Underlayment For Concrete Slab, Thoughts On My System? - Flooring - Contractor Talk

Underlayment For Concrete Slab, Thoughts On My System?

 
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Old 04-07-2020, 11:30 AM   #1
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Underlayment For Concrete Slab, Thoughts On My System?


I'm going to be finishing a basement w/ hardwood for a spec house I'm building. I'm putting down the underlayment shortly and will be installing the hardwood at a later point due to scheduling.

Here is what I was thinking for under the hardwood planks, over the slab (I don't do flooring often, and when I do it's typically not over concrete) :

6 mil poly over slab, overlapped and taped (Roberts underlayment tape, unless someone has a better suggestion).
3/4 ground contact PT plywood (ideally not KDAT due to cost, see below on my reasoning). Will be installed floating.

A few questions:
* I was going to do 6 mil poly, but, has anyone had any luck/thoughts on 15lb felt paper?
* For the plywood I don't want to spend the extra money for KDAT unless I have to. I was thinking about doing PT since I have a month or two until the actual hardwood is installed and I can setup some blowers to let it dry out for a while. I'll moisture test before installing the hardwood. Anyone else have any luck doing this, or do they just use KDAT plywood?
* Other systems/suggestions instead? I've seen other systems such as using sleepers or not doing it as a floating underlayment, but.. doesn't seem worth it as the basement is dry, and it's new construction.

Thanks!
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Old 04-07-2020, 11:38 AM   #2
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Re: Underlayment For Concrete Slab, Thoughts On My System?


Is there a vapor barrier under the slab?

What is the current rh and how long has the slab been down?

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Old 04-07-2020, 11:56 AM   #3
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Re: Underlayment For Concrete Slab, Thoughts On My System?


Haven't measured yet as concrete is green (just poured last week!)

I was going to wait a month for it to cure a bit and test at that point. Wasn't planning on doing the actual moisture barrier/underlayment install until that point assuming the moisture level is OK.
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Old 04-12-2020, 08:14 PM   #4
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Re: Underlayment For Concrete Slab, Thoughts On My System?


I've been laying hardwood for fifteen years. For my home I full trowel spread bona 880 a moisture barrier/adhesive. Tapconed the plywood to the concrete so the adhesive had a chance to bite. Used different underlayments over plywood before hardwood in a couple rooms.

One room was adhesive/3/4 ply / aquabar

One room was adhesive/ 1/2 ply (do to some door issues)/ 30 lb felt.

The 1/2 inch ply plus 30lb felt feels much more solid and is a quieter floor to walk on. The felt really breaks up the hollow feel of wood in contact with concrete.

As a rule I shoot 1.5 inch staples.

Highly recommend felt over the plywood.

The glue will help prevent moisture but also will pull the plywood tight to reduce voids and hollow points.

I'm six months in and the floor is flat, even at a door that gets a ton of rain and overflow from a hip valley.
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Old 04-12-2020, 08:19 PM   #5
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Re: Underlayment For Concrete Slab, Thoughts On My System?


In the past I've read a guy on here who swore by mopping roofing tar on the slab, then overlapping layers of 3/4 ply, then hardwood.

I personally reddid a floor in a 100 year old basement apartment back in 2016 where another contractor had installed directly over rotten joists in contact with bare earth. EVERY SINGLE FASTENER rusted through to the top of the wood. Floor looked leopard print.

We fixed the joists, laid down 6mil, coated the back of the plywood with roofing tar, 15 lb felt, then hardwood.

That was three years ago and no call backs. Knock on wood.
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Old 04-12-2020, 08:21 PM   #6
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Re: Underlayment For Concrete Slab, Thoughts On My System?


Bona also makes an easy to apply concrete moisture primer that you roll on.
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Old 04-13-2020, 12:08 AM   #7
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Re: Underlayment For Concrete Slab, Thoughts On My System?


What did you do before the concrete went down for the moisture coming up????
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Old 04-13-2020, 09:44 AM   #8
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Re: Underlayment For Concrete Slab, Thoughts On My System?


In my case it is a dry basement (new construction - vapor barrier and insulation below slab), but, I'm overly cautious (don't want call backs!) and needed a nailing surface for the hardwood anyway.

However, after looking at the numbers and talking to the designer, it looks like we'll likely be going with "luxury vinyl", which I'll probably install floating and put "floor muffler" under. Have to wait for the basement to cure some more though. Useful information for the next project though.
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Old 04-13-2020, 10:47 AM   #9
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Re: Underlayment For Concrete Slab, Thoughts On My System?


Quote:
Originally Posted by sparehair View Post
I've been laying hardwood for fifteen years. For my home I full trowel spread bona 880 a moisture barrier/adhesive. Tapconed the plywood to the concrete so the adhesive had a chance to bite. Used different underlayments over plywood before hardwood in a couple rooms.

One room was adhesive/3/4 ply / aquabar

One room was adhesive/ 1/2 ply (do to some door issues)/ 30 lb felt.

The 1/2 inch ply plus 30lb felt feels much more solid and is a quieter floor to walk on. The felt really breaks up the hollow feel of wood in contact with concrete.

As a rule I shoot 1.5 inch staples.

Highly recommend felt over the plywood.

The glue will help prevent moisture but also will pull the plywood tight to reduce voids and hollow points.

I'm six months in and the floor is flat, even at a door that gets a ton of rain and overflow from a hip valley.
1/2 ply over the concrete, and I assume (?) 3/4 flooring, and you use 1.5" staples. Do any staples rebound off the concrete?
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Old 04-18-2020, 05:31 PM   #10
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Re: Underlayment For Concrete Slab, Thoughts On My System?


No, 3/4 ply wood would be preferred but I have done probably 30 floors with half inch over concrete with the 1.5 staples and have had no problems.
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Old 04-18-2020, 06:46 PM   #11
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Re: Underlayment For Concrete Slab, Thoughts On My System?


The kitchen is where you can get in trouble. In front of the dishwasher specifically.


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