How to Fix a Sagging Floor Support Beam

By J. Cavan Barry

A building is composed of countless structural elements that keep it standing and safe for occupation. Support beams for floors are a common structural member used to carry the load of several floor joists, and when they are made from wood (which they commonly are) they have a tendency to sag over time as they wear out. While this may seem like a very serious problem, it is a common one, and by following a generally straightforward set of instructions, just about anyone with the right tools can fix this problem.

List of Items Needed

  • Electric screw gun
  • 1-inch screws
  • Masonry string
  • 4-by-4-inch wood posts
  • Circular chop saw
  • Small sledge hammer
  • Hydraulic bottle-jack
  • Tape measure
  • Plumb-bob
  • Shovel
  • Pick-axe
  • Concrete float trowel
  • 12-inch level
  • 5-gallon bucket
  • Concrete post mix (premixed)
  • 4-inch concrete post base
  • 2-by-1/2-inch lag bolts (with washers)
  • Electric impact wrench (with socket to match lag-bolt head)
  • 3-inch screws
  1. Drive a screw into the bottom edge of the beam at both ends -- the bottom edge where the side of the beam meets the bottom -- using the electric screwdriver.

  2. Tie the masonry line to either screw, then tightly stretch and tie it to the other, spanning the length of the support beam. This will show the straightest line between the two sides, and where the beam is sagging the most.

  3. Place the hydraulic bottle-jack near the center of the beam, clearing a flat area on the ground with the pick and shovel, if necessary.

  4. Measure the distance from the top of the jack to the bottom of the beam and cut a 4-by-4-inch post to this measurement, using the circular chop saw.

  5. Set the 4-by-4-inch post on the jack, tapping the top of the plank into place with the small sledge hammer, if necessary. Pump the jack until the bottom of the beam is level with the masonry line.

  6. Measure and mark the center of the support beam, using the tape measure and pencil. Hang the plumb-bob from this mark to find the corresponding location on the ground below.

  7. Excavate a hole about 6 inches deep by 12 inches wide below the center of the beam, using the shovel and pick.

  8. Pour the concrete post mix into the hole, flush with the surrounding surface. Slowly pour water over the post mix; refer to the mixing instructions on the post mix bag. Scrape the surface of the concrete pad, using the float trowel and the 12-inch level to make sure the pad is level.

  9. Fully insert the 4-by-4 post base into the fresh concrete pad. Let the concrete completely set and cure, as specified on the post mix packaging.

  10. Measure the distance from the surface of the post base to the bottom of the beam and cut a 4-by-4-inch plank to this measurement, using the circular chop saw. Set the 4-by-4-inch plank on the post base, tapping the top of the plank into place with the small sledge hammer, if necessary. Make sure the 4-by-4 post is plumb, using the 12-inch level.

  11. Depress the hydraulic bottle-jack and remove it along with the temporary post. Now the support beam will be straight and resting on the post.

  12. Fasten the post to the beam by driving 3-inch screws through the post and into the beam, using the electric screwdriver. Drive the lag-bolts through the washers and post base (pre-drilled holes) into the post, using the electric impact wrench.

Tips and Warnings

  • If the beam is very old, it may require pilot holes before inserting screws and lag bolts. Use a power drill or a handheld rotary tool with an all-purpose bit.
  • If the house is old and has settled, jacking up a sagging beam could cause new problems throughout the interior of the home such as cracked plaster walls, misaligned windows with cracked panes and ill-fitting doors. A structural engineer can tell you whether raising the beam is wise.

© Demand Media 2011