Replacing Sections of a Plywood Floor

There are a couple main reasons why you might want to replace your plywood floor. Either it’s rotting out or it’s so old and damaged it feels like a trampoline on some parts. To be perfectly honest, whether your floor is actually rotten or not, it’s a pretty BAD indication when you sink 2 inches whenever you step on “that spot” in your kitchen. Aside from your honest-to-goodness rotting out, plywood can also get damaged and eventually cease to perform its duties as a floor by not having enough joist support beneath it.

I have been genuinely shocked by the insane joist-intervals on some floors! I mean, seriously?? – 50cm from joist-center to center using half-inch plywood is unthinkable in my opinion. And the joists aren’t exactly logs either. They’re 5X5cm sticks! Anyway, it’s not the joist cross-section that’s the problem, it’s the enormous intervals. Needless to say, this floor was in such bad shape that I just had to tear the whole thing up and redo it. The plywood wasn’t even rotten, but it was beyond salvaging.

What happens is that floor traffic, over time, gradually weakens the plywood right at its most vulnerable spot between the joists, eventually breaking the bond that’s holding the wood-sheets together. Without the glue binding it all together, the plywood is no more than a glorified sheet of processed paper.

The solution to this is to never indulge your temptation to skimp on floor joists! It’ll cost you more in floor joists initially, but it’ll save you much more in plywood and labor in the long-run. The absolute minimum joist interval when using half-inch plywood – assuming your joists are in the 5X5cm neighborhood – is 40cm from center to center. But even if your joists are wider, you should never go over 35cm from edge to edge. Ideally, you should close your gaps to 25 or 30cm, but if you’re not expecting heavy traffic, it may not be necessary.

But let’s for a moment, assume you have your joists spaced according to code, but for some reason you still have a plywood floor that needs replacing. Fair enough…

1. First of all, determine the area that needs to be replaced. That’s right, you don’t necessarily need to redo everything. Maybe you do, but most likely you don’t. Say you conclude that the square meter of floor right in front of the sink is all that needs to be replaced. – More power to you!

2. Cut or peel away the lino or carpet you have over your plywood. You should cut away about 20cm more than what you’re planning on replacing, as you’re going to need space to pass your circular saw.

3. Now’s the tricky part. You’re gonna have to locate the center

2x4 joists spaced at 1 foot centers w/ plywood

of the floor joists immediately surrounding the area you want to cut out. Why? You are essentially cutting out a part of your floor and replacing it! All 4 edges of both the remaining floor AND the new piece you will be installing need solid joists to sit on. You will accomplish this by slicing 2 sides right down the center of a joist – and the other 2 ACROSS the joists, perpendicular to them. Finding the centers will be simple, being that you’ll see nails or screws giving away their position. (You may need to remove these nails or screws to accommodate the saw-cut.)

4. After determining your best cutting path, draw a straight line to indicate the area you will cut out. It is important to get this as square as possible, as it will make it easier for you to re-fit the new piece of plywood.

5. Now you’ll need to screw down a wooden straight-edge. This will guide your saw as you cut. You can only secure the 2 parallel lines at one time, as the other 2 will interfere with the saw passage. Measure the distance from your circular saw’s plate edge to its blade and secure the straight-edge to this measurement. You may need to adjust this distance several times to get it right if this is the first time you’ve attempted this method. But what’s more important than getting the plate-to-blade distance perfect, is getting the entire thing square!

6. Adjust your blade to a millimeter or 2 deeper than the plywood, turn it on, and slowly lower the blade in. You’ll find that you’re going to have to slice past the corner point to compensate for the blade curve, but that’s ok. Once the new lino or carpet is put over it, no one will ever know!

7. Measure and cut out your new piece of plywood. Once you do so, hold your breath as you lower it into your floor. That moment of truth when the final piece of the puzzle falls into place is priceless, and you should savor it! Screw or nail the plywood into the joists and you’re done! Congrats!


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