Learn to identify the most common causes of floor sagging—and what to do about it. Learn the best tools and methods for lifting and shoring a load-bearing beam and the steps to safely replace a rotted post.
Sagging floors, doors that won't close and cracks that appear in interior walls can be symptomatic of a more serious problem. You may have undersized support beams, support posts that are rotting and losing their structural integrity, or undersized post footings that are settling. It's important to stop this problem and reverse it before it gets worse and becomes more costly to repair.
We'll show you how to identify two common causes of floor sagging, the best tools for lifting and shoring (temporarily supporting) a house beam, and the steps to safely replace a post. The actual work of repairing three to four posts and footings takes three or four working days spread over 10 days. Add to that the time to plan the work and obtain any required building permits.
Raising the main beam that carries the floors and interior walls of a house is complex. Don't tackle it if you're a novice. When you raise a house even 1/2 in., the effects can ripple through the building's framing, plaster walls, rigid piping and chimneys. For this reason, only specialty contractors should raise a beam or wall more than about 1 in. or undertake extensive floor leveling.
Mark the size of the new footing and cut the outline with a circular saw fitted with a diamond masonry blade. Start by making a 1-in. deep plunge cut on each side of the post. Make successively deeper 1-in. cuts until soil pulled from underneath the slab tells you the concrete has been breached.
Floors sag because the posts supporting a house beam are failing and/or the footings holding those posts are undersized and sinking. Diagnose the problem by seeking the easiest solution first; go further only as needed. If your posts directly contact the floor, start by using a screwdriver to probe the base of your wood posts to reveal any rot that could have caused them to settle.
If the post isn't rotting, the concrete footing it's on is almost certainly sinking or has deteriorated. Compare the old footing's dimensions with an engineer's recommendations and current code requirements (see next section). To get at the footing, you'll have to saw through and strip off the concrete floor covering it, then dig down one side of the footing until you can measure its length, width and thickness (Photos 1 and 3). The 7-1/4 in. circular saw we used won't cut through slabs more than 2-1/2 in. thick. For cutting through thicker slabs, rent a 14-in. concrete saw fitted with a diamond blade. Hang plastic sheeting to block migrating dust.
Our floor sagging occurred because the post bottoms had rotted and the footings were undersized. Besides pouring new footings and restoring the posts, we added plinth blocks (Photo 12) to elevate the wood above moisture on the basement floor.
Posts and footings support the weight of your house, so you have to size them right. Consult with a structural engineer, local building officials and contractors who specialize in this work. Hire the professionals to review all aspects of your project and provide the calculations and drawings necessary to obtain a building permit (Fig. A). Look for listings in the Yellow Pages under “Engineers, Structural,” and interview those who do residential work and have experience with this type of project. You can expect them to:
Release all rigid connections between gas, heating, plumbing or electrical lines (in conduit) and the house beam. To avoid rupturing these lines while the beam is being raised, support them with a 2x6.
Repair or replace posts or footings by raising the beam with a hydraulic jack and 4x4 post, then shoring it with adjustable steel posts (Photos 4 and 5). Buy a hydraulic jack and rent the shoring posts for the project. If you're removing and resetting more than one basement post, use one hydraulic jack and move from station to station as you install and later remove the shoring posts.
CAUTION: As a house beam is raised, any rigid plumbing, gas, electrical or heating lines in the basement connected to the beam, the floors or walls above could rupture. Before jacking the beam, release the connectors holding these lines and support them with lumber resting on the basement floor (Photo 2). To monitor the total rise during lifting, measure and write the floor-to-beam heights on the beam where hydraulic jacks will be placed. Then remove or cut any nails, screws or straps that join the posts to the beam.
Position two 3-ft. long 2x10 “weight-spreader” boards a minimum of 6 in. from the sides of the hole, then center a hydraulic jack directly under the beam's lifting point using a plumb bob. Make sure the jack is perfectly plumb and sits solidly level on the boards.
Cut a 4x4 or 6x6 jacking post to fit between the jack piston and the beam. Place a 1/4-in. thick steel plate between the piston and the bottom of the post to prevent the piston from becoming embedded in the post. Snug the post up to the beam by closing the jack's release valve and pumping the jack handle up and down to raise the piston.
Center a shoring post on its spreader boards about 12 in. from the 4x4 jacking post. Then lower the post's screw head to its lowest position, raise the telescoping sections as high as they'll go under the beam and insert the metal pins to lock the sections together. Raise the screw head to its centered position under the beam and attach the top plate to the beam with duplex nails.
To avoid cracking the basement floor, lay two “weight-spreader” boards under hydraulic jacks and shoring posts (Photo 3). Place the boards flat and level to ensure a safe lifting base for the jacks and posts. If you have a concrete floor less than 2 in. thick, ask the engineer if you should substitute thicker lumber to prevent floor cracking. Check along the cut edge of the concrete floor for voids that would collapse under lifting pressure. If you find any, ask the engineer for remedies to the problem.
