Why Do My Floors Seem Weak?

Check Floors

• To check for a weak floor, go to an outside wall opposite the support beam or load-bearing wall that normally runs down the center of the house. Stand on tiptoe and drop down hard on your heels. Then go to the middle of the floor and repeat the tiptoe-drop test. You should feel the most bounce there. Then stand near the central support beam and repeat tiptoe-drop. It should feel about the same as it did at the exterior wall. If the center beam feels bouncy, consult your local building inspector or a structural engineer about reinforcement methods.

Add 'X' Bridging

• A weak floor may be the result of insufficient or missing “X” bridging. A floor joist placed under load tends to flex to one side or the other. The “X” bridging stiffens the joist by transferring some of the load to neighboring joists. There should be an “X” bridge between joists at the center of the span. To stiffen a weak floor, add an “X” bridge halfway between the center and end of the floor span on each side of the center bridge. Metal joist bridging is sold in hardware and home centers.

Plywood Bracing

• Add substantial stiffness to a weak floor by screwing sheets of 3/4-inch plywood to the joists. Start by sanding the bottom of each joist. Spread subfloor-grade construction adhesive along the joist bottoms and press the plywood sheet against the joists. Set the plywood sheets parallel to the joists, running the sheets down the center of the span and leaving the joist ends exposed. Screw the sheets to the joists, placing the screws 4 inches apart.

Add Support

• If the other methods aren’t sufficient, or the floor is sagging as well as bouncing, add a supporting beam or build a supporting wall beneath the center of the joists. Make a beam from a pair of 2-by-12-inch joists bonded together. A beam needs a supporting column every 10 feet. If you choose to build a supporting wall, make sure there’s a stud directly under each joist. If your supporting wall will have a doorway, frame it with a double 2-by-6-inch header.