I am venturing to install 3/4" x 4" pre-finished plank on plywood on concrete, and would like your opinions/recommendations on the following plan:

Remove carpet, baseboard and undercut casings

Check moisture. I see Cen-Tech at harborfreight.com for $13, is that any good for wood / concrete measurements? Any recommendations on moisture meters?

Prepare the 4' x 4' CDX plywood. Make cuts to be able to install at 45 degrees. Score the back 3/8" deep every 8". Table saw or circular saw?

Use Franklin (Titebond) sealant and adhesive system. About $1 per sft materials sound ok?

Leave 1/8" gap at plywood seams and 1/2" at ends.

Use boxes of pre-finished plank to weight down the plywood into the adhesive. Any issues if plastic has been been removed from boxes?

Roll out Insulayment

Nail down planks using 1.5" cleats (1.25" nails where top nailing required).

Install baseboard with 15 gauge nailer and attach shoe using adhesive (what kind recommended?)

Basically, your plan is sound. It revolves around gluing down plywood panels to concrete as a nailing base for 3/4" hardwood flooring. Not having any experience with that Harbor Freight Cen-Tech device, I cannot comment on it. It might work well enough, I do not know. As for scoring the backside of the plywood panels, I find a portable circular saw much easier. The scored lines do not need to be perfectly straight. You're just allowing the ply to flex a little. I'd check for flatness as well prior to installing the plywood. I think being able to get the Franklin Moisture Barrier and the adhesive for only a dollar a foot is high hopes. The moisture barrier gets 40 sq.ft per gallon and comes 3.5 gallons per pail, meaning you will get 140 sq.ft. max per container; probably a little less. The adhesives get about 35 sq.ft. per gallon, so figure 120 sq.ft per container max. To get both for a dollar a sq.ft, you'd have to find them on sale for about $60.00 for a 3.5 gal. pail. I doubt that will happen. No, you'll probably spend closer to $2.00 a sq.ft for moisture barrier and adhesive. You could just use 6-8 mil polyethylene plastic sheeting and tapcon screws. It will take WAY longer to screw down however. Personally, I'd spend the money and glue it down. I'm not sure what you mean by insulayment. Usually what is specified by the flooring manufacturer is 15 lb. asphalt saturated felt paper. Everything else sounds fine. You won't need baseshoe if you're within 1/2" of the walls and you base is at least 9/16" thick.

Thanks for the tips Gary.

The adhesive coverage seems about what I figured. But the sealant is not what I expected. They recommended the 531, which apparently gives 400 sq ft coverage for a 1 gallon container. Is this the one you had in mind? (They mentioned it was 2-part, mix them before pouring out.) If so, the sealant cost would be much less.

Actually I liked the plastic sheeting and tapcon screws approach (not looking forward to messing with glue), and I think it would be more economical, in the $0.75 per sq ft range. But when it seemed the sealant+adhesive was more like $1 instead of $2 (e.g. Bostik) I began to lean towards that as a "safer" approach. Questions/concerns I had about tapcons: (1) damage to the concrete (2) holes in the moisture barrier (have you heard of / tried dipping the tacons into calk?) (3) what tool would I need to drive the tapcons, regular Black & Decker drill/screw driver OK?

I left more than 1/2" in some rooms upstairs, hence the extra shoe base (actually a flexible polymer thing called a bead from HD) . I had a couple of questions on that: If the floor level goes up and down, should I stick the flexible bead slightly curved in places to hide the gap to the floor, or just keep it flat and parallel to the bottom of the baseboard? This polymer bead stuff only comes in 8' lengths, so I'd have to make joints in the middle of the wall which I don't really like, not sure there are any options? Also wonder what kind of adhesive to use, to attach polymer to MDF?

I'm using Insulayment to provide some thermal and sound insulation. Current floor seems cold under the carpet. Used it upstairs and it cut the sound down compared to felt paper (even though it adds about $0.50 per sq ft).

Any other reasonably priced recommendations for concrete and wood moisture meter?

I'm all for a floating subfloor, where it is not anchored or glued to the concrete.

I roll out 15 # asphalt felt paper overlappeing 4" at least.
Then I roll out 6-mil plastic and use clear plastic packing tape to seal the overlap of any seams in the plastic moisture barrier.
Then I cut my 4x8 sheets of plywood into 16"x8' planks and lay them in a staggered planks design going the opposite direction in which I want the floor to lay.

Then I use 1½" fasteners to attach the wood flooring to the plywood subfloor.

This way there is no compromising of the plastic moisture barrier, by poking holes all in it trying to fasten down the plywood sheets.
I use the asphalt felt as a cushion, and to keep the plastic from rubbing a hole in it later as the floor is put to use and years of abrasion on the concrete.

Thanks, yes the floating approach also seemed simple.

But ... I live near San Jose California, close to a creek which has overflowed and flooded once in a while in history. So I thought I would treat it as possible extreme conditions, and put in a moisture barrier underneath.

Also with the floating floor I imagined there was a possibility the whole subfloor + floor might shift in the event of a small earthquake (probably much worse is likely if it's a bigger one!).

Let's hope there's no flood ! Moisture barriers "reduce"moisture vapor, not guaranteed to eliminate it.

When you think about it.... A floating subfloor is a better option when it comes to an earthquake. The fastened subfloor is rigid , and therefore prone to damage. Where as a floating subfloor will be floating and moving with the quake, kinda goin with the flow , instead of possibly getting busted up, along with the hardwood on top

True! My wife told me the same thing . Too much imagination on my part I guess.

