I have a wood flooring project coming up in which I have to install 4000 sq. ' of hard wood
over a second floor concrete slab that is about a year old.
I would like to post what I have learnrd and my questions and would welcome any suggestions.
I first am going to fasten 3/4" cdx plywood over the slab with bostick ahesive and fasteners.
What are the fastest fasteners i could use? I understand the plywood has to be 3/4" away from the walls and 1/8 ro 1/4" spacing between sheets. I was thinking of using advantech t&g subflooring instead ?
Once I get the sub floor down i would install the floor. I'm going to use 3/4"x 5" quarter sawn white oak. As far as fastening it I was going to glue it down but was wondering if I could use a narrow crown stapler that I use for wall sheathing with the right lenth staple so it wouldn't go through the plywood or should I do both? Can anybody contribute a sq. ft. cost to the project?
j.e.

When Ken here originally brought up the subject of floating plywood subfloors on this forum several years ago, I was suspect. As I always am about new methods until proven. But once you do one your like..."what the hell was I doing spending all this time, money and energy fastening to the concrete anyway". Besides the fact that your penetrating the vapor retarder with thousands of fasteners. Just saying......

We had a massive failure of our previous glue-down over concrete. Our installer did several things wrong. Needless to say, after loosing all of that investment, I will not be glueing down again.

We purchased 1/2" thick, 2 1/4" wide solid red oak flooring. After all of our research, we have decided to do the floating subfloor method (using 3/8" plywood for both layers). However, there is no one local who has used such a method, so we have questions that no one seems to be able to answer. I would GREATLY appreciate some advice and help.

1.). After ripping up the old flooring, I cleaned the concrete as best as a I could. I used a grinder to grind off all the old adhesive and moisture barrier he had painted on and washed it down with warm water to remove most of the grit and dirt . However, with a floating subfloor, what are the requirements for the condition if the concrete? Do I still have to check within 10' for any dips below 3/16" or am I allowed a greater variance? There are a couple of divits about 1/2" wide by 3-4" long in the concrete (from a couple of slips with the grinder ), do those need to be filled flat as well? Will the two layers of plywood allow for some forgiveness in slight imperfections, or will it be like a regular floating floor where every slight bump and rise will be felt?

2.) we are going to have a 1" height differential between the wood floors and the existing tile. Obviously we are going to have to fashion our own threshold pieces, since that is not a standard height difference. However, that leads to be my next question: how do you prepare for those thesholds? when laying the plywood subfloor, do we go all the way to the existing tile (leaving only a 1/8" expansion gap), and then lay the threshold piece in such a way that it overhangs and lays on top of the tile or do we stop the plywood short thereby leaving enough of a gap that we are will nailing the threshold piece to the plywood, but the rest of the piece floats over the plastic sheathing underlayment and ends flat against the tile? Or is there an entirely different way to do this?

3. Finally, we live in Kennewick, Washington. We are in a high desert with relatively low humidity. Also, with only 1/2" thick wood and very narrow planks, do we really need to leave a 3/4" explanation gap between the walls and wood- thereby requiring not only baseboards but also a quarter round? Or, can we leave a 1/2" expansion gap around the room so that a regular baseboard will be able to cover the gap? We definitely understand the need for the expansion gap to allow for expansion, but 3/4" sounds like an awefully large gap.

Thank you so much for any help!
Bridget

I do prefer the 2 layer of 3/8 plywood or two layers of 1/2 inch if you have no height restrictions. A one half inch gap would be acceptable. Get it flat as possible, use MVP4, then 6 mil, then plywood, then 15ib roofing felt.

On the transition strip, you would want the ply to extend out further than the hardood so you can fasten it down to the ply and extend it out over the tile. No need to fasten or glue it to the tile. Others here may have different opinions.

So you want to glue 3/4" 5" wide hardwood to concrete. Humm First thing It matters not what the glue manufacturer says IF the hardwood manufacturer says don't do it, you will have no recourse if it fails. Secondly this would be predicated on the hardwood being straight enough and milled to allow them to come together neatly. This is generally why mfg's DO NOT ALLOW GLUE DOWN WITH 3/4" SOLID WOOD. It can be difficult enough when nailing 3/4" wood BUT at least you can use a screwdriver to keep the pieces tight together so that you can nail it and hold. IF the wood will not stay tight enough until the glue cures you will have a lousy job. Blue tape will not generally hold it.NWFA guidelines which most mfg's follow do not recommend trying to direct glue 3/4" hardwood to concrete. Last but not least concrete always has some moisture in it and we all should be aware of what happens with solid wood and concrete. Inspected a job 2800 sq ft glued direct wintertime by summertime there was some of the worst cupping I had even seen. Additionally it had separated from the concrete in most places. Total replacement only cure and this is in the arid southwest at Las Cruces NM. Follow NWFA guidelines or mfg if more stringent. Not worth going bankrupt over! Go to my website http://www.decker-consulting-inspections.vpweb.com I have the entire NWFA Guidelines posted there. Pretty much most mfg have instructions, warranty, and maintenance info in very box. Otherwise go to their website and download them. 85% of the time the failures I inspect are due to guidelines not being followed. Yeah, I know I have done it a hundred times and when it fails its the wood's fault or the manufacturer.

Then there's Carlisle hardwood floors who is one of the leading hardwood manufacturers who say it is safe to glue their hardwood to concrete. But they have the science down. http://www.wideplankflooring.com

Problem I see is that most hardwood guys do not understand concrete, or how to test it, and how to prepare it. They are used to working over wood substrates.