How to Avoid Sagging Floors
In some cases the degree of sag weighed against the complexity of the repair will persuade you to leave things as they are. In other cases, you may wish to take action.
Most sags occur in the first-level floor, which is a blessing, because you can make your repairs down in the basement where the job is easiest. The joists are normally exposed, and if you decide to install a permanent post or two for added support, it’s no great cosmetic loss.
Repairing sags in a second-story floor is more involved. You’ll probably have to tear down the ceiling for one thing. And you certainly won’t want to install support posts in the middle of your living room, so the fix can be more complicated, as well.
At any rate, the first step is to assess the amount of sag, and pinpoint its location.
Select a long, straight board to use as a straightedge and lay it across the floor in the area of the sag. Measure the distance from the bottom of the board to the floor in several spots, looking for the area of greatest sag.
Once you find the lowest place, find that spot in the room below the sag. If the sag is concentrated in a small area, less than three feet across, it is probably caused by a warp in one or two joists and the subfloor.
Cut several gradually tapering wedges from a piece of maple or other hard wood and drive them into place every six inches or so along the trouble spot. Have someone upstairs observe the results against the straightedge.
Sags over a wide area are caused by actual structural weakness and will need more extensive repairs. If the sag is out in the middle of a run of joists, those joists are probably undersized. If you don’t mind the obstruction, you can simply bridge the sagging joists with a length of 4-by-4 and install a telescoping steel post.
If a post would get in the way, you’ll want to jack the floor up to level and reinforce it by doubling up the old joists.
Once the jack is positioned, however, it tends to get in the way when you try to install your reinforcing joists, so it’s simplest to cut these and tack them lightly in place before you place the jack.
Cut your reinforcing joists from the same sized stock used for the originals, and tack it in place with one or two nails. Now you can place your jack.
The best type to use is a screw-type house jack. You can get one at any rental shop. Set it up as shown in the sketch at right, resting on a length of 6-by-6 to spread its load across the floor. Use another length of 6-by-6 as a post, and a third to bridge across the sagging joists. If there is bridging or blocking between the old joists, remove it.
Then slowly jack the floor up to level. By slowly I mean about 1/8 inch per day (normally one turn of the jack handle). Don’t try to rush things or you could cause structural damage.
Once the floor is up to level, nail through the reinforcements into the old joists. Use 4-inch nails, and clinch them over flat where they protrude through the old joists. If you removed blocking or bridging from between the joists in an earlier step, you can install new bridging on blocking, but I wouldn’t bother. Recent research casts some doubt on the effectiveness of this type of reinforcement.
Replacement: If the old joists are unsound (because of rot or insect damage) the fix is not so simple. You’ll have to replace and remove (in that order) every defective joist.
Start by cutting replacements of the same size as the old joists. Choose good, straight, pressure-treated stock for this purpose. To install a new joist, you’ll slide it into position on its side next to the old one, then roll it up onto edge.
But when you try this, you’ll discover there’s not enough space to roll the board on edge. So you’ll have to cut away the bottom of the joist at each end to a depth of about 1/4 inch. Just take a saw and cut off a 1/4-inch strip about 16 inches long at each end.
Now you can roll the new joist into position. Make sure it is pressed up firmly against the subfloor above. The jack will probably support the middle section, but you’ll have to drive hardwood wedges beneath the joist at the ends where you notched them.
When the joist is in place, toenail it at each end. Then go upstairs and drive 8-penny finishing nails down through the floor into the joist. After each replacement joist is in place, remove the old one.
Best way to do this? Use a reciprocating saw or a saber saw to hack off the ends of the old joist. Take a chisel and split out the ends, then pry the rest of the joist free from the floor above.
If the old joists were victims of insect damage, you should call an exterminator. The bugs are no doubt still in your house, and though they should leave your pressure-treated repairs alone, they won’t do the same for the rest of your home.
