A bounce in your step may be a good thing, but not if the dishes rattle and the gramophone skips as you tiptoe across the cottage floor. Too much bounce (“deflection” in engineer-speak) almost always means floor joists are “overspanned;” they’re too small and too far apart for the distance they bridge. It’s common in older cottages, built to less stringent building codes or no code at all, but unless there’s advancing rot, a recent structural change, or a new grand piano showroom in the cottage, a springy, overspanned floor is unlikely to collapse. Were it that weak, it would have failed years ago. If the bounce still bugs you, here are four techniques you can use (alone or in combination) to get back on a solid footing.
a. Reduce the span: A permanent support beam at right angles under joists can cut their span to a distance they can bridge without excessive bouncing. Problem is, a beam and support columns may be fine in an unused crawl space, but rather too intrusive in your open-concept great room. As well, this approach is construction heavy: The columns themselves need support, so a beam in the living room, for example, must also have a load-bearing structure, including footings, in the space below.
b. Strengthen with steel: You can, in effect, suspend the load with a perforated steel strap, wrapped down and around a joist like a sling. (Simpson Strong-Tie makes several sizes, available by special order from building centres; the cs20 model works in most applications.) To get more out of the technique, before you install the strap, place jacks and a temporary beam about 30 cm away from the joists’ midpoint to raise them slightly (just enough that the strap will be taut when you release the jacks). Starting at one end, use one joist-hanger nail per hole for 60 cm or so and one every 15 cm through the middle. Where the strap bends under the joist, hammer it flat to get crisp corners, and finish the last 60 cm as you started, one nail in every hole.
c. Stiffen the joists: Bouncy joists aren’t really weak as much as they’re overly flexible. Add backbone by doubling-up, a.k.a. sistering: Glue and screw a second joist onto the side of the first. Carpenters like to match the original in size (or use an even taller sister), but flooring nails, pipes, wires, and such can get in the way. And while bigger sisters are indeed better, engineers will tell you that even a 2×4 sandwiched alongside the joist helps. A sister doesn’t even need to extend the full length of the original, because most of the bending is in mid-span. Use the cottage engineer’s trick with a little sister: Drop it to the bottom of the joist
or raise it to the top — that’s where extra stiffness is needed.
d. Spread the load: Solid blocking —lumber installed at right angles between joists — helps knit the floor structure into a stiff, integrated system. Instead of your weight being supported by the lone joist underfoot, those on either side share the burden, too. Though less effective than other methods, blocking can squeeze in where penetrating wires and pipes make a sister impossible. The key is a tight fit: Measure and cut carefully, using kiln-dried lumber that won’t shrink and loosen after it’s installed.