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cmlcsu 11-06-2009 09:24 AM

how to fasten plywood to subfloor to even out floor height
I am going to install hardwood floors in my living room and dinning room, but to match the height of the tile in the kitchen, I will need to lay 1/4" plywood sheets to the existing subfloor to make the two floors even. What is the best way to attach the plywood to the existing subfloor? Nails, screws, glue, staples? Also, what size expansion gaps do I need to have between each plywood sheet, if any?

Thanks for your help

Daniel Holzman 11-06-2009 10:32 AM

I have always screwed plywood to the subfloor without glue, however several folks on this forum have expressed a preference for gluing and screwing down the plywood. Perhaps someone with personal experience can explain what the benefit of the glue is, personally I have never had any problem with screws only. As for expansion gaps, since the floor is indoors and the material is plywood, you can use a 1/8 inch gap between the sheets with no problem. Just as important is to make sure you have an adequate gap around the outside for the hardwood, follow manufacturers recommendations for the hardwood gap.

pyper 11-06-2009 11:41 AM

The glue holds the plywood to the subfloor and prevents it from squeaking on screw shanks.

Threre's not much of a risk of squeak, but if it happens it can be really annoying. It's more likely if the existing floor isn't perfectly flat.

cmlcsu 11-06-2009 11:53 AM

thanks for the quick info! does it matter what type of plywood I use? can I use OSB, it seems to be alot cheaper.

thanks again!

Daniel Holzman 11-06-2009 03:03 PM

See for a good discussion about the differences between OSB (oriented strand board) and plywood. They are not the same, however they both fall into the category of manufactured wood panels. Personally, I would never use OSB in an environment where there is any potential for moisture, because of the swell potential of OSB versus plywood. For that reason, I would use plywood under a hardwood floor, since there is always the potential for the subfloor getting wet (boots, coats, flood, spilled liquid etc.)

cmlcsu 11-06-2009 03:19 PM

one more question then I should be set. What length screw,and what spacing should I use between the screws?

Daniel Holzman 11-06-2009 06:17 PM

If you are using 1/4 inch plywood, and you are going through 3/4 inch subfloor, I would go with a 1-1/2 to 2 inch screw. Make sure you predrill the hole through the 1/4 inch plywood, use a bit the same diameter as the outside thread of the screw. The pilot hole is generally the diameter of the shank of the screw. If you can hit the joists, that is always best, and if you can find the joists, you can predrill through the subfloor as well.

Assuming you find the joists, and they are 16 inch OC, you can install screws every 12 inches or so along the joists. If you are concerned about keeping the plywood really tight, every 6 inches or so is better. There are some special bits made by Dewalt that are really handy, they predrill the shank hole at the same time they drill the pilot hole, saves a bit change.

oh'mike 11-06-2009 08:00 PM

Hi there.
The standard method of installing 1/4 " underlayment should work here.

Most of us simply staple the underlayment using a 1/4 " small crown stapler. Staple edges and seams about every 4" staple the field aprox. every 8".

Use staples NO LONGER than 1 1/4 inch. You do not want to be punching holes into your plumping or electrical. This is the way I have done underlayment -It has always worked well.

Good luck,have fun--MIKE--

JazMan 11-06-2009 08:39 PM

Underlayment should NEVER be glued to the subfloor. Only the subfloor is glued to the joists.

1/8" between the sheets and 1/4" at perimeter and other vertical objects.

Do NOT use luan underlayment no matter what the guy wearing a vests says. NEVER glue an underlayment.

Remember plywood is actually 1/32 thinner than the stated thickness. Use 3/8" if possible since it will add some stiffness, 1/4" adds very little. Remember no glue.

!/4" ply is usually stapled as above, but the average home owner doesn't have one in the tools box. Ring shank underlayment nails are second best. If you use screws, do not use drywall screws. Get a screws just a bit longer than the total thickness of the two layers. Do NOT use long fasteners so you hit the joists too, you wanna miss the joists, so use short fasteners. Did I mention no glue? :laughing:


cmlcsu 11-08-2009 09:58 AM

Ideas on what is better, screw the sheet to the floor or staple? I have a cheap air powered stapler/brad nailer that can handle 1/4" staples, this sounds like it would be a lot easier than pre drilling holes before I set screws.

Also, what is the point of not trying to hit the floor joists, I would think that you want to hit the joists to add in strength?

Thanks again

oh'mike 11-08-2009 10:13 AM

Staple it-no screws. You are lucky to have the stapler.

If you have any squeaks add some screws to the original subfloor at the joists.

The 1/4 inch ply will add no real strength to the flooring system,Just staple it.

If you use screws--I believe that you will have many nail jambs when you install the hardwood. Nailers don't like hitting screw heads.

Now go staple that stuff down!!-MIKE--

rredogg 11-08-2009 09:19 PM

Got a question for JAZ MAN
Why not screw into the joist to secure the underlayment?

Thanks in advance for the reply, rredogg

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