What type of plywood to use on top of a thin subfloor

Question

I'm trying to prepare floor for a new laminate flooring. I noticed that the thickness of the subfloor plywood is only half an inch. I researched it is recommended to have at least 5/8-inch or 3/4-inch. So that's more than half an inch.

I'm planning to add another sheets of plywood on top of the existing one. I need to know if that's OK and what type and thickness is sufficient?

There is all these different types:

Sanded Plywood
Hardwood Plywood
Plywood Sheathing
OSB

I know OSB is not good for floors so that's out of the picture but any advice on what I need? Also how do I make sure the floors are level? Do I use shims or maybe some mix? I'm planning to screw them.

Note: The floor is laying on wooden studs directly on the concrete. There is no basement underneath. So no joists.

I'm planning to use https://www.homedepot.com/p/15-32-in-x-4-ft-x-8-ft-3-Ply-RTD-Sheathing-132411/100067329. Is that thick enough?

Answer 1

Since you've indicated that your existing subfloor is bouncy you should use a minimum of 3/4" (1" if your pockets are deep) plywood regardless of whether you lay it over the existing subfloor or remove the existing subfloor.

You cannot just add 1/4" plywood on top of the 1/2" to add up to 3/4" because it will only be as strong as the weakest section of 1/2" plywood.

This will be a good chance to assess the health of your floor joists or sleepers. If they are unlevel then this is the perfect time to sister them or perform other repairs.

Lastly:

• Sanded Plywood
  • Generally reserved for projects where appearance matters such as shelving or a cabinet
• OSB
  • If it gets wet then it could swell permanently
  • Requires more thickness for same joist spacing when compared to plywood
• Plywood sheathing/hardwood
  • See excerpt below from Reddit

Plywood is graded by thickness, which is pretty obvious, species (some sort of pine, if not specified), and the quality of the faces, and whether its rated for exterior use.

So 3/4 ACX is 3/4 thick (nominally, usually bit less) pine or fir or spruce, one side A grade, the other C, rated for exterior use. CDX, commonly used for sheathing is grade C on the good side, D on the not good side, and rated for exterior use. Note that doesn't mean 'waterproof' just 'won't fall apart instantly'.

You want A or B for work surface, as they will be smooth, with any holes filled. C and D can have unfilled knot holes (up to 3" in diameter for grade D, as I recalll.) You're only buying one sheet of it, it's worth the money. Around here, it's about $25 for a sheet of CDX and $35 for ACX. Adds up when you're building a house, but not much for a work bench.

Also, I find painted work benches vastly superior. They're easier to clean, and easier to find things on.

Answer 2

Once the carpet and pad are removed assess the condition of the old plywood i.e. valleys between joists, signs of delaminating, etc.

Determine if your joists are springy due to being undersized or spanning further than optimal. Consider adding drop sill as needed to stiffen bouncy joists.

If you have lots of squeaks when walking the old floor after that strongly consider removing the old plywood.

If the old plywood is simply thinner than optimal you can leave it in place but understand that it provides little to the structure of the new floor. Overlay with minimum 5/8 t and g plywood.

Make certain that all fasteners are adequate length and hit the joist solidly.

Answer 3

I generally agree with MonkeyZeus--treat the situation as though you're starting from scratch.

However, if your existing subfloor isn't terribly saggy or otherwise degraded, another layer of 1/2" material (OSB would be just fine) with joints staggered would yield a good outcome. I reworked a similar situation just recently and was very happy with the results under engineered hardwood.

Answer 4

Plywood “subfloor” is rated for structural spans. Usually you’ll find a stamped number on one side (I think the lower grade side so it can be seen by inspectors after the floor covering is installed). The number looks like this: 24/16. The first number is for roof spans and the second is for floor spans.

The reason the standard construction practice provides for underlayment over subfloor is that a subfloor can be installed early in the construction process allowing workers to walk on it and scuff it up if necessary. Then, when it comes time for the final “finished flooring “ the underlayment can be installed and be without blemishes. (I know…someone will tell me they now make extra thick plywood that combines the underlayment and subfloor in one process. However, we don’t use it in rainy conditions or long construction projects.)

Evidently your subfloor is overextended. So you’ll need to over compensate by using a thicker underlayment. (Check to see that the subfloor is installed correctly..perpendicular to joists… and nailed adequately… 6” on center at a minimum per code.)

So, I’d use “Top Layer plugged and Touch Sanded” plywood underlayment (thickness depending on existing conditions, spans, etc.). Touch Sanded because of the finish flooring you’re proposing. You do not want any defects “telegraphing” through the flooring material.

Give us more info on size, spacing, etc. of joists and thickness of existing plywood and we can make a recommendation for the new underlayment.