Bulging floor? Better check joists

Q: I saw your column a while back about sagging floorboards. Well, I have the opposite problem. A couple of spots underneath my living-room carpet feel like there is a “hump” there. When I pulled back a section of carpet to check if there was something underneath, I saw nothing. The underlayment looked fine. But those areas feel out of line with the rest of the floor. Do you have any idea what this could be? — Joseph T., Ogden, Utah

A: If you’ve ruled out problems with the carpet and subflooring, the joists underneath may be a possibility. These are large boards that support the subfloor, which can be accessed from the basement or crawlspace. Like all wooden structures, homes contract and expand, and settle over time, and this affects the structure in different ways.

In this case, it’s possible that a joist has warped and is arching or bulging upward, creating that odd hump in the floor (that there are two spots, suggests two warped joists). This can be corrected by using the weight of the house to straighten the board.

•Slide a level along the carpet to find the highest point of the hump. Mark it, then measure to that point from some element that extends to the area below (such as a heating duct or pipe). This measurement will help you mark the correct spot on the joist below where the high point of the bulge is centered.

•Once you’ve found your spot, take a reciprocating saw and make a straight cut at that spot, starting at the bottom edge of the joist and going to just 3/4 of the depth of the joist (do not cut all the way through the joist).

•Leave the joist that way for a few weeks, checking the floor occasionally with a level. Don’t place excessive weight on that spot. Let the joist settle naturally. Once the spot is flush with the rest of the floor, go back down to the joist. Reinforce it by nailing a board of the same width to the joist (alongside, not underneath). The reinforcing board should be at least 6 feet long and secured with 16d nails in pairs, staggered a foot apart. Drive three nails on either side of the cut.