You are most likely to find plywood subflooring in houses that have basements, or are built off the ground, or are more than one story tall. The most common reason for rotten plywood subflooring that needs replacement is water damage. So you would expect it to be located in the kitchen or in a bathroom. If you are unlucky enough to find yourself in this situation, you are a prime candidate for plywood subfloor replacement.
Planning and Preparation
In all likelihood you are not going to need to replace the entire subfloor because damage is bound to be limited to certain areas. To find out where the damage is it will be necessary for you to remove your present finish floor, whether it is carpet, vinyl sheet goods, or something else. This is a good opportunity to do some remodeling anyway. The only exception is if you can inspect the plywood from the basement below.
Before taking any action, call your homeowner insurance agent to do a survey and document your situation. If you are covered and do the work yourself you will be money ahead since you are saving on labor.
If you have carpet and think you can save it, unhook it from the tack strip and roll it out of the way. Otherwise, just rip it and the padding out. Likewise for whatever other kind of finish floor you have.
Survey the Damage and Prepare the Site
With the finish up you can inspect the plywood subfloor and make a determination as to which areas need replacement. Inspect it visually and prod it with something to search for mushiness. Once you have determined the areas that are bad and need replacement, strike chalk lines to mark them off.
There are two things to remember at this point – the areas you mark off should be square (use the 3-4-5 method), and any cuts to be made in parallel with a floor joist should be down the center of the joist. You can strike your line through the center of the nail heads.
Remove the Old Subfloor
The best thing to use to cut the bad sections out is a Skil circular saw. Before making any cuts, attempt to look on the underside of the subfloor if possible (such as from the basement), to ensure that you will not be cutting through any electrical wiring attached to it. Set the blade cutting depth approximately one quarter of an inch deeper than the subfloor for cuts in the field and one eighth of an inch for cuts over joists.
When you get close to any wall you will find that the plate on the Skil saw will keep you away from the wall. You might get a little closer with a Roto-Zip tool but after that it will be time to get out the chisel and hammer.
With the old subfloor ripped out, it is time to get the replacement in. Two things to make note of are the thickness of the old plywood and the square footage that you will need for replacement. Plywood comes in four foot by eight foot sheets so bear in mind that you will need to buy a little extra. You can probably eyeball it to get a rough estimate. (See Different Types of Plywood)
Once you get your plywood home it is just a matter of measuring, cutting, and nailing or screwing. Coarse thread drywall screws are a good bet for this type of project. Once you get all the pieces in, use putty in the gaps.
You may find that you will need to sand or scrape the old subfloor. After that you’re done and it’s time for the new finish floor!