Ed. Note: This is the second of two articles on proper floor framing. Got Bounce?, appeared in the August 2012 issue of ProSales and dealt with mitigating floor deflection in new home construction.

For new-home builders, a squeaky or bouncy floor is a sign of substandard or code-minimum construction practices, the effect of which can erode their reputation for delivering comfortable, quality-built housing.

For remodeling contractors, though, those problems can be a significant source of revenue. Most older homes, especially those built before 1980, probably suffer from floor deflection and squeaks caused by decades of shrink-swell cycles, under-engineered joist spans, and old-school fastening, subflooring, and joist-stiffening materials and practices.

Sister Act

Assuming the joists that frame the affected floor are accessible from a basement or crawl space (ideally they’re exposed or can be exposed by the removal of drywall or other finishes), the approach that is most common and generally considered best is to attach new joists along the length of the existing ones.

Called “sistering,” this method upgrades the structural integrity of the floor frame and provides an opportunity to take up sags or joist-subfloor separation that can further affect deflection and cause squeaks and cracks in the finished floor above.

A 4x6 beam run perpendicular to and under joists in the center of the floor frame, held in place and raised slightly (probably no more than ¼ inch) by hydraulic jacks, will eliminate most of the sag and help ensure that the sistered joists will be snug to the subfloor.

Ideally, the sistering joists should run the full length of as many existing joists as possible, starting with those in the center of the floor frame. They should also be slightly deeper (e.g., 2x10s sistered to 2x8s) and as such may need to be notched to rest on a mudsill (see illustration).

It’s also okay to remove blocking between joists, if necessary, to achieve a full-length sister; blocking isn’t load-bearing and may be the cause of some squeaks.

In cases where it’s not possible to sister the full length of an existing joist, the sister joist should be attached from the center and at least 3 feet to both ends of the existing joist length.

If you have to cut a sister joist to make it fit (or just to get it down to the basement or crawl space), place the joists between adjacent sections as far away from the center of the existing joist as possible to mitigate those weak spots.

A liberal amount of construction adhesive between the old and new joists and a tight, 8-inch staggered nailing pattern using 16d sinkers from a pneumatic palm nailer will hold the sisters in place. Slowly release and remove the jacks and temporary cross-beam, and the job is done.

Double the Deck

If shoring up the existing floor frame is within the scope and budget of a remodeling project, it’s likely that the job may also entail replacing the floor finishes above. If so, or if they can upsell it, remodeling contractors can enhance the floor frame’s stiffness by either adding to or replacing the existing subfloor in addition to sistering the joists below.

Floors built in the 1950s or earlier usually feature subfloors of 1x slats fastened diagonally across the joists. Like the framing members, these slats and their connections can weaken over time, causing creaks and squeaks, and definitely deflecting with the joists.

Given the additional stiffness and strength afforded by modern-day materials and the ability to fasten them more securely with screws and glue, it is probably best to remove the old slats and replace them with at least 7/8 or 11/8 inch OSB or plywood subfloor panels.

A thicker deck than the existing subfloor, however, must be considered in light of adjacent floor and door thresholds. Those areas may require some adjustments, such as trimming the bottom of the doors, to accommodate a slightly higher floor height.

Other Alternatives

If the joists are not easily accessible for sistering, and/or the subfloor can’t be replaced or supplemented to be thicker and stiffer, there are a few other options for mitigating floor deflection.

Solid-wood blocking or bridging end- and toe-nailed between joists at or near their center span point and then every 4 feet along their length can help keep the bottom of the joists from “kicking out” from foot traffic above.

Adding a plywood or OSB “ceiling” over the basement or crawl space also mitigates joist kick-out and spreads the load across all (or most) of the floor. If adding full or large sections of subflooring isn’t possible, add 1x3 strips perpendicular to and across joists every 5 feet along a 20-foot span.

Other options include permanently adding a mid-span beam and support columns, which usually also requires new and deeper footings at the post locations.

The good news for dealers and remodelers is that all of the materials needed, perhaps including the jacks from the rental desk, are readily available and familiar, making floor deflection fixes a potential source of sales for both parties.

Rich Binsacca is a freelance writer in Boise, Idaho.

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