How To Install Plywood Over An Existing Subfloor? (5-Step Guide)

When making renovations, you may need to modify your subfloor to accommodate your new choice of flooring. ½” plywood may work for carpet, but it isn’t strong enough for heavy stone or tile. Unsure if you can install plywood over an existing subfloor? Well, good news because yes, you can!

What You Need To Know About Installing Plywood Over An Existing Subfloor?

You can only install plywood over a subfloor that is in good condition. Simply slapping more plywood over a decaying subfloor is a waste of time, money, and it isn’t safe. Make sure your subfloor is level and free from water damage or warping. If the subfloor is concrete, it should be level, free of cracks, and will require a moisture barrier. Just as a house is only as strong as its foundation and framing, flooring will only be as good as the subfloor underneath.

Laying On Top Of A Wood Subfloor

Laying plywood over existing plywood or wood plank flooring is easier than concrete. You can use galvanized screws or shanked nails to fasten your new plywood to an existing wood subfloor. They hold better than staples and smooth-shanked nails, which adds to the life of your floor. Staples and smooth shank nails are notorious for pulling out over time, leaving you with an uneven and squeaky floor.

Laying On Top Of A Concrete Subfloor

If you are laying plywood on top of concrete, you will need special fasteners and a moisture barrier. There are different ways to lay plywood on top of concrete that will call for different types of moisture barriers, fasteners, and tools.

You can nail your plywood to a concrete subfloor using a powder-actuated nail gun and concrete nails or by using concrete screws and an impact driver. Screwing to concrete will require a hammer drill to drill pilot holes for the screws. Plastic sheeting and roofing felt work best for nailing/screwing into concrete.

Plywood can be glued to a concrete floor using a liquid roll-on adhesive as a moisture barrier.

A floating plywood floor won’t require fasteners, but you will still need plastic sheeting to act as an effective moisture barrier.

Supplies You’ll Need For Installing Plywood Over An Existing Subfloor

  • Broom/shop vac
  • Pry bar
  • Tape measure
  • Utility knife
  • Hammer/nail gun
  • Ring shank nails
  • Powder actuated nailer
  • Hammer Drill
  • Concrete nails/screws/adhesive
  • Impact driver/screw gun
  • Galvanized screws
  • Plywood
  • Water barrier (if laying on top of concrete)

How To Install Plywood Over An Existing Subfloor ? (5 Steps)

Before starting, be sure to check all local building regulations to make sure that your project passes inspection. As long as you take the time to prep your floors, you will have a finished product that you can be proud of.

Step 1: Remove Existing Flooring

You may need to strip the existing flooring in order to get to the subfloor. Tearing out carpet is much easier than something like tile, but no matter the flooring, you can remove it with a little elbow grease.

Sometimes there will be an underlayment in between the flooring and the subfloor, which will usually be plywood or cement board. You will want to get rid of this as well because you don’t want this underlayment sandwiched between the existing subfloor and your new plywood. It will open your project up to problems with leveling, mold, and mildew. If your new flooring calls for an underlayment, it will go on top of this new layer of plywood.

Make sure your subfloor is relatively clean and free of anything that will cause bumps in your new layer of plywood.

Step 2: Determine What Materials To Use

Depending on your needs, decide the thickness of the plywood and other materials to buy. This will vary depending on if you are trying to match the height of flooring in another room, installing a new type of flooring that calls for a thicker subfloor, or if you are laying plywood over concrete vs. wood.

In the spirit of safety and structural integrity, you should use a fastener that is long enough to hit the floor joists or concrete. Attaching new plywood to the subfloor can lead to an uneven finished product if the subfloor moves or warps.

Regardless of the material of your subfloor, you will want to use a fastener that is at least 1” longer than the thickness of your plywood. That means if you are laying ½” plywood over a ½” subfloor, you will want a fastener that is at least 2” long.

Measure your room to determine the number of plywood sheets you will need. Make sure the room is square, level, and snap a chalk line for your first row of plywood.

Step 3: Let The Plywood Acclimate

Since wood is known to shrink and expand in different climates, it’s a good idea to let the plywood sit in the room for 72 hours. This will prevent buckling and gaps in between your sheets should they change in size.

Step 4: Laying The Plywood

This process is very similar to laying a plywood subfloor, except that this layer should run perpendicular to the existing subfloor so that the seams of your plywood line up with the joints of the existing subfloor. This pattern will help prevent any weak spots or uneven spots from crowns in the wood. Nail or screw every 6”-8” along the edges of the sheets and 12” in the middle.

Be sure to leave 1/16”-⅛” gaps between sheets to allow for some fluctuation in size so that the plywood doesn’t bow or buckle.

Step 5: Check For Imperfections

Your new subfloor should be smooth and level at this point. If there are any bumps in the plywood, you can sand them out.

Final Thoughts On Installing Plywood Over Subfloor

Adding a layer of plywood to your subfloor is much easier than installing a brand-new subfloor altogether. As long as the existing subfloor is in good shape, you can go ahead and lay another layer of plywood to achieve a strong and smooth subfloor.

Meet your Flooring Expert

Travis McCullough

Travis McCullough

Travis is a lifelong jack-of-all-trades in the construction industry with 20 years of experience in a variety of fields. He’s tackled flooring, carpentry, and everything in between on residential and commercial projects of all shapes and sizes.
Working independently and as part of a crew has equipped him with the know-how to not only complete a project but also teach others the finer points within most building professions. When he isn’t out hanging off of a ladder or crawling around on a roof, Travis spends his time educating people about the construction industry.

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