How to Strengthen Floor Joists from Beneath

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How can I make my floor joists stronger?

In modern American homes, joists are the fundamental flooring structure. However, since they don’t usually span the entire breadth of the floor, a beam or a wall is usually added for enhanced stability. Before adding a beam to support your floor joists, it’s important to ensure that it’s strong enough to act as structural support.

You can do this by standing close to the beam and stepping strongly on the floor, while listening for any bounce. Repeat the process while standing midway between the beam and the exterior wall. In the first instance, there should be little to no bounce, while you should hear a significant bounce while standing in the middle. If the level of bounce is the same for both instances, it’s highly likely that your beam is either undersized or lacks sufficient post support.

How do I fix bouncy floor [stabilize floor joists from beneath]

You can make your bouncy floor stiffer by adding bridging, adding a layer of plywood along the joists, and adding a wall or beam beneath the flooring. We go through each of these solutions in detail below:

Add bridging

Bridging allows for proper weight distribution amongst your floor joists, consequently stiffening your floor. What’s more, it’s arguably the most cost-effective method of fixing a bouncy floor.

Bridging is also quite simple and can be undertaken as a DIY project. If you already had some rows of bridging installed in the past but your floor is still bouncy, it could be that the bridging has gotten loose over time. As such, you’ll need to secure it again using screws.

For new bridging, install the rows at one-third intervals, depending on the length of your floor joists. Meanwhile, we’d recommend metal floor joist bridging over wood blocking for longer floor joists. This is because unlike solid blocking, metal floor joist bridging doesn’t get much in the way of your wiring and plumbing.

 Add a layer of plywood

Another effective way to stabilize your floor joists from beneath is to add a new layer of plywood. This fix works best if you use shorter and lighter plywood strips, as they’re more flexible, making installation a simple and straightforward process.

The ideal dimensions for your plywood strips would be eight-feet long and three-quarter-inches thick.

The process requires that you ensure maximum adhesion between the bottom of the joists and the plywood that you’re about to screw in. To do this, start off by prepping the underside of the joists by sanding with an 80-grit sandpaper.

This will ensure that the surface is rough enough for maximum bonding with the construction adhesive. Finish off the process by applying construction adhesive onto the surface before screwing your plywood sheets onto the joists.

Add a wall or beam

The final way to strengthen sagging floor joists is by adding a wall or a support beam. This fix is best reserved for crawlspaces, as beams limit headroom when used in places like the basement.

When stabilizing your floor joists with a beam, you’ll want to support it with appropriately-sized columns and footings. The columns directly support the beam, and in turn, the footings support the columns.

Ideally, you need your footings to be large enough to provide ample structural support for every column. As a general guideline, we recommend making footings that are no smaller than two-square-feet each.

You should also consider increasing the size of your footings if your columns are widely spaced out. Remember, however, that your beam will be more stable and your floor will feel less bouncy if the columns are spaced closer together.

Still- you can space them wider and save on material costs if you’re installing a steel beam- which is structurally stronger.

Does Sistering floor joists work?

To re-align saggy joists or stabilize weak ones, you can beef them up with additional lumbar. This is called Sistering the joists. For this process, contractors typically use framing lumber or engineered lumber.

Unlike adding bridging, which is a relatively easy process, Sistering floor joists is a complicated process- and that requires lots of physical effort.

To properly sister your floor joists, follow the procedure detailed below:

  1. Choose a sistering material whose dimensions match those of the joists that you’re looking to support. The material- meanwhile- can either be dimensional lumber or engineered lumber.
  2. Ensure to put on the proper personal protective gear prior to commencing your project. These include protective eye goggles and safety earmuffs.
  3. If your floor is sagging and you need to re-align/straighten it before sistering the joists, do so by cutting out a 4 x 4- inch post and raising it using a pair of hydraulic floor jacks. You should continue doing so, while gradually raising the jacks by only one-eighth of an inch daily until the cut-out post comes into contact with the floor joists above it. This is done to prevent cracks from appearing on your floor and drywall.
  4. Next, apply construction adhesive along the length of a joist to maximize the bond between the joist and the incoming sister joist. This, consequently, helps prevent the irritating, squeaking sound that’s typical in sister joists that aren’t tightly glued together.
  5. Install a sister joist that’s about the same size as the existing joist that you’re looking to support by sliding it in above the 4 x 4-inch post that you’d installed earlier.
  6. Attach the sister joist onto the existing joist by running nails or screws through both of them. While doing so, you should use quick clamps to hold the sister joists in place. You can then remove them afterward.
  7. Repeat steps three-six above for every saggy or weak joist that you wish to straighten-up or provide with additional support.
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Joe McGuinty
I’ve been working with floors for over 12 years. I started as a flooring contractor, primarily in materials selection. Then, I switched careers into accounting, so my wife and I began buying, renovating, and re-selling homes on the side. You’d be surprised how much value you can add to a home simply by adding new floors.

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