How to Strengthen Floor Joists From Beneath [5 Options]

A while back, I considered completing a task that had been on my to-do list for ages – fixing my bouncy living room floor. Even though I seemed to be the only one who noticed it, I just couldn’t stand the wobble. The main thing holding me back was that I wasn’t sure of the best method for how to strengthen floor joists from beneath.

The best way to strengthen floor joists from underneath is to make a supporting mid-span beam or wall beneath the wobbling joists. Using jack posts or 6×6 posts and 2×10 or 2×8 beams perpendicular to the joists will solve any wobble and ensure your joists never move again.

There are many other ways to solve the problem of weak joists from below the joists without building a wall with posts, but not all of them will work as well as the permanent wall with a beam and posts.

In this article, we’ll take a look at all the different ways you can strengthen your floor joists and when each method might work best in your home. We’ll also investigate ways to shore up all your floor joists without having to tear up your entire basement.

How to Strengthen Floor Joists From Beneath

What Causes Sagging Floor Joists?

There are many causes of sagging floor joists, and most center around the main beam holding up the floor joists. In older homes, the main beam may be supported by some wood posts or series of posts. These can rot at the bottom, or get soft, especially if they are sunk into concrete.

When wood posts get soft, they lose some of their rigidity and sinking occurs. The effects of this sinking are felt on the floor joists that those soft posts are ultimately supporting.

Other causes include undersized floor joists and floor joists with cut-outs that exceed an acceptable amount of removed material, such as HVAC, large plumbing fixtures, or vent pipes for various purposes.

Reinforce floor joist

In some cases, a duct inserted through a joist will only leave a small percentage of actually joist material on the top and bottom. At that point, the joist can no longer withstand the load it was designed for and will become wobbly.

Lastly, your joists may be undersized. If you have 2×8 or 2×6 joists, then it’s likely you have joists that are too small. For instance, span tables for joists do not even include 2×6 as they are too small for any span. A 2×8 will only span a building with a width of 24 feet maximum, which is smaller than many of today’s homes.

Are Bouncy Floors Dangerous?

Yes, bouncy floors are dangerous because they are indicative of larger problems. Bouncy floors may indicate your posts holding your beam are no longer structurally sound. Your joists might not also be bridged or blocked properly. Or the joists may not be big enough. If any of these issues get bad enough, you could experience structural damage to your house.

On the other hand, you will probably not get injured due to your floor collapsing anytime soon. Many, many homes have floors that “flex”. That doesn’t mean it’s OK, but it also doesn’t mean your floor is going to fall out under your feet at any moment.

Knowing what to look for to determine if your bouncy floor is in critical condition or not is key. Look for joists with large holes. Check your main beam posts and see if they are rotting or damaged, particularly at the base where they meet the concrete. Moisture from the concrete will rot even a treated beam over time.

Does Drilling Holes in Joists Weaken Them?

Drilling holes in your joists will weaken them and you must keep them at a minimum. You are allowed to drill holes with a diameter that is up to ⅓ of the depth of the joist’s depth. Holes must be at least 2” from the top and bottom edge of the joist’s depth.

You can notch joists, but only on the top edge and they must be no further from a bearing edge than the joist’s depth. The depth of the notch can only be ⅓ the depth of the actual depth of the joist.

If you have a 2×10, then your actual dimensions are 1 1/2x 9 ¼. If you divide the depth – 9 ¼ – by 3, you get slightly over 3 1/16” – the maximum diameter hole you are allowed to drill, provided it is 2” from the top and bottom edge of the joist depth. You are allowed to drill this hole along any part of the length of the joist.

Joist reinforcement

Similarly, if you have a 2×10 joist and need to notch it, you cannot make a notch deeper than 3 1/16” and it must be within 9 ¼” from a bearing. A bearing edge means an edge that supports the joist. Thus, that will likely be the main beam or your block walls in your basement. Finally, you are only allowed to notch on the top end of the joist, according to the building code.

How to Inspect Bouncy Floors

If you find that your floors are too bouncy, then you need to inspect them first to see if they are, in fact, in need of repair. But you need to know what to look for before you go into your basement and make any big decisions.

To inspect your bouncy floors:

  1. Check your joists to make sure they are all intact and solid.
  2. Use a screwdriver to check for rot. If the wood is soft and you can poke the screwdriver into it, then the joist needs repair.
  3. Check the beams and posts that hold up the joists.
  4. Check for deterioration.
  5. Measure your joists and check joist spans to ensure they are sized properly.

Here’s a quick inspection tip sheet to remember when it comes time to go downstairs and have a look at your joists:

  • Check joist sizing and ensure they are sized to code
  • Ensure holes in joists are smaller than ⅓ the depth of the joist
  • Notching should only be within the depth of the joist on either edge
  • Make sure posts holding beam and joists are not rotten at their base
  • Inspect joist bearings and ensure they are shimmed and making contact with beams or walls.
  • Use an 8’ level to check the distance of the joists to the floor. If they are lower in the middle than edges, you’ll need to bridge or sister the joists.