I looked up the NOFMA guide on vapor retarder, and it says:
Polyethylene method. When slabs are well above grade and the expected annual rainfall is light to moderate, cover the entire slab with 4- to 6-mil polyethylene film, overlapping edges 4-6" and allowing enough to extend under the baseboard on all sides.

This area probably qualifies for the rainfall level, but this actual installation is at grade. If I chose to do the floating floor, wonder if I would be "in violation" of the recommended practices, with respect to the vapor barrier? If not, I don't see any more reason not to do the floating floor.

This type of vapor barrier has been recommended long before MVP & Planniseal came into the picture.
As an added precaution I would roll the plastic up the outside walls, behind the baseboards.

Thanks again! You made my day. It's going to be so much simpler and less expensive now.

Do I still need to score the back of the plywood 3/8" deep every 8", and leave 1/8" gap between the 16" x 8' boards?

Wish you and the other helping angels on this board a happy holiday season and a wonderful new year!

Anand

It still wouldnt be a bad idea to score the back of the plywood , it will lay flatter. I would also leave a knife blade thickness gap, inbetween modules.

While prepping the floor, I've been having some second thoughts on the single 3/4" plywood floating subfloor. I remembered the weird feeling you get walking on floating floors...

Now considering glueing the plywood down (also screw perhaps, to make sure it gets embedded into the glue). I'm at grade, slab is 10 years old, looks nice and clean, and the moisture readings came in at less than 5 lbs/1000 sq ft.

If I take this route, what kind of process should I use to eliminate the stuff on the concrete (mostly a thin layer of paint droppings it looks like)?
- Angle grinder + diamond cup attachment
- 20 grit paper + palm sander
- wet the slab and try scraping off whatever possible, then dry out for 2 days

Would appreciate your inputs.

Did you know that most manufacturers of even engineered flooring want the MVE to less than 3 lbs/1000 ft. in 24 hours? So, your slab would not be within spec for a direct gluedown. I recommend using SIKA Primer MB and SIKA Bond T-55 adhesive to seal and glue the plywood down. Now, it will be expensive. An alternate is to lay out 6 mil polyethylene plastic sheeting and either float the plywood nailing base or screw or nail it down. Nailing only has a tendency to work loose. One floating method developed by NOFMA ( prior to them being absorbed by the NWFA ) was to rip 3/4" sheets of CDX plywood into 16" x 8' panels, kerfed 3/8" deep every 12" on the back side. These are laid over the plastic sheeting. They need not be tight or fastened down. Just place them on the slab in rows, offsetting the joints, perpendicular to the direction you want the wood flooring installed. Be sure to allow at least 1/4" gaps between each panel and about a 1/2" gap at the walls. The weight of the flooring and nailing the flooring into the plywood panels keeps everything flat and together. Obviously, you don't want to use warped plywood. Make sure your plywood has been stored properly. I buy it at HD or the other big box store that sounds like Crows. Both places store the plywood inside their warehouse and it's usually nice stuff at the lowest prices around. Do not use T&G subflooring. Just plain old 4 x 8 x 3/4" CDX plywood. This is about the easiest and cheapest way to get an approved nailing base for a 3/4" solid hardwood floor. Then lay out some 15 lb. asphalt felt over the plywood, staple the felt down so it's not sliding around on you, snap your starting chalk line and start nailing the flooring down. EZ, PZ

Thanks Gary. I actually got the plywood cut just that way based on the NOFMA guide (haven't scored them yet) from HD. Except they kept it outside in the rain and the ends especially got wet. I leaned them against the wall and waited 2-3 weeks now to dry them out. Not sure if I just lay them if they will be flat, and if they will be OK for floating.

If I were to put the poly and then puncture it with tapcons, would that be that much of an issue (assuming no glue)?

Even for glue-down I might need to put something heavy on it or just screw it in as well. I heard doing tapcon screws are real hard and time-consuming.

Just checked out the SIKA stuff. About $3.5 per sqft

Whenever one get's into fastening wood securely to concrete, it gets slow and expensive. SIKA's products are pretty high end and are priced high end. Mechanical fastening has worked, to a degree. Sometimes, hand driven or machine driven ( air or powder guns ) fasteners can spawl the concrete and lose their grip. Some companies use an Aerosmith and say they get good results. Personally, I do not think the fasteners will cause a problem with the vapor barrier because the plastic sheeting stretches around the fastener. Poke a hole in the film before installation to see what I mean.You would not want a brittle plastic film but a flexible one. If using tapcons or similar anchors, you could squirt a bit of silicone in the hole before installing the fastener.

Anyway, that's why so many suggest just floating the plywood. And IMHO, I would refuse to use the plywood that was stored outside in the rain. What were they thinking? It could be ruined and never lay flat. Plus you, have it standing on end instead of flat, as it is supposed to be. You should of just kerfed it and ripped it yourself. Pretty easy with a few saw horses and a skil saw. Ya live and learn I suppose.

Thanks again Gary. Got a couple saw horses, will cut the end 2" off. Wood is OK otherwise, still flat (was only standing up on the 16" side where it doesn't flex much), but will park them flat and put a few boxes of my heavy tiete till I use them. I only took them because I didn't want all the wood (trees etc.) to be wasted. I don't think I will do the glueing, will stick to poly, probably will double up the 6 mil. And if it doesn't seem flat when I lay the ply, will put in a few screws with the silicone dip as you suggested. Thanks so much again, can't tell you how much of a relief it is to have someone like you to discuss and get clarification. Keep up the great work, you're sending so much light into the world!