4" on a slab is with tar + 2x4's, then nailing the wood to the 2x4's. IF the moisture level is low enough.

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Old 06-16-2012, 12:10 AM   #24
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Hardwood over concrete slab


If as someone else recommended you opt to go with an engineered hardwood floor then you could use http://www.dricore.com/en/eIndex.aspx which is very simple to install. It allows for an air gap between the concrete and the flooring so the subfloor can breath and greatly reduces the chance of damp related issues becoming really problematic. It also helps with improving warmth in colder climates so ultimately is an energy saver.

A lack of installation instructions on the manufacturer's website would be a bit if a concern for me especially if as an installer I was expected to warranty it.

And as to why not all installers say the same thing when it comes to their recommended method ... Techniques and products and building codes change over time and also vary by location depending what is available locally and what other factors such as climate may be an issue. Some building codes or standard practices for hot regions differ from those parts of the world exposed to colder or wetter extremes, for example. Some installers may be using the latest products on the market whereas others may be happy with more traditional tried and tested methods. Personally I would recommend whichever method you feel most comfortable with skills wise and assuming you can get everything you need locally. Here in the town I am based the Dricore product just started appearing in stores so unless installers watched HGTV or did their research many do not know it exists. Any way good luck with our project.
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Old 06-16-2012, 04:43 AM   #25
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Hardwood over concrete slab


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Originally Posted by JazMan View Post
Your slab is on grade, there's dirt under the concrete which wicks water into the concrete. No one would build a house with the floor at the same level as the grass, I don't think.
I think it matches the definition of "raised slab on grade" that I have seen on a few websites.


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Originally Posted by JazMan View Post
You need a moisture test. It's simple and you do it yourself. Read the directions in the box.

Thanks for the advice. I got a moisture meter. I'll take a few measurements in different rooms and let you know the results.


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Originally Posted by JazMan View Post
Now you wanna put a sheet of plastic down, completely different scenario. If there is moisture wicking, you're gonna have problem down the road maybe.

The good old fashioned way to install 3/4" on a slab is with tar + 2x4's, then nailing the wood to the 2x4's.

Could you explain why the tar would work better than the plastic sheet, and what would be the benefit of the empty space under the plywood?

Last edited by alessandrot; 06-16-2012 at 07:34 AM.
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Old 06-16-2012, 08:37 AM   #26
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Hardwood over concrete slab


Technically speaking there should be a layer of poly vapour barrier between the dirt and the concrete slab to prevent moisture wicking up but almost guaranteed it has been penetrated to run plumbing so would never count on it 100%.

I would still encourage you to think about using an engineered hardwood which can be installed as a floating floor with a vapor barrier underneath ... But you have to check the manufacturers guidelines as far as install over concrete slab to see if they have any special conditions or if the method such as in this link is OK ... http://thehousingforum.com/how-to-in...wood-flooring/. Even though we do flooring installs all the time, we still check what each manufacturer recommends to stay within the parameters of their warranty.
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Old 06-16-2012, 10:05 AM   #27
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Hardwood over concrete slab


I did a few measurement using an inexpensive electronic moisture meter. I made sure the pins were firmly against the concrete.

I got numbers between 4% and 10% at different spots of the house (different rooms). There seems to be a correlation between the reading and the distance from the exterior walls (the closer to the exterior wall, the higher the reading) and with what I imagine to be the height of the floor relative to the ground (the higher the height, the lower the reading). The highest reading was 10.

How are these numbers?


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