Plywood subfloor question ?

[Arky217]

I can get 3/4" T&G plywood sheathing for about $22.
However, I have found a source of 1/2" plywood sheathing for just $6.50, (it has been rained on, but is still sound)

For subflooring on 16" centers, this begs the question, would 2 layers of 1/2 plywood (installed with the edges of one layer offset from the other) be as strong in bending and deflection as one layer of 3/4" T&G ?

Or if not, what about if glue were applied between the two layers ?

Thanks,
Arky

[n74tg]

Why don't you buy a couple of sheets, set up some 2x4 on 16 centers and test it out.  You may be able to resolve it to your satisfaction without spending a lot of money.  Even if it doesn't work out, I'll bet you can think of several ways to use that 1/2" ply.

[MountainDon]

With two sheets, one over the other, you stand the risk of having them rub and squeak or groan as you walk across the floor. Possibly if you used a good glue between the layers it would be fine. Unless they are glued across the entire face of the sheet it really would not have the same strength of a factory produced sheet. Add to that the extra labor and fasteners.

Trying to save money here is not a good idea IMO. You'd be better off with a solid 3/4" thick (or thicker) T&G subfloor and use the 1/2" plywood for wall sheathing. That would be great stuff for walls.

[John_C]

No glue = squeaky floor

Glue...  You'd need to nail on roughly 3" centers across the entire field to get a good glue joint (priced any decent glue lately?). The edges still wouldn't be as tight as T&G.  Way more trouble than it's worth.

Advantech is the way to go. Pricey but worth it.

[JRR]

I think the two 1/2" pieces, glued together with staggered joints, all glued and screwed (or nailed) to the joists ... may well be equal, if not superior, to the 3/4" sub flooring.

The resulting total cross section should be thicker, and the glued inter-surface will be at the "neutral bending axis"; .... i.e.,there will be very little shear on the glue.  In other words, the glue between the sheets will have very little to do.  The resulting floor should be very stiff and last a long, long time.

There is nothing wrong with doing an test-experiment, as someone suggested ... I would like to see you try the test with NO glue ... just to see how stiff two unglued layers of ply are.  I would not suggest a final installation without glue, however.

[cordwood]

 I've tried it without glue on two different floors, one in a loft and one in a 12X16 shed and in both cases it was like walking on a trampoline and squeaked like rubbing two balloons together
With good glue I don't see why it would be much different than one thick sheet? Plywood is just glued sheets anyway.

[Arky217]

Well, I decided to set up a test of the difference in deflection for a given weight.

I used a 2" wide strip of 5 ply 3/4" cdx plywood, cut lengthwise vs two 2" wide strips of 3 ply 1/2" cdx plywood, also cut lengthwise.

I set the span at 14.5" (span for 16" on center joists).

I first applied 30# to take up any play or gaps in the arrangement and measured the distance from the bottom of the plywood to the floor.

Then I applied 120# (net weight of 90# applied) and again measured the same distance.

The results:

The 3/4" strip of plywood bowed .160"

The two 1/2" strips of plywood bowed .120"

This, of course, does not take in account some factors, such as using strips vs full sheets.

But from this test, it appears that two 1/2" sheets of plywood would be at least as strong as one 3/4" sheet without even using glue.

I like the idea of putting a layer of felt between the sheets to eliminate or reduce squeaking.

[JRR]

I like the test you conducted ...and agree with your results. 

I do not agree with using felt between the plywood pieces.  The felt would act much like a slip layer ... whereas it might be quiet (time will tell), you will not get full intended strength if the two layers are moving relative to each other.  Even tho the glue won't be very "loaded" with large forces ... you don't want the two plys free to slip against each other, ... you need the glue.

The glue doesn't have to be 100% coverage ... a good heavy (1/4"dia) serpertine bead, with rows about 6" apart should do well ... assuming you're using a good construction poly glue.

[MountainDon]

I assume the plywood is rated for the purpose intended.

I assume there are no traces of mold anywhere.

I assume the local building inspector, if there is one, has been asked if this plan will pass. If there is an inspector and he says NO, then it doesn't matter what schemes are thought up. If there is no local inspector then one is free to pursue whatever they want.