Remember that a slight bounce isn’t unusual, especially if you have several kids jumping around or are doing exercise in that room. However, it never hurts to give a thorough visual inspection as your joists are a major structural component of your home.

How to Strengthen Floor Joists From Beneath: 5 Options

Below we’ll go over your options for strengthening your floor joists. You’ll find that many are achievable by even the occasional DIYer. You’ll run into problems trying to work around wiring and plumbing, however, which is why we offer 5 different options:

  • Bridge joists using plywood
  • Block joists with lumber of the same dimension as the joists
  • Use steel bridging as you would with plywood or use a steel flitch plate
  • Sister the joists with the same dimension of lumber
  • Add a mid-span beam or wall beneath the bouncy joists

1. How to Strengthen Floor Joists with Plywood

How to reinforce floor joist

To strengthen your joists with plywood:

  1. Use ¾” plywood and rip 8’ pieces with a table saw to the width of your existing joists.
  2. Lift your joists to the desired height with a couple of jacks and a temporary beam.
  3. Glue sections of plywood to the joist and nail with 10d nails.
  4. Overlap another layer in the same way, alternating seams. You can use 12d nails for the second layer.

Two layers of ¾” plywood is significant added strength to a floor joist. Be sure both ends of the plywood are sitting on bearing ends – which is a block wall and the main beam, in most cases. The plywood should run the entire length of the joist.

While nails are always the preferred fastener over screws, it is better to use lag bolts or carriage bolts versus nails. Large bolts will guarantee your plywood remains in place for the life of your house. Nail the first layer to the joist as described above, then drill pilot holes for your ⅜” lag or carriage bolts.

Reinforcing floor joists with plywood

Using ½” plywood is also acceptable, or any size greater. Your cost goes up the thicker the plywood, and anything beyond ¾” is probably overkill.

2. Reinforcing Floor Joists With Blocking and Bridging

How to stiffen floor joists

Another common and often preferred method to reinforcing floor joists is called “blocking”. It is called blocking because it calls for using lumber that is the same size as your joists to fit perpendicularly between every joist.

Just as you would with bracing, you’ll install the blocking in a row or rows depending on how long your joists are – up to 12’ you need one row, up to 18’ you need two, and anything over, you’ll need 3 rows of blocking.

When it comes to installing blocking, it is best to layout the blocking in a straight line as it will result in stronger support for the joists. Alternating allows you to nail the blocking straight into each side but results in weaker support. Toenail one side of each block to make a straight shot of blocking.

Use 10d or 12d nails for your blocking. Two nails per block are adequate, but 3 is better. Cut your blocking, so that is a tiny fraction longer than the width of the joist bay. This will ensure a snug fit. Also, ensure the bottom of the block is flush with the joist. If you jam it up into the joist bay too much, you’ll notice a hump over time from above when walking over your floor.

3. Bridging Floor Joists

Bridging refers to the process of using strips of plywood, or lumber, to connect one joist with the joists immediately adjacent to it. In many homes, this looks like a series of x-shaped connections when looked at from below the joists. Older homes use 1×2 or 2×2 lumber and newer will use ½” or ¾” plywood or 1×3 furring strips.

If you find you have no bridging between your joists and the joists are sagging, then you’ll need to install bridging. Using plywood is acceptable and will not shrink or crack over time. If you have brand new 1×3 lumber, it will shrink and potentially negate the effects of using it for bridging.

You can use ½” plywood or more for bridging joists. You’ll rip them into strips that are 3” wide. How many you need depends on how long your joists are. If your joists are 12’ or less, then you’ll only need one set of briding in the middle. Over 12’ you are looking at two sets of briding evenly spaced. If over 18’ then you’ll need 3 rows of bridging.

Double-check your diagonal length using Pythagoras’s handy theorem. Use the rise of the joist – 9 ¼ if using 2×10 joists – and the run, which would be 14 ½” if centers are 16”, to determine your diagonal length. You’ll also need to angle each end of each strip to fit plumb to the joist. You could use a speed square or just use a joist bridging angle table to help.

Once you’ve cut your bridging to length, angled the ends, and marked out where they go on the joists, it is time to install. For this job, a nail gun is ideal. Some are hard to get between joists, so if you have a pneumatic palm nailer, then the job will go much faster.

Otherwise, use 8d nails and start hammering. Structural screws are a more expensive option but avoid standard construction screws. Using furring strips uses the same process as above, but with the added benefit that you don’t need to use a table saw to rip pieces.

4. Strengthening Floor Joists with Steel

Strengthening floor joists with steel

There are several options when using steel to reinforce your floor joists, ranging from using steel bridging – some of which are nailless – to steel flitch plates that are sandwiched between two joists. We’ll go over the flitch plate method here.

