Your sink cabinet is extremely prone to rot and degradation. Eventually, even a small leak in the enclosed space beneath your sink can weaken the cabinet. Early on, the particle board floor begins to warp and sag. Then the vertical walls detach, and your Lysol and Windex bottles tumble into a pothole!

Not surprisingly, a lot of contractors say that replacing the whole cabinet is necessary. Personally, I think this is a wasteful and expensive approach.

This article and video describe a much easier, more cost efficient repair. In fact, it will even help prevent the problem from occurring again.


Tool List:

Table Saw or Magnesium Circular Saw

Drill/Driver Combo Set

Drip-free 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.


Material List:

Liquid Nails

Brad Nails

1/2″ plywood (cabinet-grade)

1x2x8 trim pine

Stain/Polyurethane or Paint

Small brush


Sink Cabinet Repair Overview

new unfinished plywood cabinet floor

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

This solves a few issues at once. It keeps the cabinet in place. That way, you don’t have to remove the countertop, sink, and plumbing. After all, all that stuff is usually OK. Also, it gives you a semi-stable surface to start with. The old floor acts as a temporary platform until you get the new floor installed and stabilized.

When I notch for plumbing, that’s probably the most difficult phase of the job. However, hole saw bits make that step a breeze. I 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 all sink cabinet problems come from some sort of plumbing failure. Leaking drain lines, loose gaskets, and damaged water supply lines are common. Now the water collects in the otherwise dry confines of your cabinet interior.

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

Here’s the key

Before you even attempt this repair, have your plumbing problem fixed. Period.

The materials that I use are more water-resistant than typical sink cabinet materials. However, anything will rot, given enough time. Save yourself a handful of problems and have your sink fixed by a qualified plumber. Periodically check it 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 the Sink Cabinet Floor

First, pull measurements on your cabinet interior. Almost all base cabinets measure 24″ deep, and something like 32″ wide. Naturally, the floor itself is slightly smaller than this. (Since it sits within the walls and face frame of the cabinet.)

You only need one piece of 1/2″ plywood big enough to fill this space. A 2’x4′ piece should do. Lowes and Home Depot sell plywood by the half-sheet and quarter-sheet. So you save money and hassle by not having to buy a full sheet.

The repair is complicated by the fact that you can’t to fit a full-sized replacement piece into the cabinet. That’s because the face frame divider in the front is in the way.

That means you 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.

This piece may meet with plumbing lines near the back of the cabinet.  If so, you can’t lay it down flat right away. Instead, you have to sort of stand it up like a ramp. Hold the front of the piece high. That way, it’s pointing down and is touching the pipes.

Reach into the cabinet with a pencil and mark the plumbing locations on the edge of panel. Draw a single line at the exact center of each pipe. Eyeball the location of the plumbing directly from the front, to be sure you’re lined up well Very importantly, be sure that your panel is pressed up against the side wall firmly. If you don’t, 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 frame. In other words, from the front edge of your piece to the pipe. To determine the pipe’s width, hold the blade of your tape up to the pipe and sight it.

It’s a tight fit

The sight lines are hard to achieve. However, getting these numbers right is crucial for making this repair work. You probably will have to stuff yourself into the cabinet to get accurate measurements. Try not to bang your head in the process. If you do, try to keep the cussing level down. You don’t want to scare the kids. Or educate them!

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 to use 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, select ones about a 1/2″ bigger than the diameter of your pipes. Use them to cut your holes.

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

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

In either case, finish 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.

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 “sweat” with condensation. That water will be absorbed by 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 other 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 can account for it.

Cut the second piece to fill this existing space as closely as possible. Is there plumbing on this side of the cabinet?  If so, repeat the steps 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  front of the cabinet. If there are gaps at the side and back, that’s okay. These gaps will be less noticeable, and you can fill them later with caulk or construction adhesive.

Step 6. Attach Both Sink Cabinet Floor 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 place the panels back into position in the cabinet. Gently, push them down into place. Try not to get adhesive on the faces of the new panels. (Additional cuss warning.)

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

Is there a big dip in the center of the floor?

If the center of the cabinet has an especially large dip in it, add a support block under these two pieces.

sagging plywood in sink cabinet

To do this, find a scrap of plywood several inches long and a few inches wide. Smear both sides with adhesive. Lift one panel and get it out of the way temporarily. Position this support block halfway beneath the edge of the panel that’s still lying down. Use a brad nailer to shoot a few nails down through it. (DON’T SHOOT NAILS THROUGH YOUR FINGERS!)

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

When the adhesive on the support block dries, these panels are locked together very securely. They’re certainly strong enough to carry the weight of whatever you store in the cabinet.

Everything should be positioned where you want it. Shoot a few nails through perimeter of the new floor pieces, 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 conceal it with a thin strip of trim.

First, rip a piece of flat 1×2 pine trim to a thickness slightly more than the thickness of ply panel. Next, cut it to length. It needs to fit between the center divider and the vertical stiles of the face frame. Make a custom piece 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. Paint or Stain the New Cabinet Floor Panels

use stain or paint on the new sink cabinet floor

If your cabinets are painted, prime and paint the new panels and trim pieces. Match the cabinet face color 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 colored caulk or adhesive. Run a bead just large enough to hide the gap around the full perimeter. Smooth it into place with a gloved fingertip. (I used more Liquid Nails in this project because it was relatively close to the stain color.)

Sink Cabinet Repair Wrap-Up

the completed repair for rotten sink cabinet floor

Let everything dry thoroughly before you put any items into the sink 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 to carry the fumes away.

Afterwards, be sure to check the sink plumbing occasionally for leaks. The new sink cabinet 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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