I maintain that a single thickness, capable of supporting the loads, is better than laminating thinner material unless you are building something with curves, like a boat hull. If you glue the panels together I can't see it saving a tremendous amount on costs, and the labor becomes greater. Maybe that's just my outlook.

If it was my floor I would be concerned about squeaks. The felt between layers bothers me. All I can say is that it just doesn't "feel" right for a house floor. The potential for flex bothers me too, in case some day somebody wants to lay down ceramic or porcelain tile or natural stone. I'm a big tile user, not everyone is I know.

[n74tg]

Why not try a more advanced test.  Get yourself a sheet of 3/4" T/G Advantech, about $20 down here  and what a lot of people swear by on this forum (though I didn't use it on my project).  Put it across joists on 16" centers like you will use and load it up good (lots of concrete blocks).  Measure your deflections.  Jump up and down on it; see how it "feels".  Walk on it (stomp on it) some and see if you get any drum-like sound.  Then do the same tests with two unglued sheets of 1/2" ply, maybe both with and without the felt in between.  Then do it once more with with them glued together. 

Yes, you will spend a few bucks here, but if you get the results you want, you will be saving a lot on the final floor. 

[John_C]

The two layers of 1/2" will be stiffer, especially if glued, but that's not the point.

The T&G along the long edge keeps adjacent sheets "working" together between floor joists.  If you use the two layers & want to keep adjacent sheets from moving relative to one another under heavy loads you will need to overlap them quite a bit & glue/fasten them securely.

Advantech in particular seems to stand up to the weather during construction.  I threw a cut off piece in the yard and 10 years later it was still sound.

I just did a storage shed roof for a neighbor using 7/16 OSB.  We had a day of very heavy rain and even though we had covered the roof with 30# felt the OSB expanded enough that 1/2" H clips wouldn't slide over the edge.  Most AC plywood I've seen de-laminates at the least excuse.

In older carpentry books 1/2" plywood is shown in a single layer as subfloor. I have no idea if it would meet current code but in Wagner's Carpentry  ©1983  it meets FHA standards.  Often blocking was placed between the joists so that all the joints landed on solid wood.

I built strip planked and cold molded boats for many years but I can't muster much enthusiasm for the laminated sub floor.  If the floor is not going to be insulated you could put the joists on 12" centers and use a single layer. With blocking for the long edged to land on that would be stiffer than 3/4" on 16" joists  and the few additional joists would be cheaper & faster than the whole laminating thing.

If it's in a utility building it won't make much difference how it's done. In a residence there are better ways.

[Arky217]

n74tg,

I noticed that you are in Hot Springs; where is the Avantech 3/4" T&G available and what is the price ?
What makes it better than the typical 3/4" T&G that the big box stores have ?
It doesn't seem to be available in Ft. Smith or else I haven't found the right place.

Thanks,
Arky

[John_C]

What makes it better than the typical 3/4" T&G

It stands up to the elements better than anything else I've seen.  For an inside jobs that absolutely wouldn't get rained on there are other products that are abou 1/3 cheaper.

[n74tg]

Arky:
I know Lowe's has it here in Hot Springs, though it may now be about $22 per sheet.  And I think Home Depot in Benton (about halfway between here and Little Rock) has it too.

Seems like in the reading I've done on Avantech, they use smaller more consistently sized wood fibers and a better glue in the making of Avantech. 

Where are you located (if not in Ft Smith)?

Tony

[MountainDon]

It's the resin glue they use.

http://www.huberwood.com/main.aspx?pagename=advantechflooring

[jb52761]

When I put down a new subfloor in my old fishing cabin, after I ran my floor joists accross the span accordingly, I then took scrap boards and fastened them at random locations throughout  the entire place....so basically, I ended up laying plywood on top of a giant checkerboard....16" on center, yes, but then every 16, 20, or 22 or so inches, another support board......and it certainly looked like a checkerboard....works for me..... ...jb

[tc-vt]

If you are thinking of doing two layers of 1/2 inch plywood for the subfloor, think about doing soundproofing.  One technique of soundproofing is two layers of sheet goods (sheetrock on walls, too) with Green Glue in between them.  The cost of the Green Glue may surpass the savings over a single layer of 3/4 inch tongue and groove.

http://www.greengluecompany.com/newConstructionFloors.php

Tom