Adding flooring to concrete

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Leftside
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Adding flooring to concrete

Post by Leftside » Sat Aug 12, 2017 9:53 am

Hi all, I would like to add flooring to this room. The room has carpet on top of concrete slab. The room's floor is also lower than the rest of the house.

I'd like to raise the floor by adding 2x4s, sheathing and then tiles. Is this possible?

Can I just glue the 2x4s to the concrete? I am concerned that screwing them in will weaken the concrete which is only a few inches thick.

Here is a pic:

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Shannon
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Re: Adding flooring to concrete

Post by Shannon » Sat Aug 12, 2017 1:55 pm

It is possible for sure construction adhesive should work but really once the 2x4s are installed and if you install a couple rows of blocking between them and add the plywood it should be pretty sturdy. If the concrete is not to flat you will need to shim low areas to stop the 2x4s from flexing when people walk on them.
Tile floors usually are required to have 1" of plywood under them or use an uncoupling membrane over min. 5/8" plywood with joists 16" o.c. This should prevent cracking or broken tiles
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Leftside
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Re: Adding flooring to concrete

Post by Leftside » Sun Aug 13, 2017 9:31 am

Thanks Shannon, I will have to do some research on the membrane. Os there an advantage to using membrane over plywood?

Also, do I frame the joists up against the studs in the wall? So i would build a box with 2x4s then put the joists inside like i was framing a wall? Or just run the joists along the footers in the wall?

Thanks again.

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Re: Adding flooring to concrete

Post by Shannon » Sun Aug 13, 2017 2:07 pm

The advantage is usually you can get a thinner overall floor thickness in the case where you have existing 5/8" or 3/4" plywood you either would normally add 1/2" plywood over it or add the 1/4" membrane which is usually closed set to 3/8 including the thunder used to lay it.

Yes build the floor just like a wall .also use joist hangers
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Leftside
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Re: Adding flooring to concrete

Post by Leftside » Tue Aug 15, 2017 8:57 am

So the issue that I see mow is that the new floor may be higher than the existing floor in the home. This is assuming i use 2x4, 1 inch plywood and then lay the tile down on top.

What can I do to try and make the floor level with the homes existing flooring? Can I use 2x2s (or similar) instead of 2x4s?

Also, what is this thunder?

Thx.

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Shannon
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Re: Adding flooring to concrete

Post by Shannon » Tue Aug 15, 2017 9:51 am

So how much higher are we talking?
Oh that was a mistake, that should have been thin set mortar.
Also a correction on the Ditra thickness it is available in 5/16" and 1/8" thick. Usually the thin set mortar bed used to place the Ditra would add about 1/8" extra thickness.
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Leftside
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Re: Adding flooring to concrete

Post by Leftside » Tue Aug 15, 2017 10:16 am

Well the 2x4 and 1inch plywood meet the height of the original floor. The membrane, mortar and tile would all exceed the height.

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Re: Adding flooring to concrete

Post by Shannon » Tue Aug 15, 2017 1:11 pm

Ok so if you use membrane you do not need 1" plywood, you would need a min. of 5/8" T&G plywood with the joists at 16" O.C. So I would say that you could cut the 2x4s down to what you think you need, but you need to be sure they are shimmed or tight to the concrete every 2' so there is no movement.
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Leftside
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Re: Adding flooring to concrete

Post by Leftside » Thu Aug 17, 2017 8:06 pm

Thanks Shannon, I assume I have to remove the carpet, right? Any chance I can leave it since it is glued on?

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Re: Adding flooring to concrete

Post by Shannon » Thu Aug 17, 2017 9:21 pm

No, pull it up for sure if you get moisture in there you will be sorry you left it.
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Leftside
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Re: Adding flooring to concrete

Post by Leftside » Tue Oct 10, 2017 9:43 am

So, i pulled up the carpet. And am thinking about adding the subflooring. Few questions though:

1. I don't have to scrape the adhesive off do I?

2. Should i use a self leveler to level the floor?

3. I plan on using glue to install the joists or framing, but i would like some nails or screws. What nails/screws do i use? How do i install them (do i hafta drill or is there a power tool for this?)?

The exterior door will be lower than the floor. Should I raise it? Create a step in the room? Or just leave it?

Thanks again.

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Re: Adding flooring to concrete

Post by Shannon » Tue Oct 10, 2017 7:29 pm

The adhesive can stay
No leveler need on the old floor if you are building it up anyways
So you would be using 2x4s I believe from our earlier conversations so you will need fasteners as well as the construction adhesive . I would likely use tap cons driven through some blocking placed between the joists laying flat. The blocking can be nailed to the joists and then shim things if they need it and fasten to the floor.
For the door I would likely raise it if you can and have the step outside.
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Re: Adding flooring to concrete

Post by Leftside » Wed Oct 11, 2017 4:00 am

Thank you. One more thing. Do i need to seal the concrete or add any moisture barrier?

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Re: Adding flooring to concrete

Post by Shannon » Wed Oct 11, 2017 7:16 am

Sealer will not really work with the old adhesive still stuck down but it would be a good idea to lay down a heavy black plastic but because you will be using construction adhesive that won't really work . I would just use P.T. lumber for the joists. Besure to use ACQ screws though or Hot dipped galvanized nails for anything that will be penetrating the P.T. lumber.
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Leftside
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Re: Adding flooring to concrete

Post by Leftside » Wed Oct 11, 2017 10:03 am

So if I didn't use adhesive, I could lay down black plastic? How many inches apart would the screws need to be?

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Re: Adding flooring to concrete

Post by Shannon » Thu Oct 12, 2017 9:11 am

I would say every 4'-6' and for sure at any spot that is shimmed to hold the shims in place
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Leftside
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Re: Adding flooring to concrete

Post by Leftside » Fri Oct 13, 2017 10:35 am

Ok thanks Shannon. Looks like if i lay the 2x4s flat, add 1” subflooring I will be about half an inch off (give or take depending on tile and tile adhesive) from being level with the other room. Can i add half inch shims to compensate? If so, how far apart do they need to be? I am still looking for 2x2s or something, but I was just curious about the shims.

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Re: Adding flooring to concrete

Post by Shannon » Fri Oct 13, 2017 8:01 pm

Yes you could use 1/2" plywood if it works. Since the 2x4s are laying on the flat they are not that strong that way so I would shim every 12" to be sure you have no deflection in the floor which in turn would cause cracking tiles
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