Jacks apply tons of hydraulic force to lift the wood jacking post and beam, so select a virtually knot-free Douglas fir or Southern yellow pine 4x4. Don't use 2x4s or weaker woods like cedar or redwood. Use a 6x6 if you're jacking houses with two or more stories. Once the spreader boards are laid and the hydraulic jack is set in place, cut the 4x4 to length. Measure the height between the support beam and the top of the jack's lowered piston, then deduct 1/2 in. and cut the post to length.
Snug up the 4x4 to the beam and position the shoring post (Photo 5). Now you're ready to lift.
Raise the house beam by pumping the hydraulic jack very slowly using short strokes. Raise it no more than 1/2 to 1 in. total to release the weight from each post. Measure from the floor to the beam to monitor the beam's rise. Have an assistant simultaneously raise the shoring post as tight as possible against the beam to provide backup load support. When the old post loosens, remove and label it for reuse.
I have never lifted a house beam without having the hairs stand up on my neck as I pumped the jack and heard the beam, floors and walls above groaning in response. Before lifting, double-check your equipment, visualize your plan and think safety. Follow these rules:
Excavate the footing hole to the specified size, digging the sides and bottom of the hole square and level. Scrape loose soil from the bottom of the hole until you reach undisturbed soil. If the soil caves in along the edges, install 1/2-in. plywood to hold the dirt back. Remove them while pouring the concrete.
Install rust-free, 1/2-in. dia. reinforcing rod (rebar) to strengthen the concrete footing. Suspend the rebar in the lower third of the footing and tie the rebar together using 16-gauge wire.
Pour concrete into the hole to the level of the existing floor and smooth it with a steel trowel. Suspend a plumb bob from the post's center point under the beam above and place a 1/2-in. x 12-in. anchor bolt in the wet mud.
Level an 8x8 concrete block in freshly laid concrete, centering it around the anchor bolt. Completely fill the core with concrete.
If you have to replace undersized footings, it's time to excavate, place steel reinforcing rods and pour concrete for them. With the post removed, break out an undersized footing with a sledge and shovel (wear goggles and gloves) and pour the new one. Follow Photos 7 – 10 and these tips for building solid footings and plinths:
Measure the height between the house beam and the plinth, subtract 3/16 in. and cut the old basement post to length. Saw a squared bottom from the rotted end, using a circular saw to girdle the post. Finish the cut with a handsaw.
Set the post on its plinth block (use sill sealer as a vapor barrier), matching the post top to its prior contact position with the house beam. Then reset the hydraulic jack, lower the shoring post, and very slowly open the valve of the hydraulic jack to carefully lower the beam onto the post.
Using a hammer drill fitted with a masonry bit, predrill two 3/16-in. holes 1-1/4 in. deep into opposite sides of the plinth block. Install 1-1/4 in. concrete screws through metal anchor straps into the plinth and then nail the straps to the wooden post with galvanized 10d box nails.
Join the support beam and wood post by toenailing them together and attaching two metal T-braces to opposite sides of the beam with galvanized 10d box nails.
Install the post on a new plinth and footing only after the concrete has cured for seven days.
Wood posts are easier to work with, so stick with wood rather than metal. Home centers and lumberyards carry steel “mono-posts” and steel posts with telescoping sections, but you need special skills and techniques to properly support wood beams with steel posts. Consult with engineers and/or local building officials before using them.
Your local building officials may insist you install only new, pressure-treated 6x6 posts set on plinths. We had no such restrictions and reused our old wood posts because they were still tall enough after sawing off the rot (Photo 11). Most old posts don't have a square, level top. Don't change that. For the best contact and load transfer, reinstall the posts exactly in their original position under the house beam, with all those old irregularities fitting like a hand in glove. To prevent any moisture from wicking into the post from the plinth, lay sill sealer or galvanized sheet metal between the two (Photo 12).
In your excitement to reset the posts and finish the job, don't take risks. Use an assistant and lower the jack very slowly, leaving the shoring in place while putting weight onto the posts.
Complete the job by installing the specified metal straps and T-braces (Photos 13 and 14) to connect the plinth block, post and house beam all together—especially in earthquake- prone areas. Once that's done and the support strapping is reinstalled on all gas, electric, plumbing and heating lines, you'll have the satisfaction of completing a job that will last for the life of the house.
Have the necessary tools for this DIY project lined up before you start—you’ll save time and frustration.
Hydraulic jack, Shoring post, Plumb bob, Diamond blade for circular saw
Avoid last-minute shopping trips by having all your materials ready ahead of time. Here's a list.