Level an Uneven, Crowning Subfloor by Planing / Sanding Joists

June 28, 2010 | by Fred (email) |

Up first in hardwood flooring week is tackling a particularly challenging section of our subfloor that couldn’t be leveled using our asphalt shingle subfloor leveling method. If you want background on the entire leveling project, I suggest reading the shingle-method post first.  It gives information on the subfloor situation covered in this article, and the steps required to level a subfloor without taking up the existing subflooring material.

In some cases, it is necessary (or at least beneficial) to address an uneven subfloor issue at the joist level. In our case, this occurred in a particularly uneven spot near our garage entry door. The problem: one of the joists close to the end joist of the house had a very significant crown in it. Between it and the end joist (just 12 inches away), there was a 5/8 inch drop! The joist on the other side of the crowned joist was properly level (meaning it was in plane with the end joist). Essentially, a hump on the floor was created by the excessively crowning joist.

To address this unique area of the floor, we decided to remove the existing 5/8-inch OSB, plane the joist using a standard handheld planer, and install new OSB over the joists. This process was fairly simple and straightforward–the details are below.

This article is part of our series on installing hardwood flooring. Click that link for an index of all the articles in this series.

Safety Issues

Before getting into the process, it’s worth noting that whenever you plane a joist you reduce its weight-bearing capacity. Accordingly, it may be unsafe in any given situation (especially if you plane too much). In our situation, this joist was very close to the neighboring joists on both sides and we felt comfortable planing. Before tackling something like this in your own home, you should discuss and obtain approval on the project from a structural engineer.

Also note that you could encounter electricity, plumbing, ductwork, alarm wires, telephone wires, and more when doing this project. There are also other safety hazards–such as the gaping hole left by the subfloor being removed mid-process. We can’t anticipate the problems or safety issues you might encounter in their own home, so make sure to contact a professional if you aren’t qualified to perform this job. As with every article on our site, use this information at your own risk!

Tools and Materials Used

• Circular Saw
• Chalk Line
• Drill Driver with 1/32″ bit.
• Planer (or sander, although planers are better for this).
• Screwgun (we like this Senco Screwgun)
• New Subfloor Material that Matches Existing Subflooring
• Subfloor Screws
• Construction Adhesive (Liquid Nails for Subfloors works)

Steps to Level a Subfloor At the Joists

Step 1: Locate the centers of the joists on either side of the offending joist and mark these on the floor using chalk line. Identifying the exact location of joist centers can be difficult. We used a 1/32 inch drill bit poking through the subfloor at 1/4″ increments until we found both sides of the joist, then we marked in the middle. We did this on both sides of the floor and then snapped a chalk line between them. This process is much easier if you know the approximate location of the joists (we can see them from our basement).

(Update: Note that it is better if you can go further out than one joist on each side of the offending joist, as span-rated plywood or OSB is rated so when it spans multiple joists.  In our case, existing walls prevented this from being accomplished easily, and we were planning to lay 3/8″ plywood on top, and 3/4″ hardwoods on top of that – so we weren’t concerned).

Step 2: Set the blade on your circular saw to the subfloor depth (no more!) and cut along the chalk lines on the centers of the joists on either side of the offending joist. The goal is to remove the subfloor back to the centers so that installing new subflooring material is simple. If you are able to preserve the existing subfloor, you may be able to re-use it (we didn’t). Note that at some point you will have to cut 90 degrees to the parallel chalk lines. We got as close to the walls as practical in our situation.

You can see in the picture below that the subfloor has been removed back to the centers of the joist on either side of the crowning joist (you can also see some of the spray foam insulation in the rim joist in the basement through the hole). The single joist completely exposed will be planed.

Step 3: Identify a method to determine what constitutes “level.” We decided to use a piece of the hardwood we are installing which registered as flat against our 4 foot level. We were able to cut this wood to fit between the joists and drag it along the surface.

Step 5: Plane the joist until level with the neighboring joists. The key in this step is to not over-plane. If you do, you’ll have the exact opposite problem on your hands. Plane slowly and regularly check the joist height relative to the neighboring joists.

Step 6: Cut and dry-fit the subfloor material (OSB or plywood at the same width as the existing subfloor). We also checked the floor for level to make sure our work was good. You must make precise cuts on the subfloor material since it will be resting only on half of the neighboring joists.

(Update: note that plywood and OSB span ratings are based on the wood being run across the joists as opposed to parallel to the grain.  In our situation we laid the subfloor parallel to the joists which creates a weaker subfloor. However, we are laying 3/8″ plywood on top of this OSB, and 3/4″ hardwoods on top of that for a very high strength floor. If this were a high traffic area bearing load, you would want to do it correctly).

Step 7: Lay a bead of construction adhesive on the joists, place the new subfloor piece, and fasten the new subfloor to all joists using 1.5 or 2 inch subfloor screws. Our Senco Screwgun made this step a snap, but you could do it with a drill driver and subfloor screws.

What do you think? Have you tried this method in your own home? Let us know in the comments.