Subfloor Repair: A How-To Guide

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Level subfloors are key to gorgeous floors.

You could be walking on the finest tile or hardwood that money can buy, but the truth is that the flooring in your home is only as good as the subfloor beneath it. Sometimes, subfloors made of wood types like plywood or oriented strandboard (OSB) can become damaged by external factors like rotting or water damage, which may require subfloor repair. Fortunately, repairs can be made to these subfloor types rather than fully replacing them depending on the amount of damage to the subfloor material.

Different Subfloor Types

Two common types of subflooring made from wood materials are plywood and OSB. These two materials may look similar since they are both made of pressed wood, but they are quite different when it comes to how they hold up as a subfloor option.

Plywood is made of thin strips of wood veneer that are pressed together using heat. Plywood is the most common type of subflooring used in homes and boasts a quick drying time if it does become exposed to moisture. Because plywood offers a quicker and more complete drying process, it won't warp under wet conditions like OSB, which is known to swell at the edges once it has finally dried. However, plywood is lighter than OSB and can contain more inconsistencies within the board, like knots.

Along with its cousin OSB, plywood is the most common type of subfloor.
Image Credit: Jens_Lambert_Photography/iStock/GettyImages

OSB is made from wider pieces of wood, usually about 3 to 4 inches, which are layered to form a board using heat-cured adhesives, according to the Tile Council of North America. OSB is a popular option because it is cheaper than plywood and is more dense despite being the same thickness. Although plywood does dry out quicker than OSB, the glue that holds it together can become compromised by wetness since it does absorb moisture more quickly. Overall, plywood is known for being more long-lasting than OSB.

Assessing Your Subfloor for Repairs

How can you tell if your subfloor needs to be repaired? There are a few telltale signs of a damaged subfloor, including damage to your flooring — look for cracked grout in tile flooring, hardwood that's cupping or laminate floors that appear to have bubbles underneath. Additionally, floors that appear to be sagging or feel like they're sinking or bouncing when you walk on them are all indicators that your subflooring may need to be repaired or replaced.

One of the most common reasons plywood or OSB material is in need of subfloor repair is due to water damage. If you notice a musty or wet smell in the room, hear your floorboards creaking or know that your floors have been exposed to excessive moisture be that from flooding or a consistent leak, you'll want to inspect your subfloor for repairs immediately, suggests Old House Journal.

Additionally, the presence of rotting joists, which are the wooden beams that lay horizontally and are used as structural pieces when framing a space, can be an indicator that your subfloor could use repairing. To check for rotting joists, look for wet or dry rot, insect damage and rust around hardware near the joists.

You can repair joists that have less than 1 inch of rot by scraping away any rotted wood and filling the space with epoxy. All epoxied joists should be brushed with a few coats of wood preservative before laying anything on top of them. For joists with more than 1 inch of damage, sistering the wood with identical lumber may offer a repair; otherwise, replacement may be necessary.

Things You'll Need

  • 2x4 or lumber blocks (as many as needed to reinforce exposed joists)

  • Plywood

  • Chalk snap line or pencil and straightedge

  • Circular saw

  • Tape measure

  • Pry bar

  • Level

  • Hammer

  • Nails

How To Repair a Subfloor

Replacing an entire subfloor can be an expensive and time-consuming task. Fortunately, many subfloors exposed to water damage can be whipped back into shape with a basic subfloor repair. If you have flooring like tile or laminate over your subfloor, it will need to be removed before you begin any of these steps so that you can access the subfloor. Common areas of the home that show water damage to plywood subfloors are kitchens and bathrooms, especially around areas that hold water, like tubs.

Step 1: Indicate the Area You Need to Remove

When repairing water-damaged plywood subfloors, look for water stains that are discolored from the rest of the surrounding plywood. You'll need to mark the area you will be removing with a chalk line or straightedge and measure the area so that you can cut a new piece of plywood to fit perfectly in its place.

When identifying your damaged area, take notice of where your floor joists sit under the subfloor so that you don't cut into them when you saw through your existing subfloor. This can be determined by following the line of nails you see on your subflooring. Plywood subfloor is nailed into the joists.

While repairing your subfloors is generally best left to the professionals, small repairs can be DIYed.

Step 2: Remove the Damaged Flooring

Using a circular saw, remove the damaged patch of subfloor by following the lines you have marked. To do this without causing potential harm to anything that may be under the subflooring, like electrical wires, measure the thickness of your subfloor and then set the depth of your saw to just a tiny bit deeper than that measurement to ensure a clean cut. Plywood subfloors vary in thickness from 5/8 inch to 1 inch. The wider the space between your floor joists, the thicker your subfloor will be. Use a pry bar to pull up your rotted subfloor.

Step 3: Reinforce Floor Joists

Before you add your patch, you will need to reinforce your floor joists using a technique known as "sistering." To do this, measure the length between the two floor joists that rest under the damage piece of subfloor you just removed. Then, nail 2x4 pieces of wood or lumber block to the floor joists you just measured, which will create a beam that connects the two joists under the piece you just removed. If you had to remove a large amount of flooring, you may need to apply a few beams between your joists — just be sure to try to space them out as evenly as you can. Use a level to make sure your sistered joists are balanced and make sure that the edges of the new wood pieces lay flush with the existing joints to ensure that your patch will be fully supported when installed.

Step 4: Apply Your Patch

Use a hammer and nails to attach your plywood patch to the floor joists. When measuring your patch, be sure to leave an extra 1/8 inch between your old and new subflooring, as the wood will expand slightly.

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Krissy Howard is a NY-based freelance writer who specializes in creating content regarding pet care, skin care, gardening, and original humor. Her work has appeared on Reader's Digest, Hello Giggles, and Reductress.

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