How to Repair a Sagging Floor

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A guide on how to repair a sagging or sloping floor with screw jacks, jack posts, and temporary beams. Sagging can be due to rotted wood, poor design, settlement, or shrinkage.

As a home ages, wood shrinks, structural members shift, and foundations settle. Due to the nature of construction, each home is unique in how it settles and depends on how it was built, the time or year it was constructed, soil features, weather conditions, and how it has been used and maintained.

Along with normal settlement, damaged structural components can also lead to slopping and sagging floors. Insect damage and rot can cause sill plates and floor joists to lose their strength and compress under the load of the home and its contents. Split or cracked floor joists can cause localized sagging in floors.

It is important to correctly diagnose the cause of any floor sagging before you or a contractor attempt to perform any repairs. You should have a licensed inspector or structural engineer inspect the foundation and structural supports or your home before you do anything else.

This article will discuss the most simple repair of a sloping or sagging floor by jacking up the floor section and adding additional support.

It is important to understand that most repairs to sagging floors may take several days or weeks to repair. A floor that is out of level more than an inch will need to be lifted back into position slowly to avoid damage to walls, windows, doors, and the floor system itself.

Why is the Floor Sagging?

As stated in the introduction, floors can sag for several reasons; wood rot, insect damage, damaged floor joists, columns, or beams, excessive wood shrinkage, or foundation settlement. As long as the floor have not shifted any more then you should be able to raise the floor to a nearly level position. Continual sagging and settlement is a serious problem that should be inspected by qualified professionals.

You may also realize that original structural components have been modified or moved during a past renovation or addition that have caused additional settlement. Again this is a situation where a qualified inspector or engineer should investigate the structure and determine the best method for repair.

Damage from Rot

A Standard Configuration of A Basement (Support Column with beam on top in center)

Locating the Problem

For now let’s assume you do not have a crumbling foundation, or excessive wood rot or insect damage, but the floor joist have compressed the sill plate or the beam has shrunk to the point where the floor is not out of level.

To locate the cause, place a 4-foot level across the floor joist from below to see if an individual floor joists are bowed more than the others. Also place the level on the lower edge of each joist to determine if their slope is uniform. If the foundation is solid and the beam has shrunk or settled, the joists will slope towards the center of the home. If the floor is higher in the center by the beam and slopes to the foundation wall, the sill plate is most likely the cause.

Checking for a Bowed Joist

It the sagging is not too severe, less than one inch, you should be able to jack up the floor with some help and a lot of patience.

Tools and Materials

12 or 20-ton Screw Jack – Can be rented at most home improvement or rental centers

Temporary beam – either steel I-beam or solid wood beam such as 4 x 6 or 6 x 10.

4 x 4 or 6 x 6 post – preferably pressure treated


4-foot level

Reciprocating saw, if you need to cut away any wood

Permanent beam if required. Needs to be sized by an engineer

Lally Column or Jack Post is additional support is required

2 lb. Sledge if you need to insert new sill plates or joists

Steel plates and shims

Diagram for Jacking a Floor


Jacking up a floor is usually a 2 or 3 man job. Not only will it make the work easier, it is also a safety consideration in case there are any accidents. You may also want to contact your local building department to determine if there are any permits required before attempting the repairs. They may also be able to provide some guidance to help your repair go smoothly.

If you have determined that the floor system needs a new beam or lally columns installed, don’t set up the jack at this point. Allow yourself at least 3 feet on either side to work, if possible, to set a new beam or install a new concrete pier and column. Screw floor jacks can lift an entire house so it is not necessary to place them exactly where the problem is.

The most important thing to remember before using a screw jack is that you need a stable base. Use a large steel plate, at least ½ inch thick as a base plate for your jack. The concrete slab does not have the required strength to support the load from the floor above once the jack starts to lift the floor into a level position. You can also use thick wood beams to distribute the weight of the floor. It is best to use two alternating layers of beams for the base and then use a steel plate between the jack and the beams.

The post used to jack up the floor can be a wood post that is either 4x4 inches square or 6x6 inches square. You can also spike together three 2x6 studs. Typically you should not use an old lally column unless it has a steel plate welded to the top and bottom of the steel casing to protect the concrete inside the column.

Hydraulic jack placed on wood beams and a steel plate.

Many jacks have very small top plates which are inadequate for use with a wooden post. You need to place a thick steel plate on top of the jack to evenly distribute the force from the jack head.

The next issue is to distribute the load from the jack to multiple floor joists. This is typically down be using a temporary beam which can be a single solid or engineered beam at least 4 inches thick. The size depends on the amount of flooring you are attempting to jack at one time. A steel I-beam can also be used but they can be difficult to work with. Wood beams can be temporarily attached to the floor joists with steel joist hangers or rafter ties. After the floor has been lifted back to its normal position, your can remove the hangers and reuse the beam on another section.

Various Screw Floor Jacks

Screw jacks use a solid steel bar inserted into a hole just beneath the top plate. Turning it counterclockwise raises the jack and lifts anywhere from 5 to 40 tons. Usually 12 or 20 ton jacks are sufficient for this type of job. Depending on how long of a section you are working on you may want to use two identical jacks to raise a long continuous beam.

Slowly raise the jack until the post is snug against the temporary beam.

With your 4-foot level check the initial slope of the floor. You may want to run a string across the bottom edge of the joists at the lowest point to track how high you are raising the floor.

As you continue raising the floor you will hear creaking or popping. This is normal as you are transferring the load from its current location to the temporary beam. Have someone go to the floor and see if there are any problems such as doors sticking, windows cracking, or damage to the floor. If any damage of this sort is present you should stop immediately.

Keep in mind that as the floor has sagged slowly over time, doors may have been trimmed to accommodate the floor level. This will have to be repaired once you have completed raising the floor.

If the floor sagging is due to a rotted sill plate, you should be able to remove the sill plate and insert a new one. Use pressure treated lumber either 2x4 or 2x6 in size. You may have to lift the floor slightly beyond the level point to insert the new sill plate.

Before you begin any repair work, you should install a few temporary posts to support the temporary beam instead of the jacks. The jack can remain in place but you should not rely on it to carry the load while you are working. You may inadvertently hit the post or jack while working and cause the floor to suddenly drop. It will also allow you distribute the weight of the floor to more than one point. If you are sistering new joists onto the old ones this is a very important step. You can also use adjustable jack posts for temporary supports. They are basically lally columns with an integral screw jack inside.

Two lally columns supporting a temporary beam

Securing an Adjustible Jack post

If you need to install a new lally column you will need to locate the poistion on the floor with a plumb bob, cut out the slab and dig dow to the required depth for the support pier. An inspector or engineer will be able to determine the size of the pier and reinforcing bars for the pier.

Diagram for a finished lally column, pier, and beam


You need to continually check the floor level after you raise the jack. It can take several rotations for the jack to raise even 1 inch. Once the floor is leveled to your satisfaction, or as well as it can be, complete your permanent repairs to the sill plate, floor joist, beam, or columns. Special attachment plates can be used to secure the permanent columns to the beam. If you are not using an adjustable column, you will need to insert steel shim plates between the end of the column and the beam. Consult with your local building inspector on how to proceed.

Although repairing a sagging floor can be a time consuming and possibly expensive process, you will have the satisfaction of knowing that your home is stable and safe and you will improve the function and value of your home.

1 comment

Jerry Walch
Posted on Feb 12, 2011