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 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 in the wrong place (see our Guide to Notching and Boring Joists) or are weakened by decay or insect damage.
Any moisture problems that contributed to decay in the structure should be addressed before making structural repairs. Then rotten or damaged joists can be repaired or replaced, as needed. Assuming the joists are in good condition, the solution to both sagging and bounciness problems is to reinforce the floor. Depending on cost and practicality, they could mean beefing up the floor joists, or adding new support underneath with new or reinforced posts of support beams.
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 repair 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 larger beam supported only at the two ends. This could be 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 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 ones. Attach the sisters to the existing joists with nails, screws, or bolts (bolts are best). Sistering is similar to adding a flitch plate, but typically uses framing lumber rather than steel or engineered lumber.
Solid blocking. If you have access the joist spaces from below, adding solid blocking between the joists at 4 ft. intervals will help stiffen the floor. If the joists are damaged or undersized, blocking will not usually be sufficient to solve the problem. But it will help in combination with sistering or plywood facing, as described below.
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 (in the living room) 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.