Fixing Bouncy and Sagging Floors

Gary writes: The first and second floors in our house seem overly bouncy and sag toward the middle.  How can we evaluate whether the floors need  reinforcement and, if so, what is the best way to do this?

Steve Bliss of responds: If the floor is sagging visibly in the middle or is very bouncy, you may want to reinforce the floor to eliminate the sag and reduce the bounciness (for example, dishes rattling in a cabinet when you walk by). This is common in old houses and is usually more of a nuisance than a safety issue. Still it  is a good idea to have the problem evaluated by an expert, such as a structural engineer or experienced contractor. It should only take an hour or so of his time — ask ahead of time what the cost will be to evaluate the problem and recommend a solution.

Bounciness is usually caused by undersized floor joists. Sagging can be the result of undersized joists, an undersized support beam, or support posts that have rotted at the bottom or settled into the ground. Other causes can be joists that were notched or drilled improperly  (see our Guide to Notching and Boring Joists) or are weakened by decay or insect damage.

Rotten or damaged joists may need to be replaced, and the moisture problems that caused the decay should be addressed.  Assuming the joists are in good condition, the solution to both sagging and bounciness problems is to reinforce the floor by either beefing up the floor joists or adding new posts and/or a new or reinforced support beam where this is practical.

A small amount of sag in joists under a load is normal “deflection.” A large amount of sag that has permanently set into the wood is called “creep.” To reduce this type of sag, the wood must be very slowly forced back to level, typically using hydraulic or screw jacks pushing up on a temporary beam. This is done slowly over a day or more to minimize damage to flooring, plaster, and other finishes. The same approach can sometimes be used to straighten a sagging beam if the deflection is not too great.

Once the sag is removed, the floor will need to be held in place with a new or reinforced beam. In a basement or crawlspace, posts can be placed along the beam to support it. The posts must rest on properly sized concrete footings to prevent settling. If adding posts is not an option, then you may need to add a new correctly sized beam made of wood, engineered lumber (LVL), or steel.

If you are able to straighten the support beam with a jack, and don’t want to replace it, another option is to reinforce the straightened beam with a steel flitch plate.

Flitch plates. Existing beams can be reinforced by bolting on one or more long pieces of ¼ or ½-in. steel plate, called a flitch plate, along the length of the beam.  Short  flitch plates of steel or LVL can also be bolted on both sides of a joist to repair a cracked or damaged section.

Sistering. Where sagging is not the main problem, bouncy joists can be stiffened by “sistering” a new wood or engineered-wood joist alongside the existing one. Attach the sisters to the existing joists with nails, screws, or bolts (bolts are best).

Add plywood below. Many years ago, I contracted a remodeling job where the plumber cut a series of deep notches in the tops the floor joists, right across the middle of the span. Notches should never be made in the middle third of the span. The floor had started to sag and the plaster ceiling below was cracking. We brought in a structural engineer who had us remove the plaster ceiling below and reinforce the entire ceiling with heavy plywood sheets screwed and glued to the joists, creating a diaphragm — essentially a huge box beam. Above each notch, we also added a rigid steel strap. Adding a beam or posts below was not an option.

Which solution is best for your project will depend on the cause of the problem, access to the framing, and whether adding beams and posts is a option. Best to get an experienced contractor to do the work.

Q&A Index


  1. Stiffening Bouncy Floor With Plywood

    I have a kitchen floor that bounces whenever we walk on it. The area in particular is in the center of the room, between the fridge and the dishwasher. It’s a span of 10 feet.
    The basement is finished and I don’t want to tear out the dry-walled ceiling.
    Would screwing another sheet of plywood stiffen it up? If so, what thickness?

    • buildingadvisor says:

      The best way to reduce bounciness is to add support from below (a beam or load-bearing wall) or to reinforce the joists if they are accessible. Solid bridging can help, but “sistering” new joists alongside the existing joists is much better.

      Adding support from below is also very effective. Shortening the span by adding a center beam or load-bearing wall will stiffen the floor dramatically. Even building a closet along one wall in the basement that shortens the span from 10 ft. to 7-8 ft. could help a lot.

      Adding a layer of plywood to the underside of the joists can also be very effective as you are turning the floor into a giant box beam. Use 5/8-3/4 in. plywood with construction adhesive and screws that penetrate the framing by at least one inch. Orient the plywood as described below.

      Adding plywood above, as you plan to do, can be moderately effective. I would start be refastening the existing subflooring with subflooring screws or decking screws that penetrate the framing at least one inch. The standard fastening schedule for subflooring is 6 in. on-center around the edges and 12 in. on-center along each joist. Since you are trying to stiffen things up, however, you should reduce the fasteners to no more than 6 inches on center throughout.

      To get the maximum benefit, laminate the new layer of plywood to the existing plywood (after cleaning it and sanding it with 60 to 80-grit paper). Use yellow carpenters glue spread evenly across the whole surface — not construction adhesive which will leave gaps between the beads of glue.

      Orient the new plywood panels along the length of the joists to help stiffen the span, and offset the joints from the existing plywood joints by at least 6 inches. You want the full sheets to be more-or-less mid-span, so a staggered pattern of 8 ft. sheets and, 2 ft. sheets should work well.

      Tongue-and-groove subflooring will give you the stiffest floor. Use a 1/8 in. glue bead on the joints, which are designed to leave a 1/8-in. gap. Otherwise, you will need to gap the joints yourself.

      Attach the new subflooring to the joists with screws long enough to penetrate the joists by at least one inch. The screws should be no more than 6 in. on-center. Also screw the new layer of subflooring to the existing subfloor between the joists. Use screws long enough to penetrate both layers of plywood. Place these screws at 4 to 6 in. on-center in both directions.

      As for what thickness of plywood, I would use the thickest layer you can use without causing problems at doorways and threshholds. A total thickness of 1-1/8 to 1-1/4 in. of plywood is recommended for most types of finished flooring, but you may need more than this to compensate for undersized floor joists. If you have the space, I would use 5/8 – 3/4 in. plywood.

      If you are planning to install ceramic tile or vinyl, you may need another thin layer of underlayment on top of your thickened subfloor, so make sure you take that into account when choosing the plywood thickness.

      Best of luck with your project!



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