How to Put Down a Subfloor When the Walls Aren't Square

Starting the Installation

• Because subfloor panels must run perpendicular to the joists, it doesn't make sense to line them up with a wall, especially one you think isn't straight. A better procedure is to snap a chalk line down the middle of the floor after determining its direction with a carpenter's square. The first course of panels goes against the line, and it sets the standard for the rest of the subfloor. You'll have to take some measurements to find the middle of the floor; take them from the centers of the walls that run perpendicular to the joists.

Trimming the Ends

• The ends of each panel must intersect a joist, and adjoining panels share the same one. You install the panels somewhat like you do hardwood floorboards, butting them against each other and trimming the ends when you get to the walls. Even if you think the walls are straight, it's a good idea to measure from both sides of the second-to-last panel to the wall so you can compensate for any difference in length when cutting the end panel. Draw the cut line on the end panel and cut it with a circular saw. You should leave a 1/4-inch gap to allow for expansion.

Trimming the Sides

• As you install the panels, staggering the joints by at least one joist for stability of the overall floor, you eventually come to the walls on each side. Cutting the panels to fit in the gaps and finish the installation follows basically the same procedure as cutting the ends. Take at least two measurements, one from each end of the panel you're fitting, and make the cut with a circular saw. If the wall is curved or irregular, it's helpful to trace the shape onto a piece of kraft paper and use the paper as a template for your cut.

Compensating for Narrow Edges

• After determining the center of the floor, it pays to divide the measurement in inches from that point to either wall by 48, which is the width of a panel. If the result has a remainder of less than 2 inches for either wall, move the center line a little farther than that distance toward that wall. Doing this avoids having to fit a narrow piece of subflooring against the wall, which can affect floor stability. Don't forget to make sure that moving the line doesn't create a similarly small gap at the other wall. If it does, move the line at least 2 inches farther.