The floor rests on floor joists unless your home is built on a concrete slab. These are usually 2”x 8”, 2”x 10”, or 2”x 12” dimensional lumber, depending on the span between supports and the spacing between the joists themselves. The building code has mandated this for years.
Nominal Joist Size
Joist Spacing on Centers
Lumber Grade: Select Structural
Lumber Grade: No. 1
Lumber Grade: No. 2
2”x 6”
16
11’ 4”
10’ 11”
10’ 9”
20
9’ 11”
9’ 7”
9’ 1”
2”x 8”
16
15’ 0”
14’ 5”
14’ 1”
24
13’ 1”
12’ 4”
11’ 6”
2”x 10”
16
19’ 1”
18’ 5”
17’ 2”
24
16’ 8”
15’ 0”
14’ 1”
2”x 12”
16
23’ 3”
21’ 4”
19’ 11”
24
20’ 3”
17’ 5”
16’ 3”
In recent times, engineered wood joists, essentially a thin I-Beam, have replaced dimensional lumber in many newer homes. These joists consist of a 2”x 2” square piece of lumber at the top and bottom, with a “web” of plywood or OSB between them. This allows the home’s building to have less lumber, conserve resources, and lower costs. Being made from engineered wood products, an I-Joist is less likely to cup, twist, bow, or warp. This eliminates many floor squeaks that you will find in older homes.
Those squeaks generally indicate a problem with the floor joists, although in ancient homes, it can happen because of shrinkage of the plank flooring and nails pulling out. These older homes were not built with modern flooring nails, which have a twist to keep them from pulling up with time and flexing the floor.
Floor joists can twist or warp slightly with age, especially if installed in an area exposed to a lot of moisture. The plywood subfloor tends to hold them in place, but the twisting or warping will pull on the nails, either pulling them through the subflooring or out of the floor joist, which depends on what else is happening to the joist. If the joist is beginning to rot due to moisture, then the nail is more likely to pull loose in the joist.
Besides creating squeaky floors, joists that have become somewhat rotted become weaker, allowing more flexing of the floor and joist. When you feel sponginess on a floor, you should not assume that it’s just loose floorboards or subflooring; it is also a potential problem with your floor joists, which needs to be investigated. Should there be any damage to the joists, they should be reinforced.
While you may already know that a sound plywood subfloor supports your home, you may not know that reinforcing joists with plywood can add structural integrity. If you notice that one or more joists are squeaking, or if you’re starting to feel some bounce when walking, it’s time to get to work.
How to Reinforce Joists with Plywood
If a joist is cracking or sagging, it needs to be repaired quickly. The longer you leave it, the worse it will get. Sagging floors can also have other effects, such as causing the walls to sag, resulting in cracked drywall.
Some people reinforce floor joists by scabbing on another piece of the same-sized dimensional lumber as was used initially, essentially making a double joist. While this is functional, it is also an expensive means of strengthening the joist, using more material than necessary. Using a smaller piece of dimensional lumber is never advised, such as scabbing a 2”x 6” onto the side of a 2”x 8” floor joist, as the new piece won’t be as strong as the old one.
A better option is scabbing ½” thick softwood plywood to both sides or ¾” thick softwood plywood to one side; it is just as effective and ultimately cheaper. If you’ve ever looked at the engineered wood joists they use today, you’ll notice that the web is usually only ½” thick plywood or OSB, so ½” thick plywood is sufficient.
Plywood is a strong laminate composed of several thin layers of heavy wood glued together under compression. Some consider it inferior to standard lumber, but the opposite is almost always true! A quality piece of plywood is typically more potent than the corresponding amount of standard wood.
Measure the distance from the bottom of the floor to the bottom of the joist. This measurement should be 5 ½”, 7 ½”, 9 ½”, or 11 ½”. If it comes out to be an even dimension without the ½”, it is an ancient house where they didn’t use sanded dimensional lumber.
Measure the length of the joist. You don’t want to repair only a part of the joist but to go the entire length. Adding only a partial reinforcement will create a stress point at the end of the reinforcement, allowing the damage to the original joint to accelerate.
Rip two strips of ½” or one strip of ¾” plywood to match the measured dimension of the existing floor joists. If the joists are over eight feet long, you’ll need multiple plywood strips to support them. In this case, you can either:
To strengthen the joint, add an additional layer over the plywood reinforcement. Make the additional reinforcement four feet long.
To avoid creating a stress point, use ¾” plywood on both sides of the existing joist and offset the joints by at least two feet.
Drill a line of ¼-inch pilot holes at six-inch intervals down both long edges of each plywood reinforcement strip.
Check the joist with an 8-foot level or other long, straight edge. If the joist is cracked or markedly sagging, you will need to raise it with ahydraulic jack. While doing this, it’s a good idea to put a heavy 6×6 block of wood beneath the jack to support it.
Measure the distance between the bottom of the sagging floor joist and the top of the jack.
Cut a 4×4 piece of lumber to fit the distance measured.
Place the 4×4 between the jack and the floor joist. It should be a snug fit. If not, raise the jack slightly. It is often more manageable with a helper to position the 4×4.
Jack the joist up very slightly. This process can be lengthy, as you should jack at a rate of no more than ¼ inches per day. Jacking more than that can damage the drywall or walls in the house.
