How to Repair Floor Joists in Sagging Kitchen Floors

Location and Preparation of the Affected Area

• With a circular saw, cut an exploratory hole, roughly a 4-inch square, into the lowest point of the sag. Set the depth gauge on the saw's base plate to 1 inch to account for a 3/4 inch thick subfloor and the thickness of the flooring material to avoid cutting into any undesired utility, material or component beneath the surface. If this does not penetrate all the way, adjust it 1/4 inch deeper and try again. Use this hole to locate each floor joist.

Layout of Repair Location

• Mark the floor with a carpenter's pencil 3/4 inch in from the edge of each joist to establish its center. You need to mark a straight line down each joist at this centerline location, so at this point, extend your hole in the direction of the damage and mark two more joist locations at least 6 inches further down, then connect them with a straight edge or chalk line and mark the entire length of the damaged area. Once you have marked all of the damaged area, connect the two parallel lines with 90-degree lines, forming a rectangular cutout. Cut off the repair along the most natural edge of the flooring material to aid in matching the pattern with your repair.

Demo of Affected Material

• Measure the actual thickness of the undamaged, unswollen flooring from the surface to the top of the joists, then set the saw's baseplate accordingly and cut out the damaged rectangle. Thoroughly inspect the damaged area and both joists. If either joist appears rotted or is also sagging across the damaged area, extend the repair to the next joist out, using the steps above. As always, wear adequate safety goggles when cutting. While inspecting the area, try to determine the source of the water leak. If the cause is not immediately obvious, call a professional plumber to inspect and address this situation before finishing the repair, or it will only be a matter of time until it happens again.

Sistering the Rotted Joists

• Any compromised joists are now open from above and accessible on both sides. Cut a matching piece of straight structural lumber -- usually 2 x 6 or 2 x 8 pine -- to a length that spans the damaged area with an overlap of at least 2 feet into the undamaged board in both directions. Be sure to span the entire repair area such that the new, or "sistered" board now reaches just under the existing subfloor in both directions. Secure the board at this level and nail it into the damaged board, nailing only into the undamaged regions on both sides of the rotted area. Fasten in an "X" pattern -- through the new board and into the old one -- on both sides of the rot using 16d framing nails or 2 1/2-inch wood screws.

Cut Out Rotted Area and Replace

• Setting the baseplate on the circular saw for 1 1/2 inches to account for the board thickness, mark a 90-degree line on both sides of the damage and cut straight down on those lines to remove the rotted wood from the new member. Now cut a piece of fresh lumber to this size and shape and fasten it into place, making sure that the top edge is flush all the way across the repair. Depending on the integrity of the old joists, at this point you may consider adding a fresh sister board onto both of the sides using the same basic method. It is a cheap way to add additional support to high-traffic areas prone to this type of damage and provide additional wood to screw the new subflooring into.

Patch the Subflooring and Replace Flooring Material

• Using a caulking gun, apply a smooth bead of construction adhesive to the topside of all visible joists and sisters. Cut out the replacement plywood, matching the existing subflooring exactly, and press it into place before the glue dries. Mark the subflooring patch for all the joist locations, then screw the patch into place as tightly as possible, using one 2 1/2-inch wood screw every 4 inches along the seams and no less than one every 6 inches out in the body of the piece. Always use fine-thread wood screws and adhesive instead of nails to eliminate squeaky spots and movement from expansion and contraction due to temperature and humidity changes that could cause the patch to lift and buckle, deforming the flooring material above it. Replace the flooring to match the existing material.