Joist Stiffener for Bouncy and Sagging Floors & Rafters increase stiffness up to 80%

Undersized and over-spanned floor joists are common, especially in older homes. The result is a floor that bounces or flexes as you walk across it, which is uncomfortable and unsettling. The floor may be plenty strong; it just isn't stiff enough. Strength and stiffness are different things.

The Joist Stiffener is the result of more than two years of development and testing, and it can stiffen floor joists and roof rafters by up to 80%. Spread across a series of joists, that added stiffness can make a dramatic improvement.

Joist Stiffeners are made from 10-gauge G90 galvanized steel. Every strap has our patent-pending interlocking key on both ends, so the straps lock together into one continuous steel flange along the bottom of the joist. Each kit includes fasteners, a pilot drill and a driver bit. All you need is an ordinary drill/driver.

How Joist Stiffeners Work

When a joist bends, its top edge squeezes together (compression) and its bottom edge stretches (tension). Stop the bottom edge from stretching and the joist can't bend as far. Steel barely stretches at all, so once the strap is screwed tight to the bottom of the joist, it works the way the bottom flange of an I-beam does.

Most of the bending happens in the middle of the span, so that's where the steel goes. The last few feet near each support hardly move, which is why the straps don't need to run the full length of the joist.

Two straps and an interlocking key

The system uses two strap lengths, 46" and 23". Both ends of every strap have the same interlocking key (patent pending), so the straps lock together end to end and any strap can go in the middle or on the end. There are no splice plates or bolts at the joints. Each strap is fully screwed to the joist on its own, so a joint never carries the load by itself.

Keeping every strap under four feet means a kit ships in an ordinary box and fits down a basement stairway without a fight.

Pick your kit by joist span

Measure the span from support to support, not the length of the board. Kits are priced per joist. Most floors need every joist in the bouncy area done. With joists 16" apart, a 12-foot-wide room has nine or ten. For spans over 19 ft, give us a call.

Which joist thickness do I need?

Measure the thickness of the bottom edge of the joist.

• 1-1/2" joist (modern lumber): Most homes built after about 1950 have standard dimensional lumber, which measures 1-1/2" thick.
• 2" joist (full-size, older homes): Many older homes have full-size lumber that measures about 2" thick. Rough-sawn joists vary, often anywhere from about 1-3/4" to 2-1/4". If your joists measure in that range, choose the 2" kit.

Not sure? Send us a photo with a tape measure across the bottom of the joist and we'll tell you which kit fits.

What's in each kit

• Three straps, 10-gauge (0.135") G90 galvanized steel
• #14 x 1-1/2" structural screws with T25 Torx heads: 73 in the 8 to 13 ft kit, 93 in the 13 to 19 ft kit (one spare per strap)
• Pilot drill
• T25 driver bit
• Installation instructions

Installing it

1. Find the middle of the joist span and mark it on the bottom of the joist.
2. Center a 46" strap on the mark and screw it on.
3. Lock the next strap onto each end and screw it on. The 8 to 13 ft kit adds a 23" strap on each side. The 13 to 19 ft kit adds a 46" strap on each side.
4. Move to the next joist. Do every joist in a continuous run across the bouncy area, never every other joist.

Rebuild the blocking. Old bridging and blocking between joists often works loose over the years. Tight-fitting solid wood blocking the same height as the joists, screwed through the joists into the ends of the blocks, spreads the load across the stiffened joists so they work together.

Adhesive is optional. In our testing, construction adhesive under the strap didn't add stiffness over screws alone. If you'd like to use it anyway, we recommend Loctite PL Premium MAX.

Each joist takes 70 to 90 screws, so a whole floor adds up fast. An impact driver is the right tool for the job.

Good to know before you order

• Tested. In our lab, Joist Stiffeners made floor joists up to 80% stiffer.
• It's for sound joists that bounce or sag. If a joist is cracked, broken, notched or has a large hole cut in it, repair that first with one of our joist reinforcers.
• It won't lift an existing sag. It stiffens the joist from where it is now and helps keep the sag from getting worse. We don't recommend jacking old joists before installing.
• It stiffens joists, not everything else. It won't fix a springy subfloor between joists, a sagging beam or a failed post.
• Thinking about tile? Ceramic tile and grout crack when the floor under them flexes too much, and natural stone is even less forgiving. Stiffening the joists before the tile goes down is a lot cheaper than redoing a bathroom floor.
• It adds about 1/4" below the joist (strap plus screw head). Drywall can still go up underneath; furring strips beside the strap give it a flat surface.

Joist Stiffener straps are made in our Roanoke, Virginia factory.