This article is part of our series on How to Install Hardwood Floors ^[1].

While preparing to lay 1100 square feet of hardwood flooring, we discovered significant variation in the OSB subfloor that will serve as the nailing surface for the hardwoods. This article provides detailed instructions for leveling a subfloor based on our project.

Project Overview: Uneven OSB Subfloor

The subfloor beneath our first floor is 5/8″ oriented strand board (OSB) laid over 2 x 10 joists that are spaced 16 inches on-center (o.c.). The joists run from the front of the house to a steel I-beam in the middle of the house, and then from that I-beam to the back wall. Each span is approximately 14 feet.

Before we pulled up the previous flooring surfaces (carpet and vinyl), we knew there were some spots on the subfloor with significant peaks and valleys.  Our kitchen, for instance, had a noticeable dip around a cabinet peninsula.  We think the house was built with ‘green’ lumber that may have been exposed to moisture before it was installed. When it dried, the bowing became very noticeable.

Also, every piece of dimensional lumber has a natural crown to it (the direction it bows along the long edge of the wood). The joists should have been laid such that all the wood was “crown up,” but we suspect that several of the joists were incorrectly installed “crown down.” When this happens between two joists that are “crown up,” the depression can be significant.

The depth error in the worst spots on the floor was as much as 3/4″ over about 4 feet. This is well outside the tolerance for hardwood flooring, which should be no more than 3/16″ variation over 8 feet. You can see the dramatic slope on just one part of the floor in this picture. The level is sitting on the crown peaks, with the same measure in a depression between them.

What is surprising about this floor is that the actual OSB is in very good shape. Sometimes, you’ll find OSB swelling at the joists where moisture contacted the subfloor. This wasn’t the case in this house. We did have to replace one piece of OSB due to wood rot where a persistent leak existed, but the rest of the floor was in pristine condition.

If you’re dealing with a swelled OSB situation, we think the best course of action is to use a belt sander to grind down the swelled edge. Just make sure this doesn’t cause any structural instability. If the piece is rotted or just beyond reasonable repair, the best course of action is replacement.

The Solution for Leveling a Subfloor

Searching around the internet for solutions, we eventually landed on this article ^[2] from Ask the Builder that suggests using construction felt and asphalt roofing shingles to level an uneven plywood or OSB floor ^[3] for hardwood installation. This was a novel idea to us, but it made sense.  Both roofing shingles and construction felt are dense, not prone to compression (a key element for leveling), and sufficiently thin that they can be used as incremental steps to get to a flat surface.

Since we have such dramatic slopes on the floor, we decided to use the shingles and construction felt solution along with a new layer of 3/8″ plywood. We chose 3/8″ plywood because it is rigid enough to absorb very minor depressions underneath it, and not so heavy as to be unwieldy.

We are planning to lay 3/4″ exotic hardwoods on top of the plywood, so what we’re looking for from the new plywood isn’t an “ultra-rigid” surface, it simply needs to bond to the layer beneath it, absorb very small gaps where the leveling provided by the construction felt and shingles isn’t perfect, and allow sufficient “grab” by subfloor screws to get a tight bond with the 5/8″ OSB below.

To fasten the plywood to the OSB, we considered using a combination of 2″ subfloor screws and construction adhesive (i.e., Liquid Nails for Subfloors). This is generally referred to as “gluing and screwing,” and you’ll see it suggested on some ceramic tiling sites as a method to increase the rigidity of a floor for tile installation. Since we’re going to be laying the 3/4″ hardwoods on top, which itself will entail 15 gauge staples more or less every 18 inches on the floor, we decided to omit the construction adhesive and instead go just with the screws. Our primary motivation was speed and less mess, and so far after walking on the floor, we’re happy with the result.

For the subfloor screws, we used a Senco Screwgun ^[4] (reviewed at that link) to fasten the 3/8″ plywood to the OSB. The screwgun is an absolute must for this job. We put a screw in approximately every one foot square, which means approximately 1100 screws on this floor. Driving with a drill-driver is simply not an efficient option.

Alternatives to Shingles and Plywood for Leveling

Before getting into all the details of the leveling job, I should say there are other options for leveling a floor. For instance, we could have pulled up the OSB, shimmed the joists underneath, and refastened the OSB to the shimmed joists. This would have the advantage of not putting any additional load on the subfloor, and addressing the cause of the problem directly at the root.

In our situation, however, we believed that this effort would be much more significant than a new layer of 3/8″ plywood, because the problem was pervasive throughout the floor in varying degrees, and as you’ll see from the pictures below, the shingles give a lot of options for how to address a slope over a wide range of floor.  If we were dealing with a single, significant depression in one or two parts of the floor caused by joists being incorrectly crowned, we probably would have opted for this solution since it is much less expensive.

There’s also self leveling concrete ^[5] as an option for leveling a subfloor. We used self leveler to level the concrete slab ^[6] in our basement with great success. We opted against this solution on the first floor because most self levelers aren’t rated for installation over OSB, and because they are expensive and messy. Plus, we figured we would need to install 3/8″ plywood over the self leveler to prepare for hardwoods (this may have been overkill, but we weren’t comfortable with the solution of driving staples directly through the leveler). Effectively, the asphalt shingles in our solution are a rigid leveler that we think works as well as any liquid self leveler would have, and they are easier to control.  If you’ve never worked with self leveler, it is a miracle product, but it has its drawbacks, including short work times.

