How To Replace the Cabinet Floor Under a Sink Base

Depending on how your under-sink plumbing arrangement looks, turning off, disconnecting, and removing drain and supply pipes may be helpful. At the very least, shut off the water supply to avoid a huge mess if you damage the supply lines during the project. Put on your safety glasses and work gloves, and use a combination of pry bar, jigsaw, and oscillating saw to remove as much of the material as possible from the floor area. 

Remove any leftover debris from demolishing the floor and clean the area with a wet/dry vacuum or rags. The more rotten material you can remove, the better. Spray the entire area with an antimicrobial spray or a mix of bleach and water to get rid of mold that may exist. Allow the area to dry completely before moving onto the next steps. 

Now that you can see the entire area without rotten materials in the way, you can make a plan. In most cases, your new cabinet floor will require you to create a support structure for it to rest on. A simple way to build one is to attach pieces of the same ½-inch plywood you're using for the floor material to the inside perimeter of the cabinet. 

The pieces will create a lip for each edge to rest on. You may also consider installing a 2x4 and some scrap material to support the floor's center seam if you need one. If your cabinet has a center door stile, cutting the new floor in half may be necessary to install it. 

Measure the interior dimensions of the cabinet where the new floor will rest. Also, measure the diameters and locations of each pipe or other penetrations in the floor. Make marks on a sheet of ½-inch treated or untreated plywood and cut out the shape and details with a circular or table saw and a combination of spade bits and your jigsaw. Cut along a center line between the two long sides of the new floor if you need to install the pieces while a cabinet door stile is in the way. 

Measure the distance between the kitchen floor and the height of the bottom of your new cabinet floor. Out of the leftover ½-inch plywood material, cut one piece for each of the four sides as wide as the distance from the kitchen floor to the bottom of the cabinet floor and as long as each side of the cabinet's inside perimeter. Use exterior grade screws to attach them to the existing cabinet frame, so the top edge height equals the height of the underside of the new floor. 

Cut and use screws to attach a 2x4 plus scrap material, if necessary, centered between the two long sides of the cabinet floor at the same height as the other support pieces. If you need to cut the floor in half to install it, this piece will support the seam between the two pieces.

Carefully place the floor, or each half of a cut floor, under the sink—pre drill at least three screw holes on each short side and at least four on the long sides. The holes should extend through the cabinet floor and into the supporting material below. Also, make at least three holes per side along the seam of the two halves if you cut the floor in half to install it. Use exterior-grade screws to secure the floor in place. 

Using a paintable caulk, fill the gaps between the new cabinet floor's edges and center seam. Also, seal areas around pipes and any other penetrations. 

If your new floor is untreated plywood, sealing it is necessary. If the material is the treated variety, you may choose to seal it or not. Allow treated wood to dry out for several weeks before sealing, however. Seal the wood with primer and paint or stain and polyurethane. Allow it to cure sufficiently before replacing the under-sink items.