Reinforcing Joists with Plywood

Unless your home is built on a concrete slab, where the slab is the subfloor for the first floor, the floor itself rests on floor joists. 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. This is mandated by building code and has been in place for years.

Nominal Joist SizeJoist Spacing on CentersLumber Grade: Select StructuralLumber Grade: No. 1Lumber Grade: No. 2
2”x 6”1611’ 4”10’ 11”10’ 9”
209’ 11”9’ 7”9’ 1”
2”x 8”1615’ 0”14’ 5”14’ 1”
2413’ 1”12’ 4”11’ 6”
2”x 10”1619’ 1”18’ 5”17’ 2”
2416’ 8”15’ 0”14’ 1”
2”x 12”1623’ 3”21’ 4”19’ 11”
2420’ 3”17’ 5”16’ 3”

In more recent times, engineered wood joists, which are 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 building of the home with less lumber, conserving resources and lowering 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 really old 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 in them, to keep them from pulling up with time and flexing of the floor.

Floor joists can twist or warp slightly with age, especially if they are installed in an area where they are 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 pulling them out of the floor joist. Which of those 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 be pulling loose in the joist.

Besides creating squeaky floors, joists which have become somewhat rotted become weaker, allowing more flexing of the floor and joist. When you feel sponginess in a floor, you should not assume that it’s just loose floorboards or subflooring; but 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 good plywood subfloor adds support to your home, you may not be aware 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

In the event a joist is cracking or sagging, it needs to be repaired quickly. The longer you leave it, the worse it will get. Not only that, but sagging floors can 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 originally used, essentially making a double joist. While this is functional, it is also an expensive means of strengthening the joist, using more material than is necessary. It is never advised to use a smaller piece of dimensional lumber, 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 is just as effective and ultimately cheaper. If you’ve ever looked at the engineered wood joists they are using 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 heavily wood, glued together under compression. Some people consider it to be inferior to standard lumber, but the opposite is almost always true! A quality piece of plywood is typically stronger than the corresponding amount of standard wood.

  1. 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 a very old house indeed, where they didn’t use sanded dimensional lumber.
  2. Measure the length of the joist. You don’t want to repair only a part of the joist, but rather, to go the full 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.
  3. 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 longer than eight feet long, you’ll need to use multiple strips of plywood to support the joist. In this case, you can either: 
    1. Add an additional layer over the plywood reinforcement to strengthen the joint. If you choose to do this, make the additional reinforcement four feet long.
    2. Use ¾” plywood on both sides of the existing joist and offset the joints by at least two feet, to avoid creating a stress point.
  4. Drill a line of ¼-inch pilot holes at six-inch intervals down both long edges of each plywood reinforcement strip.
  5. Check the joist with a 8 foot level or other long straight edge. If the joist is cracked or is markedly sagging, you will need to use a hydraulic jack to raise it. While doing this, it’s a good idea to put a heavy 6×6 block wood beneath the jack to support it.
  6. Measure the distance between the bottom of the sagging floor joist and the top of the jack.
  7. Cut a 4×4 piece of lumber to fit the distance measured.
  8. Place the 4×4 between the jack and the floor joist. It should be a snug fit. If not, raise the jack slightly. This is often easier with a helper to position the 4×4.
  9. Jack the joist up very slightly. This process can be a lengthy one, as you should jack at a rate of no more than ¼ inch per day. Jacking more than that can damage drywall or walls in the house.
  10. Once the joist has been appropriately straightened, it’s time to get back to the process of 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.
  11. 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.
  12. Repeat steps 10 through 12 on the other side of the joist.

Reinforcing Joists When You Can’t Get Underneath

Some crawlspaces are not DIY friendly; not providing 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 is going to be a much bigger job and is best left for doing when you are going to 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, once the flooring is up, you’ll be able to see the subfloor.

Working on the joists from above will require cutting out sections of the subflooring. The temptation will be to cut as little as possible, so as 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.

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, if you are reinforcing from only one side. The reinforcement should be made of softwood plywood that is at least ¾ inch thick.

joists, floor, wood, repair, tools
Reinforcing floor joist, Ben Freeman

All subfloor replacements must be made in such a way that the new subflooring is properly supported. This means that the edges which are 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.

  1. Identify squeaky spots by carefully walking the floor. Mark each of the squeaky spots 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. Those which are spongy 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.
  2. If damage is extensive, get in touch with your homeowners insurance agent before doing any more work. You may need to file a claim.
  3. 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.
  4. Use a circular saw to remove damaged areas of plywood, setting your blade to 1/8 inch deeper than the thickness of the plywood. Are there a lot of problem spots? You might save yourself some time and effort by prying up entire pieces of damaged subfloor and replacing them instead of cutting lots of holes.
  5. 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 there are nails running through the subflooring, which can get caught in your saw blade and either damage the saw blade or sever the nail, sending pieces of it flying off.
  6. 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 part of the work from above. Once any problems have been addressed, you can move on with installing new subfloor for patching damaged spots.
  7. 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.
  8. Measure all the damaged areas and cut the new plywood subfloor accordingly. You want to ensure a nice tight fit with adjacent subflooring.
  9. Before setting the new plywood in place, apply a layer of construction adhesive to the upper edge of the joist.
  10. 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.
  11. Lay a bead of construction adhesive into the joints between the plywood panels. This will add strength and help prevent future problems.
  12. Re-install the finish 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.

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