Tools Required for Leveling a Floor with Plywood and Shingles

• 8 foot level – for getting an idea of where the high and low spots are on the floor.
• 4 foot level – for moving in tighter areas and judging how level a local area is.
• String and nails – for pulling a string above the floor both before and after the installation to get floor-wide perspective on the problem.
• Utility knife with roofing blade (2 knives are nice if you’re working with two people).
• Tape measure
• Circular saw (recommended) and/or table saw for cutting plywood.
• Screwgun (we like this Senco Screwgun ^[4] with square head driving bit).

Materials Required for Leveling a Floor

• Asphalt roofing shingles – the 3-tab kind that are about 1/16″ thick (cheapest available is best). You can find these at Home Depot, Lowes, roofing supply stores, and other home stores.
• 3/8″ plywood – this will be sold as 11/32″ plywood. You will need a sufficient amount to cover the subfloor.
• 2″ or 2.5″ collated subfloor screws for the screwgun – we used square-head screws – expect 1 screw per square foot.
• 30 lb. construction felt.

Steps for Leveling a Subfloor

Preparation: Ensure the subfloor is clear, clean, and free of debris, with plenty of space to work. Remove molding and baseboard at the edges so you can get the plywood close to the walls.

Step 1: Determine and mark high and low spots across the floor. We did this two ways, first by dragging our 8 foot level across the floor to try to determine where the high and low spots were in local areas, and then by stretching a string across the whole floor, secured by a nail on each end.  The string does a better job of showing high and low areas across the whole floor. We marked high and low spots using a non-scientific method, sharpie marker on the floor with the words “high” and “low” and sometimes “very low” in the worst spots.

Step 2: Set a strategy for filling in low spots to “even out” the floor. The floor doesn’t have to be level, it has to be flat. Also, it doesn’t have to be perfectly flat – 3/16″ rise or fall over an 8-10 foot area is acceptable.  We decided to start at one end of the floor near the fireplace that had a significant low area and a nearby high area. Near the front of this fireplace, the depression is almost 3/4″, so we’ll have to stack the shingles over one another.

Step 3: Fill in low spots with shingles and construction felt. For our efforts, we decided to use shingles exclusively under the 3/8″ plywood, with the possibility of using the construction felt between the 3/8″ plywood and the hardwoods if necessary.  This can be hard to get perfect, and we found ourselves frequently going back and forth between our 8 foot and 4 foot levels.

If you picture the floor as a topographical map, the idea is to try to fill in the low spots, which may require overlapping some shingles to get to the right depth, and will certainly require cuts… especially since you are always leveling in two dimensions. You can picture that if you were filling a perfectly round depression, you would probably start in the very middle and work your way out, layering shingles until you get a perfectly flat surface.

Step 4: Once the low spots are filled in with shingles, fasten 3/8″ plywood to the surface using 2″ or 2.5″ subfloor screws. Try to hit the joists as often as possible and put the screws about every 12-14″ along the perimeter and about once every square foot in the field. In this picture, Ethan is checking to sure that the shingles underneath my feet are filling the gap in the floor correctly.

We found it best to “dry fit” the plywood in place first, walk on it and have another person use the level to check for uniform flatness.  Once you start putting screws in, you’ll eventually create a very difficult sheet to remove (remember, each piece of plywood ends up with about 45 screws in it if you screw every 12 inches square and along the perimeter).

Step 5: Continue moving out from the first piece of plywood, continually leveling and re-evaluating extended areas of the floor. It is a good idea to regularly check and re-check the height of the floor and to make sure that seams between the pieces of plywood do not create ridges.  Much like regular tongue-and-groove plywood on joists, we recommend staggering the joints on the long edge.

Step 6: Repeat steps 3-5 throughout the rest of the floor.

Floor Loading / Weight Considerations

Any time you are adding weight to a floor, there can be a loading concern.  Most modern homes are built to handle 40 lbs. / square foot average loading, but every house is different, and your individual situation could be very different.  3/8″ plywood weighs about 1.5 lbs. / square foot, so we were not concerned about it.  Asphalt shingles can get heavy, however, especially when filling in a low spot. We think the most weight that would be placed across a single joist with the shingles is about 15-25 lbs along the span, or about 3-4 lbs. per square foot.

In general, plywood does not add strength to a floor, except that it can help distribute the load from the center of a floor to the edges and to nearby joists, which tends to increase strength a bit. If you are at all concerned, it is wise to consult with a structural engineer before trying this in your own home.

Additional Thoughts & Tips

• Try to avoid relying on the 3/8″ plywood to level the floor. 3/8″ plywood will span about a 1 inch gap before it exhibits considerable localized weakness… Instead, try to use the shingles and/or construction felt to eliminate air gaps between the true subfloor and the additional 3/8″ plywood layer.
• Remember that the subfloor being “level” is considerably less important than being “flat” – if there’s a slight incline (3/16″ or less) over 8 feet, that will likely be imperceptible to visitors and you especially once furniture is placed in the room.
• Think of the project as trying to “smooth out” hills and valleys in the floor to acceptable tolerances. Nothing has to be perfect.
• Plan to lay the hardwood floors across ^[7] the joists rather than parallel to them. This will further reinforce an even flooring surface.

Does it Work?

Absolutely. Take a look at the finished floor in this space ^[8]. It’s beautiful, and almost perfectly flat. We’ve had the floor installed for 1-1/2 years now and have had no problems with leveling. Guests marvel at the new floor, not knowing all the work that went into the installation.