A: Joist bridging is the right term for bracing between floor joists, but it doesn’t usually look like what you have. Typically, the bridging consists of solid pieces of wood as thick and deep as the joists but fitted tightly between them, at a right angle, or it’s made of slender pieces of wood or metal installed in pairs diagonally between joists, creating a series of X shapes. Some manufacturers offer single-piece cross bracing that has the X shape already fabricated, and there are styles that come as one piece but then get broken at the job site into two sections that slide together and lock into half of the X at the precise length that’s needed. But there’s always an X, not the inverted V you have.

Both solid bridging and cross bridging serve the same purpose and are considered equally effective. During construction, the bridging keeps joists vertical so they can’t twist out of place. After construction, for the life of the house, the bridging helps stiffen and strengthen the joists by tying them together so that some of the load on one transfers to neighboring joists. Without adequate bridging, you might hear rattling as you walk across a floor or get significant sag if something really heavy, such as a piano, weighs down on a single joist.

Metal cross bridging comes in various styles, each one requiring installation details validated by the manufacturer’s engineering department. There are nail-on types as well as nailless styles, which help reduce the risk of floor squeaks; they have claws that grip into the joists. Some fasten to the tops and bottoms of joists, and others to the sides; some give installers the option of whichever place works best.

Darrel Hewitt, an engineering technician for Simpson Strong-Tie, one of the leading manufacturers of metal bridging and other fittings useful in framing buildings, looked at the pictures you sent and said he has never seen bridging that was designed to be installed like yours is. He didn’t recognize the pieces as Simpson products, but added: “If those are our bridgings, they would be installed incorrectly.” The empty nail holes in the metal leave no doubt, he said. “These are nailed types, and there’s no nails. So that’s a problem.”

But even if your bracing was nailed on, that wouldn’t make it function as typical cross bracing does, because the top connects to the floor, rather than to the top or near the top of an adjoining joist. “Bridging is for tension,” Hewitt said, referring to a force that pulls objects together through a taut string, rope or piece of metal. “If the bridging is nailed to the subfloor and then to the bottom of the joists, [the tension force] would be pretty insignificant,” Hewitt said. It would tie the flooring and joists together, rather than accomplish what should be the main goal: tying the joists to each other so they can share loads.

Even if the floor above seems to have no issues now, it would be worth fortifying your floor so it doesn’t sag over time. You can leave the bracing that’s there, but add metal or wooden bridging aligned nearby in a straight line perpendicular to the joists. (With cross bracing, the pairs should be on either side of the line and be arranged so top and bottom edges press directly through the joist against the top and bottom edges of the next X.)

Simpson’s nailless bridging, which has NCA in product names, is among styles that can be installed after joists are in place, but it comes in sizes only for joists spaced 12 or 16 inches apart. (The 12-inch-spacing style for joists 10 0r 16 inches deep costs $1.03 per piece at Home Depot.)

Hewitt said solid bridging is usually simpler to add once flooring is in place. If you choose this approach, buy lumber the same thickness and depth as the joists and cut it into sections that fit snugly between joists. Measure as you go in case the spacing varies a bit from joist to joist. Angle the nails — a process known as toenailing — so you can install the pieces in a straight line. Staggering the pieces would allow you to nail straight through the joists, which is easier but not as good; however, even that would probably be sufficient, and it would certainly add more stiffness than the system you have now, Hewitt said.

If the spacing between joists is too cramped to allow swinging a hammer, use three-inch screws at an angle; pre-drill to avoid splitting the wood. Or attach L brackets with shorter nails or screws. Or consider getting a palm sander that operates on air pressure. A Porter-Cable 0 Degree Mini Impact Nailer is $42.98 at Home Depot. Some Home Depot stores and many tool-rental companies have compressors available for rent by the half or full day.

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