I will be installing 3 14" wide, 3/4" thick solid maple with an MIIIFS stapler. I see both 1-1/2" and 2" staples are available. The subfloor is layers of plywood, 1/2" + 7/16".

Figuring in the angle of the staple, the 2" staples will go through both layers of plywood, but not quite poke through the bottom. The 1.5" staples will go through the top ply layer, but barely into the bottom layer.

I am guessing I want to use the longer 2" staples, but wanted to get your opinions.

Thanks

use the 2" it's a better bet.

I am assuming since you have staples that you have a pneumatic. With a decent whack the pneumatic will drive a 2” down pretty well. If you sub-floor is over a slab a decent whack will cause the staple to bend out under the sub-floor, when it hits the concrete which holds it in pretty tight.

But what is it doing to the moisture barrier, or did you use one?

I posted twice

Good catch Floorguy, if you plan to use 2" staples on a slab with a plywood profile that will allow penetration a cut-back mastic should be applied to the slab.

I always use a barrier, a great deal of the manufacturs now say it is OK to just attach the plywood to the concreet but I figure $0.10 a sq. ft. lets me sleep better at night.

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I've alway's wondered this myself. If you look at the NOFMA website, when you shoot the 3/4 plywood subfloor into the slab, you're essentially puncturing the polyethylene film moisture barrier, so what's the point of using a barrier if you're going to punch holes in it. I do like KLS's idea, though, of using roofing caulk on the plywood before shooting the nail into the floor.

The transmission from the concreet nail (not floor staples) holes is really no greater than the transmission that happens at the over-lapped seams. BTW I tape the seams which is not required according to NOFMA. The thing to remember is that the wood is always going to experience some moisture retention and lose the objective is to regulate those changes. The barrier is to regulate that transmission from the sub floor. The fact that it covers a majority of the concrete means that it will serve the purpose. The standard transmission rate for 6-mill poly is .004 in a 24 hour period most wood is rated to withstand a transmission of .02 so there is quite a bit of room to breath if the slab is within normal conditions. If done correctly the punctures in the paper will result in a minimal increase if any increase in the transmission of vapor. Vapor tends to like to move through porous material a nail is not a good conductive material for it to travel through, as well the nail tends to pull some of the plastic into the void with it further sealing the puncture, further the plywood sub-floor will slow transmission as well if you have an asphalt slip sheet it will slow transmission. What you are trying to accomplish is to get vapor transmission down to the point that the top wood has enough time to expel it before more vapor transmission is absorbed. That rate is around .02. There is a good article on vapor films that can be found here:

http://palimpsest.stanford.edu/waac/wn/ ... 4-204.html

The reason I like to use the asphalt caulk to shoot through is not because I am afraid of transmission from the concrete into the sub-floor from the holes in the 6-mill but rather that the nail going through the plywood can create a void from the bottom of the plywood to the top of the plywood. It them becomes like an elevator for vapor. Again the asphalt paper layer should retard this transmission but I add the caulk to insure that the void is sealed.

The best system going, is a floating plywood subfloor, over a moisture barrier. I have yet to see one failure from that method of installation. It feels and performs just as a fastened down subfloor.

I am not installing over a concrete slab, but on a main floor area which has a full basement underneath. I am installing with a pneumatic stapler. So is 2" still the right choice?

Yes, 2" is what you need.

Thanks, KLS,

What an excellent and detail explaination.


Best...Stan

Since we don't want to overwelm the wood with too much vapor, I wonder if anyone has tried drilling very small, scarcely placed, holes in the hardwood, and possbly the plywood (but not the vapor barrier), to allow a sort of "fast path" for the vapor, should you have a situation where the slab absorbed too much water. Perhaps the holes go go where you have a gap between two plywood sheets. I wonder if the would help promote the same humidity both above and bleow the hardwood?

Also, while I am asking, when doing wood over slab, does anyone seal the wood on the backside and ends as well?

discalmer: I have no idea or experience, so this could be complete BS for all I know.

Moisture vapors are not under any pressure.

Your Idea wouldn't matter.

While NOFMA and the NWFA, nor any flooring manufacturers that I am aware of, do not recommend applying a sealer to the backside of solid wood flooring, common sense would indicate that it would provide some benefit to limit moisture absorbtion. This has been bandied about for quite a few years in my area. Personally, I have never done it as it would require opening all the flooring up, spreading it out upside down and spraying or brushing on something ( poly, shellac, laquer, ? ). Then letting it dry and then stacking it all up and out of the way so one could proceed with the install. Quite a bit of extra work that many homeowners may not wish to pay for. Plus, there's no guarentee that it will be completely effective. However, I did install a Bruce Maple plank floor about a year ago that was finished on the backside! This was the first factory finished floor I'd seen the had a finish applied to the back. The flooring was manufactured in China and perhaps they did that to prevent damage to the flooring on it's long trip across the ocean. I don't know but I thought it was a good idea nevertheless.