How to Measure a Sagging Floor

Walk around the room to determine the rough area of the sag. Note any dips in the floor that make you feel as if you walk downhill then uphill again. Or roll a marble or pen around and look for acceleration.

Set a laser level on a few books on a level area of the floor adjacent to the sag. Ideally the laser level's beam should sit a few inches off the floor.

Rotate the laser level so the beam projects across the center of the perceived sag.

Measure the depth, in inches, of the distance from the floor to the beam at several points along the beam. Record the largest distance. To make sure you have the lowest point of the sag, change the beam's direction slightly and measure other points of the sag. The largest measurement is an accurate depiction of the sag of the floor.

Calculate the sag. According to the experts at Construction Calc, the maximum amount of floor sag for most building codes is L/240. The "L" stands for the length of the joists in the room in inches. If the joists in your room are 18 feet long, then the maximum amount of allowable sag is 216 inches divided by 240 or 0.9 inches. If you do not know the length of the joists in your floor, or if you determine the sag is significantly higher than the code limit, contact a professional contractor.