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 DIY Chatroom Home Improvement Forum Need tongue/groove subfloor if placing over old subfloor?

05-09-2020, 12:05 PM   #1
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## Need tongue/groove subfloor if placing over old subfloor?

I'm installing new hardwood in an older house with 5/16" subfloor. The house was built in 1954 and 5/16" is too thin of a subfloor to install hardwood over. So I want to put an extra layer of 1/2" OSB over the 5/16". My question is, should the additional layer of 1/2" OSB be tongue and groove? Or is that not necessary since there is already an existing subfloor already there?

05-09-2020, 12:13 PM   #2
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## Re: Need tongue/groove subfloor if placing over old subfloor?

5/16" are you sure?
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05-09-2020, 06:31 PM   #3
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## Re: Need tongue/groove subfloor if placing over old subfloor?

Are you sure you can even get T&G in 1/2"? I've usually seen it in 3/4" and over, and occasionally available in 5/8", but I've never seen 1/2" T&G. If the existing is actually 5/16", it will contribute next nothing for strength or stiffness. Even adding 5/8" panels over 5/16" will not provide the same strength as 3/4" panels alone. Strength is calculated based on the square of the the thickness, and stiffness as the cubic.

.3125"^2 + .625"^2 = .0977 + .3906 = .4883
.75^2 = .5625.

.5625 / .4883 = 1.152 - 3/4" alone is 15% stronger than 5/16" + 5/8"

.3125"^3 + .625"^3 = .0305 + .2441 = .2747
.75"^3 = .4219
.4219 / .2747 = 1.536 - 3/4" alone is 54% stiffer than 5/16" + 5/8"

The strength and stiffness for 3/4" vs. 5/16" + 1/2" are much worse; 62% stronger and 171% stiffer. Put another way, 5/16" + 1/2" is only 37% of the stiffness of a 3/4" panel. If you're going to do hardwood flooring with just adding 1/2" panels to the 5/16", the flooring better be fairly thick, or your floor will feel pretty squishy.

Last edited by HotRodx10; 05-09-2020 at 06:34 PM.

05-09-2020, 08:11 PM   #4
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## Re: Need tongue/groove subfloor if placing over old subfloor?

I bet you mean 5/8" subfloor, not 5/16. Yes you can put 1/2" over that, and you don't need tongue and groove, which is lucky cause there is no such thing as 1/2" t and g.

Just make sure the joints of the two layers of plywood don't line up.

05-09-2020, 08:16 PM   #5
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## Re: Need tongue/groove subfloor if placing over old subfloor?

We do 5/8 T&G subfloor and they put hard would on it all the time with out a problem.
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05-10-2020, 07:15 AM   #6
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## Re: Need tongue/groove subfloor if placing over old subfloor?

Thanks for all the replies. That makes sense about the reply above that the old 5/16" subfloor plus 1/2" new subfloor won't be stiff enough. And that 1 sheet of new 3/4" OSB would be stiffer.
Yes, it's only 5/16" existing subfloor. And it's very squishy and squeaky. I only weigh 155 pounds and I feel like I'm going to fall through it. And no, I have not seen 1/2" T&G OSB either. I was hoping to use 1/2" (actual thickness .469) plus the 5/16" (.3125) = .7815". I don't want to make the floor too thick.

I SERIOUSLY considered cutting out the 5/16" subfloor, and replacing with 3/4" T&G OSB. I even bought the toe-kick saw to cut it at the wall edges. However, I believe (but not sure) that I would then need to install joist hangers and additional floor joist supports (maybe 2x8s) near the wall edges to support the new subfloor? If I replace the existing subfloor.
Thoughts?

Last edited by toddlimelight; 05-10-2020 at 07:41 AM.

05-10-2020, 07:53 AM   #7
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## Re: Need tongue/groove subfloor if placing over old subfloor?

OK, that's weird. Best would be to rip it up and replace with 3/4, though it may be a lot of work. Hard to know until you rip it up, but worst case you would need to add blocking on the walls parallel with your joists (it might already be there). You could probably get away with a flat 2x4 from the joist, underneath the wall, and then screwed in from the top.

I did recently remodel a 30 year old house that was framed with a 1/2" OSB subfloor with oak strip flooring, and it looked fine. So maybe you could get away with that. I wouldn't worry about the edges as much as getting that flooring nailed well. Use the longest fasteners you can find and make sure each joist gets a staple/cleat.

05-10-2020, 04:22 PM   #8
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## Re: Need tongue&groove subfloor if placing over old subfloor?

I forgot to mention.

The house had 3/4" hardwood originally installed over the 5/16" subfloor. The hardwood was pet stained badly in areas over a period of 9 years so I'm tearing it up and replacing it.