Using a steel flitch plate requires you to purchase a ½” or ¼” steel plate the same width and length of your floor joist(s) and then bolting it to the joist. Some opt to sandwich a joist or joists with these plates and bolt them together for even more strength.

To properly attach a fitch plate reap the benefits of the steel’s strength, you’ll want the plates to run the length of the joist. That way they are supported on either end. There should be precut holes for you to bolt your plate into your joist. ⅝” carriage bolts are often used in these applications.

Keep in mind that attaching a flitch plate limits your ability to drill through the joists for wiring or plumbing. And while steel can significantly strengthen your joist, it only provides a limited benefit in terms of strength versus simply using another joist – sistering – instead of the steel. And a sistered joist can still be drilled through.

Thus, if you consider using a flitch plate, make sure you need that extra strength before you spend money on steel that you don’t need.

5. How to Reinforce the Floor With Sistering Joists

Sistering Joists

Sistering floor joists require you to use lumber that is the same size as your existing floor joists. Remove all obstructions from the existing joist, raise the joist back to the desired height using a jack post, and apply construction adhesive to the old joist’s face. Apply the new joist and nail it in an x-pattern every 6 inches using 10d nails.

Sistering joists provides a huge increase in floor joists’ strength and still allows you to drill through them for your wiring and plumbing.

If you don’t want to use nails, you can bolt your joists together using ⅝” carriage bolts that are 4” long. Bolts are a better choice as they’ll keep the joists tighter over time and can be adjusted if in the future. They are also more expensive.

If using engineered I-beam type joists, then use ¾” plywood or OSB to fill the joist “web” – the part between the top and bottom “I”. Make sure you fill the entire joist on both sides, staggering the plywood seams on each side as you go. Use construction adhesive and nails to attach.

For extra strength, you can then apply yet another layer of plywood along the length of the entire face of the I-beam, from top to bottom, on either side. Again, alternate as you go to ensure maximum strength. This option would only be necessary if you had something incredibly heavy on the floor above.

Add a Mid-Span Beam or Wall

Floor joist support beam

Adding a mid-span beam or wall beneath the middle of your floor joists will permanently fix any sag or bounce you might have in your floor. As this solution uses posts and beams, it affixes the center of the joists to the floor below, providing arguably the best solution for wobbly floors above.

To add a mid-span beam, use two jacks to lift your joists to the desired height. Once in place, use two lally posts and an appropriately sized beam to perpendicularly run the length of your joists. Use shims to ensure contact is made between all the joists and the beam. Adjust the posts to ensure a snug fit and remove the temporary jacks.

You could also erect a wall, with 2x4s and 16” on center spacing, to lift the sagging joists. Use two jacks and a temporary beam to lift the joists into place, then measure the height from floor to joist bottom. Once you know your height, construct the wall on the floor and lift it into place. Use shims to ensure a solid fit beneath the joists and remove the jacks.

How Do You Support a Floor Joist in a Crawl Space?

A floor joist wobble is difficult to fix if you have limited space beneath, such as a crawl space. You may not be able to or have the desire to get a full-sized joist into the crawl space to sister the joists. If not, then bridging or blocking is a better option.

To block joists in your crawl space, follow the instructions above. The method does not change and it would arguably be the fastest if you have to get down and lie on your back, especially if you can run an air nailer down there.

If you’d rather sister the joists in your crawl space, then use the directions above. Be sure to remove any obstructions. You’ll want to lift the joists you intend to sister first, so you need a jack of some kind and a solid surface for the jack to sit on. Once you have raised the joist, you can glue it, install the new joist, nail, and get out.

How Do You Jack Up Floor Joists?

Many of these methods stated above require you to either permanently or temporarily jack up your floor joists. We do this because reinforcing a sagging joist requires us to prop it up. Otherwise, we are reinforcing a joist while it sags. We want the original joist back in its original position, so to do that we need to jack it up.

Using two jack posts is the best method to jack up a floor. They are adjustable steel posts that have a variety of heights. A steel pin usually sets the general height, then a screw plate at the top to get the height precise and snug. Place a beam or piece of lumber across the two posts to lift several joists at once.

Conclusion

Reinforcing joists can be a relatively straightforward task that can improve the stability of your floor and give you peace of mind every time you travel over it. Since there are so many options for strengthening your joists, there isn’t a situation when you wouldn’t be able to fix a wobbly floor.

Remember that when blocking, bridging, sistering, or using flitch plates, your fasteners are crucial. Avoid screws unless they are structural, and use galvanized fasteners if your basement experiences damp issues. Whenever possible, oversize your fasteners. Otherwise, your efforts may end up in vain.

Finally, if you don’t think you can manage this project on your own, get a friend who is handy with wood. You’ll be surprised how fast you can manage this job once you get going.

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