No More Bounce: How To Strengthen Floor Joists From Below

Kenneth Wilson

If fixing your bouncy floors has been on your to-do list for quite some time now, it might be time to strengthen the floor joists from below. There's nothing more bothersome than the constant wobbling– so it's best to act on it at once.

Fortunately, you have different methods to strengthen the floor joists underneath. Each of these may help you solve the "bouncy floor" problem.

Let's discuss all the different solutions you can consider without having to tear up your entire basement.

Gather the Right Tools

First off, you must gather all the necessary tools for this project. Here's a list for your reference.

  • Hammer
  • Safety glasses
  • Stepladder
  • Tape measure
  • Required Materials for this Project
  • Drywall screws
  • Joist hanger nails
  • Nails
  • Plywood
  • Sandpaper
  • Construction adhesive
  • Metal bridging

5 Options to Strengthen Floor Joists From Underneath

As previously mentioned, there are different methods you can consider if you want to strengthen your floor joists to stop the bouncing.

Method 1: Use Plywood

To strengthen your joists with plywood, you will need to fasten a layer of ¾ inches of plywood to the joists' undersides. Doing so prevents the joists from bending side by side.

The critical task here is to sufficiently fasten the upper edges of the joists to the subfloor. If you're dealing with squeaky floors, chances are the subfloor is loosened from its joists. Hence, you will need to ensure the new plywood addition is bonded with the undersides of the joists– using multiple screws and adhesive (for construction use).

Start by sanding each joist's underside using coarse sandpaper, around two or three passes just enough to leave a clear surface for the construction adhesive.

Next, screw the plywood parallel to the joists. You can choose between CDX or BC plywood for this. Just ensure to use an adhesive that's designed for subfloor use.

Method 2: Blocking

Next, you can consider the "Blocking" method to reinforce your floor joists. It is often the most preferred method by many homeowners. It uses lumber (around the same size as the joists) to fit perpendicularly.

Upon installation, lay the blocking materials in a linear direction to ensure substantial support. In contrast, alternating it may enable you to nail the blocking into each side– but it will give you weaker support.

Method 3: Bridge the Floor Joists

Another method to strengthen the floor joists from below is Bridging– which uses lumber or plywood strips to connect a joist with the other immediately adjacent ones. When viewed from below, it's common to see this method as x-shaped connections.

This method enables the joists to share the weight. A portion of the force is distributed to the surrounding joists with every footstep. Not just one.

Although this third solution on strengthening floor joists won't ensure a 100% stiff floor, it is relatively affordable and easy. You can always give it a try and resort to other options if it doesn't work.

Method 4: Strengthen Floor Joists with Steel

Steel bridging is another solution to consider in reinforcing your sagging floor joists. Before bolting it in, this involves a steel plate that measures the same as your floor joists.

Some are nailless, with steel flitch plates sandwiched in between two joists. You can have them bolted together to ensure maximum strength.

To do this, have the plates run the entire joist length, so they are supported on both ends. Pre-cut holes help you bolt the plate in the joists quickly. Once you attach a flitch plate, this limits your chances of drilling through the joists for various plumbing or wiring purposes.

Steel is known to help strengthen the joist, but it falls behind the strength offered by other solutions, such as a sistered joist, which enables you to drill.

Method 5: Reinforce the Floor with Sistering Joists

Lastly, you can also opt to "sister" the floor joists, which require lumber (about the same size).

Start by removing any obstruction in the way and raise the joist to your desired height with a jack post. Then, apply adhesive to the old joist's surface. Attach the new joists and follow an x-pattern as you nail it in, preferably every 6 inches.

Sistering the joists allow for increased strength in your floor joists. Unlike steel bridging, it enables you to drill through for wiring or plumbing purposes.

Homeowners looking to add additional strength may opt to include another plywood layer along the l-beam on either side, from the bottom to the top. Work in an alternate pattern if you want to get the most strength.

This is ideal only for heavy items above, such as appliances or huge cabinets.

How to Inspect Bouncy Floors

Before making any big decisions, it's crucial to assess your floors to confirm that they have a severe bouncing problem.

Don't worry; the steps are relatively quick and easy to do. Here's how you can inspect your bouncy floors:

  • Check if the joists are all intact and solid in place.
  • Inspect the sizing of the joists if they are sized to code appropriately.
  • Check for any signs of deterioration. You can use a screwdriver to check for rot. Poke the screwdriver, and if you notice the wood is soft, it might be a sign that the joists could use some strengthening.
  • Ensure the holes in the joists are smaller than ⅓ of the joist's depth.
  • Check the joist bearings to ensure they are shimmed and contact beams/walls.
  • Using an 8' level tool, inspect the distance of the joists to the floor. If you notice they are lower in the middle than the edges– opt to either bridge or sister the joists accordingly.

Note that there are different causes of sagging floor joists. The main beam may have insufficient support by the wooden posts below (as observed in older homes), or it may rot and get soft.

In that case, the wood's rigidity is lost, and they begin to sink. It will be noticeable and felt on the floor joists of these soft posts.

How to Test if You Have Bouncy Floors

Some floors will always have a slight bounce, which is okay. It is commonly observed in homes with kids jumping around or one or two adults doing Zumba exercises in the room.

Before you find a solution to strengthening the floor joists below, you may need to do a visual inspection to determine if your beams need one.

Here's how you can do it:

  • Find your way to the exterior wall that's opposite the beam. Ideally, you shouldn't feel any bounce once you stand on your tiptoes, then drop hard onto your heels.
  • Stand in between the beam and the exterior wall. Do the same tiptoe drop test, which will likely cause a significant wobble.
  • Lastly, stand somewhere near the beam. If you notice any bounce in this area, it should be minor and nothing noteworthy. However, if it gives you the same wobbly feeling as what you felt in the middle of the floor– chances are the beam underneath may be undersized or insufficiently supported by the posts.

To solve the problem, you can consult your local building inspector to know about the correct sizing of the beam.

Final Thoughts

Bottom line, you may come to realize that reinforcing the floor joists can be a straightforward task that gives stability to your floors and removes any wobbling.

Fixing a bouncy floor is now easier than ever with five different methods to strengthen floor joists.

Kenneth Wilson
December 8, 2021
Contractor Tips, Flooring

Kenneth Wilson

Retired contractor. Currently residing in Southwest Florida. Now in semi-retirement, I write and manage this blog focused on helping home owners make savvy decisions when it comes to finding contractors and getting their projects done. I also operate remodeling design service for homeowners.

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