Once the joist has been appropriately straightened, it’s time to get back to reinforcing with plywood. Begin by applying construction adhesive to the face of one of your plywood boards. Position the board against one side of the joist. Use clamps to keep it in place.
Drill 3/16” pilot holes into the joist, centering them in the holes you already have in the plywood reinforcement. Insert ¼” lag bolts into the pilot holes and tighten them into the joist with a ratchet and socket.
Repeat steps 10 through 12 on the other side of the joist.
Reinforcing Joists When You Can’t Get Underneath
Some crawl spaces are not DIY friendly and do not provide enough room to work. In these cases, you can still reinforce joists and eliminate weak or squeaky areas by accessing the joists from above. However, this will be a much bigger job and is best left for when you replace the flooring anyway.
Doing this requires removing the surface flooring. In some cases, such as carpeting, the flooring can be saved and reused, but with other types of flooring, you may as well plan on doing a remodel, as there is no way of removing the flooring without damaging it. In either case, you can see the subfloor once the flooring is up.
Working on the joists from above will require cutting out sections of the subflooring. The temptation will be to cut as little as possible to minimize the amount of subflooring that you will need to replace. However, to properly reinforce a floor joist, you will need to reinforce a large span, at least six feet long.
If you are reinforcing from only one side, any reinforcement will require removing the plywood subfloor from the centerline of the floor joist on one side of the joist to the centerline of the joist to be reinforced. The reinforcement should be made of softwood plywood that is at least ¾ inches thick.
All subfloor replacements must be made so that the new subflooring is properly supported. This means that the edges parallel to the direction of the floor joists must rest on the joists themselves. In new home construction, adjacent pieces of subflooring share the top of the floor joist, with each piece set so that it goes halfway across the joist. This allows enough surface for both pieces of subfloor to be nailed to the floor joist.
Identify squeaky spots by carefully walking the floor. Mark each squeaky spot for further investigation. While investigating the subfloor, look for signs of dampness or rot. Poke suspect areas with something sharp to see if they feel mushy. Spongy areas should also be marked for replacement. Now is the perfect time to make repairs and solve problems that are causing issues, so be thorough in your investigation.
If the damage is extensive, contact your homeowners insurance agent before doing any more work. You may need to file a claim.
Once you’ve marked all the bad spots, draw chalk lines so you’ll know where to cut. Any cuts you make parallel to floor joists need to be down the nearest joist’s center line. You have to have a way to attach new plywood to reinforce the joists.
Use a circular saw to remove damaged areas of plywood, setting your blade 1/8 inch deeper than the thickness of the plywood. Are there many problem spots? You might save time and effort by prying up entire pieces of damaged subfloor and replacing them instead of cutting lots of holes.
For areas close to walls where your saw can’t reach, use a hammer and chisel or a multitool to cut through the damaged wood. Keep in mind that nails run through the subflooring, which can get caught in your saw blade and either damage it or sever the nail, sending pieces of it flying off.
Inspect the joists underneath the areas you have opened up to provide access. If they appear to be sagging, you may have to bite the bullet, go under the house, and follow the instructions for bracing joists with plywood above. Luckily, you can do some of the work from above. Once the problems have been addressed, you can install a new subfloor to patch the damaged spots.
Plywood usually comes in 4×8 foot sheets, although some venues carry different sizes. Choose the same thickness as the existing subfloor and purchase enough to account for all the damaged areas, plus a little extra in case you’ve made erroneous measurements.
Measure all the damaged areas and cut the new plywood subfloor accordingly. You want to ensure a nice tight fit with adjacent subflooring.
Before setting the new plywood in place, apply a construction adhesive layer to the joist’s upper edge.
Next, screw the plywood into place. Drywall screws work well for this. Place the screws at four-inch intervals to add strength. If you prefer to use nails, rather than screws, then be sure to buy screws with a twist, so that they will not loosen over time.
Lay a bead of construction adhesive into the joints between the plywood panels. This will add strength and help prevent future problems.
Reinstall the finished floor or install new flooring.
While these tasks can be labor-intensive and time-consuming, reinforcing joists with plywood protects your investment in your home. Doing it yourself will save money – even if you purchase the best plywood, screws, and construction adhesive available.
You’ll need specific materials and tools to reinforce your floor joists effectively. Here are some expert-recommended items available on Amazon:
½-Inch Softwood Plywood Sheets Used to scab onto existing joists, adding strength and reducing flex. Ensure you select the appropriate size based on your joist dimensions.
Construction Adhesive Provides a strong bond between the plywood and joists. A high-quality adhesive ensures long-lasting reinforcement.
¼-Inch Lag Bolts Essential for securely fastening the plywood to the joists. Using lag bolts ensures a tight and durable connection.
Hydraulic Jack A reliable jack is useful for lifting sagging joists back into position before reinforcement. It ensures safety and precision during the process.
Joist Hangers Provide additional support to the joists, ensuring they remain securely in place. These are especially useful if you add or reinforce existing joists.
Before purchasing, ensure you accurately measure your joists and workspace to select the correct sizes and quantities. Additionally, safety guidelines should always be followed when performing structural repairs.
These products will aid in effectively reinforcing your floor joists, enhancing the structural integrity of your flooring system.
Metwood Hole Reinforcer This two-part plate reinforces joists with existing holes for piping or ductwork, restoring their load-bearing capacity.
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