So the total floor thickness was 1.06".

05-10-2020, 05:01 PM   #9
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## Re: Need tongue/groove subfloor if placing over old subfloor?

Quote:
 ...I would then need to install joist hangers and additional floor joist supports (maybe 2x8s) near the wall edges to support the new subfloor?

I'm assuming you're talking about at a wall that's parallel to the joists (with a wall that crosses the joists, you don't need to do anything). For that it depends on how far it is from the wall to the first joist in front of the wall. Unless it's been remodeled and walls moved, I would think there would be a joist under the wall (I could be assuming too much), but the next one may not be the normal 16" away. If the next joist is 8" away or less, you probably wouldn't need to do anything to support the cantilever. If it's more, then joist hangers or cleats for 2x4 blocking between the joists should be adequate. If you're adding a joist (parallel to the wall), then matching the depth of the existing joists should be fine.

05-22-2020, 09:00 AM   #10
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## Re: Need tongue/groove subfloor if placing over old subfloor?

Quote:
 Originally Posted by HotRodx10 I'm assuming you're talking about at a wall that's parallel to the joists (with a wall that crosses the joists, you don't need to do anything). For that it depends on how far it is from the wall to the first joist in front of the wall. Unless it's been remodeled and walls moved, I would think there would be a joist under the wall (I could be assuming too much), but the next one may not be the normal 16" away. If the next joist is 8" away or less, you probably wouldn't need to do anything to support the cantilever. If it's more, then joist hangers or cleats for 2x4 blocking between the joists should be adequate. If you're adding a joist (parallel to the wall), then matching the depth of the existing joists should be fine.
Hi. Thanks for the detailed response. Are you saying, that if the nearest joist from the wall is more than 8", then I should be able to attach 2x4 blocking between the joists using joist hangers. What would the spacing be between the 2x4 blocking? 16"? Because the floor joists are 16" apart. Since the floor joists are 2x8, shouldn't the blocking be 2x8 also?

05-22-2020, 10:09 AM   #11
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## Re: Need tongue/groove subfloor if placing over old subfloor?

You all have to think about WHY you are doing something. Joists 16” OC, you would not need anything at the ends where you are going ACROSS joists. When you hit the side that is parallel to the joists, you need to support the plywood edge....and here is the think about why. You are not Aiming at keeping the joists from twisting because that is already done with blocking, bracing, whatever.. all you are doing is supporting the overhanging piece of plywood. Therefore, no need for joist hangers, matching the supports to joist size, etc., although it might be easier to screw in a two bye eight block than a two bye four. Basically screw in any size block every 16” and you’re done. And while you’re at it, be sure to GLUE the new floor down (I like the PL line) and to use either screws or ring nails to attach. Ron

05-22-2020, 10:21 AM   #12
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## Re: Need tongue/groove subfloor if placing over old subfloor?

What's the "PL line" mean?

05-22-2020, 02:22 PM   #13
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## Re: Need tongue/groove subfloor if placing over old subfloor?

Quote:
 Originally Posted by HotRodx10 I'm assuming you're talking about at a wall that's parallel to the joists (with a wall that crosses the joists, you don't need to do anything). For that it depends on how far it is from the wall to the first joist in front of the wall. Unless it's been remodeled and walls moved, I would think there would be a joist under the wall (I could be assuming too much), but the next one may not be the normal 16" away. If the next joist is 8" away or less, you probably wouldn't need to do anything to support the cantilever. If it's more, then joist hangers or cleats for 2x4 blocking between the joists should be adequate. If you're adding a joist (parallel to the wall), then matching the depth of the existing joists should be fine.
Many of the old houses had the decking put down before the walls were put up.That means the outside joist is several inches from where you can cut. So you would need blocking or the floor will sag next to the wall over time.
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05-22-2020, 05:57 PM   #14
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## Re: Need tongue/groove subfloor if placing over old subfloor?

Quote:
 What would the spacing be between the 2x4 blocking? 16"?

16" spacing will be fine. You could probably go wider, but it's difficult to say how much.

Quote:
 Since the floor joists are 2x8, shouldn't the blocking be 2x8 also?

Doesn't need to be. The 2x8 joists are spanning probably 8' or more, where the blocking is only spanning 14-1/2". 2x3 blocking would be plenty adequate for that span.

05-22-2020, 06:50 PM   #15
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## Re: Need tongue/groove subfloor if placing over old subfloor?

I've seen installers use SQUARE head screws to install a subfloor. Do you guys have a preference for the type of screw head? Why not just Phillips head screws? For 3/4" T&G OSB, what size screw?