Sink cabinets are extremely prone to rot and degradation. One small leak in the enclosed space beneath your sink is often all it takes to weaken the cabinet permanently. Soon, the particle board floor begins to warp and sag, the vertical walls detach, and your Lysol and Windex bottles tumble into a pothole.

A lot of contractors will tell you that replacing the whole cabinet is necessary. But, I think this is often a wasteful and expensive approach.

This article and video will describe a much easier, more cost efficient repair–and one that will even help prevent the problem from occurring again.

(The content below contains some affiliate links.)

Tool List:

Table Saw or Circular Saw

Drill / Driver

Caulking Gun

Best Picks:

HOLE SAW BITS give this project a great finished look. Dewalt’s quick-change system has great reviews:

If you want a more multi-functional option, a jigsaw will work just as well!

BRAD NAILER / COMPRESSOR combos are great for this repair and pretty much all DIY:

Hole Saw Bits or Jigsaw

 

Material List:

Liquid Nails

Brad nails

1/2″ plywood (cabinet-grade)

1x2x8 trim pine

Stain/Polyurethane or Paint

Small brush

Project Overview

new unfinished plywood cabinet floor

To tackle this problem, I usually like to install a new plywood floor over the old floor.

This solves a few issues at once. It keeps the cabinet in place, preventing you from having to remove countertop, sink and plumbing just to address structure below. Also, it gives you a semi-stable surface to start with. The old floor (or whatever is left of it) can act as a temporary platform until you get the new floor installed and stabilized.

Notching for plumbing is probably the most difficult phase of the job. However, hole saw bits make that step a breeze. I’ll explain below what they are and how they’re used.

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Step 1. Fix Your Plumbing

damaged cabinet floor caused by water dripping from sink plumbing

99% of cabinet sink problems come from some sort of plumbing failure. Leaking drain lines, loose gaskets, and damaged water supply lines can all result in water collecting in the otherwise dry confines of your cabinet interior.

This is a serious problem, because modern cabinets are often made of shoddy materials that wick water rapidly. And, given that you might go days without looking into this unsightly area, the problem can run rampant before you even realize its happening.

So, before you even attempt this repair, it’s important that you have your plumbing problem fixed. 

The materials that we will be building with are more water-resistant than typical cabinet materials. But, anything can rot given enough time. Save yourself a handful of problems by having your sink fixed by a qualified plumber. Then, check it periodically to make sure nothing else has failed.

(Here’s a good link that explains basic sink plumbing components, in case you want to know a little more about how you sink works.)

(And, if you’re an adventurous DIYer and are considering attempting a sink repair yourself, this video might help…)

Step 2. Measure Cabinet

Go ahead and pull measurements on your cabinet interior to start out. Almost all base cabinets measure about 24″ deep, and something like 32″ wide. (The floor itself will be slightly smaller than this, because it sits within the walls and face frame of the cabinet.)

You really only need one piece of 1/2″ plywood big enough to fill this space. Lowes and Home Depot sell plywood by the half-sheet and quarter-sheet, so a 2×4 piece should do.

The repair will be complicated by the fact that you won’t be able to fit a full-sized replacement piece into the space because of the face frame divider in the front.

Therefore, you’ll need to install the new floor in two separate pieces that meet in the center.

Pull a measurement from the left wall to approximately the center of the cabinet (behind the face frame divider). This will be the width of your first piece.

(For precision, wait to measure the second piece until the first has been installed.)

Step 3. Cut First Piece, Mark For Pipes

cut plywood panel to width and length using table saw or circular saw

Use a table saw or circular saw to cut the first piece to length and width. Then, bring it into the house and work it roughly into position inside the cabinet.

If this piece meets with any plumbing lines near the back of the cabinet, then you won’t be able to lay it down flat right away. Instead, you’ll have to sort of stand it up like a ramp, with the leading edge pointing down and touching the pipes.

Reach into the cabinet with a pencil and mark the plumbing locations on the panel. Just draw a single line at the exact center of each pipe. Eyeball these locations directly from the front to be sure you’re lined up well, and be sure that your panel is pressed up against the left wall firmly. If you don’t, then the panel won’t sit where you want it to.

measure for plumbing pipes inside cabinet by using tape measure

Finally, measure the width of each pipe, and where they sit in relation to the front wall of the cabinet. Butt your tape to the pipe and pull to the inside edge of the cabinet face to determine their depth in the cabinet. To determine the pipe’s width, hold the exposed blade of your tape up to the pipe and sight directly at it.

You’ll probably have to stuff yourself into the cabinet to get accurate measurements, and sight lines are hard to achieve when your banging your head on a drain pipe. However, getting these numbers right is crucial for making this repair work.

Step 4. Cut Holes For Pipes

cut clearance for pipes in plywood using jigsaw or hole saw bits and electric drill

There are two ways you can handle these cuts. The easiest of the two is definitely using hole saw bits.

