Q:
My husband and I live in an 1850s 1-1/2-story, balloon-frame house. Floor joists for the second-floor bedroom are 2x6s that span 12 ft. The joists have a definite sag to them (more than 1-1/2 in. in places), and the floor is like a trampoline when walked on. We are thinking of jacking the floor from underneath to take out the sag, then beefing up the joists. Do you have any suggestions?
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A:
John White, a carpenter and woodworker in Rochester, Vermont, replies: I’ve fixed a lot of bouncy floors in old houses over the years, and the first thing I strongly suggest is to resist the urge to jack up the floor from underneath. Here’s why.
Those old joists have taken many decades to develop the sag you describe, and they have undoubtedly become set in that position. My experience with jacking sagging joists is that the sag usually stays, and the ends just tear away from where they’re anchored and move up. Unless you absolutely cannot live with the bowed ceiling below, I’d let the old droopy joists stay in their comfortable positions and work to fix the floor from above.
Your floor is bouncy because 12 ft. is beyond the 9-ft. 9-in. recommended maximum span of 2×6 joists with 16-in. o. c. spacing. Increasing the depth of the joists is the best way to increase their spanning abilities, and it’s also exactly what you need to do to eliminate the sag in the floor.
Begin by tearing off the old flooring down to bare joists.Next, stretch strings from corner to corner along the walls running perpendicular to the joists. If yours is like most old floors, you’ll probably discover that the plates where joists are attached are also sagging, which plays right into our strategy to give the joists more depth.
To bring the floor up to the level of the strings, each joist will have a second 2×6 sistered to the first. I recommend using kiln-dried lumber to sister to the existing joists. It costs a bit more, but it won’t shrink as much as green lumber. I’d also get the straightest boards available.
Cut each joist to length, giving yourself plenty of room at each end. Slip 1x spacer blocks under the strings at each corner; then use the same thickness block on top of the new joists at each end to bring them to the level of the strings.Use a generous amount of construction adhesive between the new joists and the old, and then nail the new joists to the old at the proper height.
As added insurance, I’d install solid blocking between the joists across the middle of the floor. Blocking will keep the joists from flexing sideways as they take weight. Finally, I’d skin the whole floor with plywood to tie the system together further. If the added minimal floor height doesn’t bother you, use 3/4-in. tongue-and-groove plywood or oriented strand board glued and screwed to the new joists.
Notice that one tool I haven’t mentioned in this process is a level. Floors in old houses such as yours are not expected to be level, or flat for that matter. But remember that the primary goal here is to strengthen the floor, not to make it level.And being flat just makes it a whole lot easier to set up the furniture.
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I had an issue with bouncing floors and searched forever. I did find this company called structure Lock that uses metal Ijoists.
Try them out structure lock.com