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 BuildingAdvisor.com 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.

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 with a sagging beam.

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 are not an option, then a correctly sized beam made of wood, engineered lumber (LVL), or steel will be needed.

Sistering. Where sagging is not the main problem, bouncy joists can be reinforced 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).

Flitch plates. Existing beams can similarly be reinforced by bolting on one or more long pieces of  ¼ or ½-in. 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 joist to repair a cracked or damaged section.

Add plywood below. Many years ago, I worked on a remodeling job where the plumber had made a series of deep notches across a floor,  right in the middle of the joists. 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  and reinforce the entire ceiling with heavy plywood sheets screwed and glued to the joists, creating a diaphragm. 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 in a option. Best to get an experienced contractor to do the work.

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