How To Strengthen A Floor Joist With Steel

Jessica Stone
by Jessica Stone

Over time, floor joists in older homes can experience sagging and even cracking. When it comes to newly constructed homes, wear and tear to the floor joists can still occur. In this case, it’s very common for long stretches of joists to bounce or wobble due to foot traffic.

Floor joists are absolutely vital to the structural integrity of the home and are not easy to replace when they become damaged. However, there are a few ways that you can strengthen the joists and expand their life. One of the options for strengthening floor joists is to use a steel reinforcing plate, or flitch plate, on one or both sides of the beam.

In addition to steel, let’s examine all of the various ways that you can reduce wobbly or sagging floors by making your floor joists stronger.

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Step One: Testing the Floor First

While floor joists are the main structural member of your floor, they seldom span the entire width of your house. In fact, you generally have a beam or wall that supports the joists near the center. If, in your situation, there is a beam, it’s important that you ensure that it’s not too frail before you reinforce the joists.

You can check the beam by performing a very simple test. Head to the exterior wall that is opposite the beam. Stand and rise up on your tiptoes then, drop your heels down quickly. At this spot, you should not feel any bounce.

Try the same test on the area of the floor halfway between the beam and the wall. This spot should produce the biggest bounce. Next, stand as close to the beam as you can. If the beam is sufficiently supported, the bounce should be very little. However, if it’s more intense, you likely have a beam that is not supported enough or is undersized.

Most of the methods outlined below, require you to have access to the underside of your floor joists. This means, having a basement with an easily removable drop ceiling, an accessible crawlspace, or simply an unfinished basement. However, we do offer one option that will not require you to rip out any drywall, should you have a drywall ceiling in your basement.

Step Two: Apply One of These Methods

First Method: Steel Bridging to Strengthen Floor Joists

The first method for strengthening your floor joists involves the use of steel bridging, or “X” bracing to provide more support. Implementing bridging allows the weight to be evenly distributed throughout the joists.

If your joists already have bridging, they can still benefit from additional rows being placed on either side of the existing bridging. Although this option decrease floor bounce, it will not make the floor as stiff as other alternative methods.

Steel bridging can be purchased at most home improvement stores to fit perfectly between joists that are either 16 inches or 24 inches apart.

Second Method: Sistering Joists

The most common way to reinforce floor joists is by securing another length of wood, or other material, to the existing joist. This is referred to as “sistering,” and while it’s generally done with wood, some experts suggest that steel can be a great option as well.

To sister using wood, start by measuring the length and the width of the current floor joist. Although most new construction uses 2×8 lumber for joists, purchase the appropriate width that corresponds to your original joist. Use the length you measured to cut your new joist to size.

With the new wood joist suitably sized and cut, apply multiple thick drops of construction adhesive onto the wide side of the existing joist. Use a plastic putty knife to smoothen out the adhesive. Then, lift the new joist into position and finish off by driving three 16d nails at 16-inch intervals, so that they penetrate through both of the joists.

If you prefer to use steel, special care should be taken in choosing the thickness, width, and how the steel will be fastened. When used in this way, the steel is referred to as a flitch plate.

Third Method: Flitch Plates

A flitch plate, made of either steel or engineered lumber, can be used to reinforce floor joists. You can install one plate of steel or two on either side to effectively support the beam.

Use one or more pieces of ¼ or ½ inch steel, cut to the necessary length, and secure them along the length of the beam with bolts. Another option is to bolt short flitch plates of steel or LVL on both sides of a joist as a way to mend a damaged or cracked section.

Fourth Method: Solid Blocking

Installing block inserts between joists to reduce wobbly or sagging floors is called “blocking.” If you have easy access to the joists from underneath, add solid blocking every 4 feet to help stiffen your flooring.

Calculate the width between the span of two joists. Using this measurement, cut a piece of 2-by-6 or 2-by-8 wood. Position the lumber block in between the two floor joists and fasten using 16d nails. Drive the nails into each side of the blocking for a sufficient hold.

Repeat the same process every 4 feet along the joists. However, once you reach the next set of joists, stagger the blocking within the cavity to prevent the new nails from interfering with the ones you already placed.

Step Three: Securing Plywood Below

If the above methods simply aren’t an option, you can reinforce your floor and the joists by installing heavy plywood sheets underneath. A layer of ¾ inch plywood securely attached to the undersides of the joists can successfully stiffen the floor and prevent any wobbling or sagging.

However, for this to work, a rock-solid bond between the joists and the new plywood is absolutely essential. Start by sanding the sides of the joists that will attach to the plywood. Then, apply a construction adhesive along each joist and lift the plywood into position. Secure the plywood using several screws.

The plywood should run parallel to the joists, not across them. With the sheets centered, you will have the ends of the joists exposed. You also have the option to reinforce further by implementing rigid steel straps.

Step Four: Adding a Beam or Wall

The final option for strengthening your floor joists does not require you to have access to the undersides of the joists and can be done over top of drywall ceilings. By building a wall or installing a beam that spans the joists, you will stiffen the floor and reduce the likelihood of bouncing or sagging.

Depending on where you place the wall or beam, you can decrease defection by up to 98 percent. In general, the longer your floor joist is without any support, the more it will bend. By building a wall, you are dividing this span and for best results, position the wall in the center of the joists.

This method does require quite a bit more time and effort. If you build a wall, make sure to place a stud under each joist. Whereas, if you go the beam route, it will need to be supported by wood or adjustable steel posts throughout its length.

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Wrapping It Up

There are many options to strengthen floor joists with both steel and lumber to fit your particular situation. Whether you opt for steel bridging, flitch plates, plywood, or even a wall, you will effectively diminish floor wobbling, bending, and sagging, to create a stiff and secure surface.

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Jessica Stone
Jessica Stone

Jessica considers herself a home improvement and design enthusiast. She grew up surrounded by constant home improvement projects and owes most of what she knows to helping her dad renovate her childhood home. Being a Los Angeles resident, Jessica spends a lot of her time looking for her next DIY project and sharing her love for home design.

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