These circular, saw-toothed bits chuck into your drill and cut by spinning like any normal drill bit. They come in a range of sizes. If you have hole saw bits, then select ones that are about a 1/2″ bigger than the diameter of your pipes, and use them to cut your holes.

If you don’t have hole saw bits, then you’ll probably have to cut these holes with jigsaw.

This is a lot more difficult. You’ll have to find circular objects (appropriately sized) to use as tracing templates for laying out the holes. Then, you’ll have to use the jigsaw to follow these guide lines.

In either case, you’ll have to finish off this step by cutting straight lines from the holes to the back edge of the panel. These straight cuts will provide clearance for the panel to slide into the cabinet around the pipes.

When you’re finished, test the piece to make sure that you’ve laid everything out accurately. Also, make sure that the panel is not touching copper pipes. These metal lines can “sweat” with condensation, and the residual water will be absorbed into the inner edge of the panel.

partial new plywood floor for sink cabinet wood rot repair

Step 5. Measure And Cut Second Panel

With the first panel lying in place, pull measurements to it from the right side of the cabinet. Measure at the front edge of the first panel, and the back edge. This way, if the cabinet is slightly out of square, you’ll be able to account for it.

Cut a second piece to fill this existing space as closely as possible. If there are more plumbing protrusions on this side of the cabinet, then repeat the steps detailed above to lay out pass-through holes for the second panel.

Test fit the second panel in the cabinet. Focus on getting the two panels to fit tightly together at the center, and at the front of the cabinet. If there are gaps at the side and back as a result, then that’s okay. These gaps will be less noticeable, and you can fill them later with caulk or construction adhesive.

Step 6. Attach Panels

liquid nails adhesive spread around perimeter of cabinet floor for new wood rot repair

Remove both panels from the cabinet and run a thick bead of construction adhesive around the perimeter of the old floor. Use big blobs in any places where the floor seems to have sagged below level.

Carefully move the panels back into position in the cabinet. Push then down into place, but try not to get adhesive on the faces of the new panels.

Spread a little adhesive on the two edges where the two panels meet. This will help hold them together more securely.

If the center of the cabinet has an especially large dip in it (as the one in this project did), you might need to add a support piece below these two pieces that will lock them together.

splice plywood repair panels together with a sister piece glued beneath the two

Just take a scrap of plywood several inches long and a few inches wide, and smear it with adhesive. Lift one panel and get it out of the way momentarily. Position this support piece halfway beneath the edge of the panel that’s still lying down, and use a brad nailer and compressor to shoot a few wire nails down through it. (BUT DON’T SHOOT NAILS DOWN THROUGH YOUR FINGERS!)

Now lay the other piece back in place, and shoot through it into the support piece as well.

When the adhesive on the support piece dries, these panels will be locked together far more effectively. They’ll certainly be strong enough to carry the weight of whatever you store in the cabinet.

When everything is positioned where you want it, shoot a few brads through perimeter of the new floor into the old floor. This will hold them in place while the glue dries.

Step 7. Cut And Attach Face Trim

plywood edge in cabinet is left exposed and must be covered up with pine trim wood

The edge of the new panel will now be visible from the front. No one likes to look at an exposed plywood edge, so I like to conceal it with a thin strip of trim.

Just rip a piece of flat 1×2 pine trim to a thickness slightly more than the thickness of your ply panel (about 9/16″), and cut it to length to fit between the center divider and the outer pieces of the face frame. Make a custom one for each side of the cabinet opening.

use custom pine trim piece to hide plywood edge in finished cabinet floor

I don’t even bother to nail these pieces into place sometimes. I just cut them so they fit tightly, spread glue on the undersides, and push them down into place to let them dry on their own.

Step 8. Finish The Panels

use stain or paint to finish new cabinet floor for wood rot protection

If your cabinets are painted, then prime and paint the new panels and trim pieces to match the cabinet faces as closely as possible. Use an exterior paint to help protect against future leaks, and apply at least two coats.

If your cabinets are stained, get a stain/polyurethane combo that will match the cabinet exteriors. Close enough is good enough here–this isn’t a place that will be seen often. Again, two or more coats are necessary.

Finally, hide the gap around the edge of the panel with a similar-colored caulk or adhesive. Run a bead just large enough to hide the gap around the full perimeter, and smooth it into place with a fingertip. (I used more Liquid Nails in this project because it was relatively close to the stain color.)

Wrap-Up

finished plywood panel repair for rotten sink cabinet floor

Let everything dry up thoroughly before you put cleaning products and such back into the cabinet. Also, try to provide as much ventilation as possible while the wet paint and adhesive set. Open a door, or run the HVAC fan in your whole house to clear the fumes away.

Afterwards, be sure to check the sink plumbing occasionally for leaks. The new floor panels will be able to stand up to a lot of abuse, but why test them? Sitting water in a house is never a good thing!

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