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I am ripping up tiles on the ground floor of my home and would like to install 1/2 inch engineered hardwood. It is over a concrete slab on grade. What is the best and most economical way to do this once the tiles are removed and the concrete is cleaned and leveled?

-Can I float a plywood subfloor and then nail the hardwood to it? If so should I leave expansion gaps between the plywood and the walls? I have seen this suggested on other forums but doesn't seems to make sense to me. Shouldn't the subfloor be screwed down if the flooring is to be nailed to it?

-Or should I secure the subfloor to the concrete and then nail the wood flooring to that? What is the best way to go about doing this?

-A modular subfloor like Dricore seems great but expensive and maybe overkill for an on grade floor?

What is the best recommendation for this type of project? What subfloor should I use, how should it be installed, where should the vapor barrier be placed and what is the best way to secure the engineered hardwood flooring to the subfloor?

  • What's wrong with the tiles? – iLikeDirt Apr 12 '15 at 16:54
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If you're not wedded to a nail-down install, the best route to go with here is a floating floor with nothing but a soft underlayment between the flooring and the slab. This is an accepted install method for engineered hardwood over a slab, and your flooring manufacturer/seller should be able to provide you with instructions for how to do it and not void the warranty.

As for vapor barriers, you only need to worry about that if your concrete slab doesn't already have a vapor barrier underneath it. Obviously there's no way to easily check, but if the slab was poured in the last 20 or 25 years, there's a very good chance it already has one. In any event, once the current flooring is off, you should have a calcium chloride moisture test performed on the slab. Where you live matters, too. I live in the desert, so soil moisture is a non-issue here, and my slab's lack of a vapor barrier underneath it is irrelevant. If you're in the pacific northwest, it's a different story.

If the moisture test shows that you need a vapor barrier, choose a closed-cell foam underlayment, not something water-sensitive like cork.

  • I've been told that floating floors can be problematic with heavy furniture, cabinets or appliances on top of them. I will be installing about 900 sq feet in a mostly open floor plan. Would a floating floor installation be ok over that sq footage or would I have problems with spaces opening up or buckling? Is it possible to nail down the flooring to the floating subfloor? Or is that not recomended? Thanks! – Kate Apr 12 '15 at 15:29
  • I don't understand that claim. The underlayment will be like 1/32" thick; it's not going to sag, especially if the flooring is a glue-together tongue-and-groove type. The planks will basically be held together as one solid very rigid mass. And remember that you're going to be holding it down at the edges with baseboards. – iLikeDirt Apr 12 '15 at 15:56
  • So you would recommend both a floating subfloor and floating engineered flooring with the flooring glued together? The flooring I have is 1/2 inch engineered tongue and groove. It says it can be nailed, glued or floated. Would you recommend any underlayment between the subfloor and the floated flooring? – Kate Apr 12 '15 at 16:02
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    You don't need to build a floating subfloor. It would be a waste of time and money. Install your flooring over the slab with foam underlayment between them, and glue the boards together, applying glue to the grooves. Leave a 1/4" gap between the boards and the walls, and undercut the door frames. Just follow the manufacturer's instructions, really. – iLikeDirt Apr 12 '15 at 16:46
  • Wouldn't a hardwood direct over concrete get cold in the winter? I was thinking that a subfloor would keep the temperature of the floor closer to that of the room in general and if any small amount of water does seep into the floor from the outdoors the subfloor would prevent the planks from coming in direct contact with the water. What do you think? Still a waste of time? Should I just go with the foam and then the engineered hardwood? I live in New York so the weather gets pretty cold but is not overly humid like the pacific northwest. Thank you for all your advice. – Kate Apr 12 '15 at 17:04

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