Why is it recommended to lay plywood over plank subfloor???

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Old 09-29-13, 05:58 PM
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Why is it recommended to lay plywood over plank subfloor???

I am replacing the wood floors in my 1920 home. Right now, I have 3/4 TG hardwood, which is not in the best shape, over a 2.5" wide x 3/4" inch think plank subfloor, which runs perpendicular to the joists (2x10, 16 oc).

Ideally, I would tear out the existing finish flooring, screw down the existing plank subfloor where loose, repair the plank subfloor where needed and then lay my new flooring (engineered hardwood) right over the existing plank subfloor. However, the conventional wisdom seems to be that you should install a layer of plywood, at least 1/2 but ideally 5/8, on the existing plank subfloor before installing the new finish floor.

My question is why? If the existing plank subfloor is in great shape, what am I getting with the additional layer of plywood that I don't already have? I'm sure there is a reason why most people recommend adding plywood on top of plank subfloor. I'm just trying to understand.

Thanks!
 
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Old 09-29-13, 06:08 PM
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Modern flooring technology has gone beyond the plank type of subflooring, in that planks only have strength in one direction. Plywood on the other hand, seeks its strength in both directions and pushes and pulls to give that strength. I will have to agree that you should fasten the planks to the joists and install the plywood with screws intentionally missing the joists to afford "slip" between the layers. Then install your flooring. You will have much stronger floor with less chance of squeaks.
 
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Old 09-29-13, 07:05 PM
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Thank you.

If the planks are in good shape, could I get away with 3/8 plywood or do I need at least 1/2?
 
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Old 09-29-13, 07:09 PM
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If I were installing a floor in my house over planks, I would not go less than 1/2". Do you think it is going to create problems?
 
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Old 09-29-13, 07:25 PM
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Height issues, especially with stairs.
 
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Old 07-19-14, 09:49 PM
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I am in the same situation. I am going to be installing 1/2" plywood over the existing planks.
My question is, should I also place roofing felt between the two in order to eliminate squeaks? I read that in another forum, wanted to get your advice here.
 
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Old 07-20-14, 04:43 AM
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In order to eliminate squeaks, screw down the existing subflooring to the joists, then apply your plywood over it screwing it in the field, intentionally missing the joists. It will provide a "slip" between the two layers. No felt necessary, nor recommended between the strata. What will be your final flooring? Tile, hardwood, laminate (), engineered flooring?
 
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Old 07-20-14, 09:10 AM
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Plywood

Could you not use 3/8 plywood on decking planks the planks are probably 3/4 of an inch. The use of 3/8 plywood with plenty of glue, and nails. Just a thought
 
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Old 07-20-14, 09:43 AM
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The final floor will be 6x18 porcelain tile, on top of thinset, on top of a Ditra Heat mat, on top of more thinset, then the 1/2" plywood that I will screw to the planks as you suggest. No liquid nails needed I assume? thanks for your reply!
edit: additional question, when screwing the 1/2" to the 3/4" plank, that's a total of 1 1/4"..so does that mean a 1 1/4" screw should be used, or will it poke out slightly since the screw should be sunken in a little?
 
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Old 07-20-14, 11:08 AM
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Yes, 1 1/4 to 1 1/2" deck screws, not sheetrock screws will be fine. If it pokes out a little below, no harm. Mowpar, you don't glue down the top layer. The real test of whether or not the floor will take the tile is to let us know the size of the floor joists, their spacing and the total run of unsupported joist in the area. Too much bounce and the tile will crack no matter how many layers you apply.
 
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