How to Repair a Bowed Floor Joist

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Steps for repairing a sagging or crowning floor joist in your home.

Older homes are apt to have more “character” than more modern homes, and by character I mean slight defects that homeowners have become used to. Doors that stick, windows that have to be propped open, and floors where you can roll a ball from one end of the room to the other just by placing it on the floor. While some of these defects are serious and potentially unsafe and require extensive repairs to remedy, a simple bowed of sagging floor joist is a repair that do-it-yourselfers can tackle with the right tools and a little help.

This article will discuss the steps involved in splicing or “sistering” a floor joist to repair a floor joist that has a problem with the crown being too high or too low.

Floor Framing

Not all floor joist problems are due to age. Wood is a natural building material and no two pieces are exactly alike. Modern wood framing is more uniform, but high demand, new-growth trees, and kiln-drying have led to some sub-standard material making its way into a home. Carpenters select the best joist material for the floor, but when the material arrives at the job site it can sit for several days, become wet or muddy, and can be dropped from the end of a truck with a ton or more of other framing sitting on top of it.

When a carpenter installs floor joist, or any horizontal framing member, they look down the length of the board to see which way it is crowned. Crowning is the normal bend in the wood caused by the grain drying and the milling process. Since floors need to support the weight of walls, furniture, fixtures, and people, the floor joists are always installed with the crowns up. As the live and dead loads are placed on the floors, the crowns are forced down to a relatively level position.

The foundation walls, beams, columns, sub-flooring material, and bracing all play an important role in the function of the floor.

Solid Bracing

Cross Bracing


The first order of business is to determine what the source of the problem is. The joist can be cracked, twisted, installed with the crown down, of have some problem at the rim joist connection.

If you're concerned about the structural integrity of sagging, cracked or twisted joists in your house, first call in a structural engineer to inspect the floor and recommend strengthening measures that'll take out the sags and bring the framing up to code. The inspection will inform you if there are any structural problems, such as undersized joists, that will require professional repairs. A typical inspection can run anywhere between $250 and $500.



Tape measure

Circular saw

Caulk Gun

Safety glasses

Drill and bits for installing bolts


16d common nails

Joists (Typically 2 x 10 or 2 x 12)

Construction adhesive

Hydraulic jack

2 x 4 for making a post

Sistering a Joist Crack

A severely damaged, cracking, or under-sized joist should be removed and replaced.

Use caution when working with power tools and around electrical wires and plumbing lines.

Many times joists crack where plumbers or electricians have notched the joist to allow for piping or cables. These will have to be removed temporarily to perform the repair and you may need the services of a qualified contractor to relocate them if necessary.

While it is best to install a new floor joist the same length of the existing one, most times a repair can be performed by attaching a new piece of joist on either side of the damage that extends at least 3 feet past; this would require two 6 foot long pieces of wood to repair a vertical crack.

For a joist with an upside-down crown, you would need to install a full-length joist.

Cut the pieces to length and attach on either side of the crack by applying construction adhesive on the old joist, nailing the new joists on either side with 16d nails. I also like to drill through all three pieces of joists and install 3/8 inch carriage bolts with washers and nuts. Install the bolts in an “X” pattern starting about 4 inches from the end of the short piece. (See illustration below)

An alternative to using standard floor joist material, you may want to use LVL, Laminated Veneer Lumber, which is basically a thick slab of plywood. You can usually purchase this material by the foot at lumber yards and various sizes are available. In this application, a 1 3⁄4 inch thickness with a depth of 9 1⁄2 or 11 7⁄8 would be used.


In cases when the floor is sagging, you will need to level the floor first before making the repairs. This typically involves bracing or jacking up the joist.

Set a hydraulic jack and post under the lowest point of the joist, and jack up the joists slowly. It is a good idea to toe-nail the post into the bottom of the joist to prevent accidental slipping. Hydraulic jacks can be rented at tool centers or home improvement centers. Screw jacks are used for jacking up large areas and should not be necessary for repairing a single floor joist.

An alternative method to using a jack is to make a post out of two 2x4s nailed together to form a long “T.” The T-post should be cut so that it fits snugly under a joist that is level. Wedge the post under the sagging area and gently hit the bottom of the post with a large hammer or 2-pound sledge. This will gradually bring the joist up even to the other joists.

In either case, leave the jack or post in place as you perform your repairs.


Another common problem is a floor joist that has crowned significantly to the point where it is well above the adjacent joists. In this case you will need to cut the joist at the highest point in the crown and then push the joist back into a level position. Usually a few good stomps will force the joist back into place, but in some cases you will need to use the T-post method to force the joist down from above. Use a scrap piece of 2x4 or 2x6 to span the ceiling joist in the room above so you don’t make a hole in your ceiling. Place another scrap under the post to protect the flooring.

With a circular saw, make a vertical cut at the point of the crown. Finish the cut with a small hand saw or reciprocating saw. Use caution when using power tools above you head and always wear safety goggles.

Follow the same steps as repairing a cracked floor joist nailing, gluing, and bolting a new sister-joist on both sides of the saw cut.


Patrick Regoniel
Posted on Jul 27, 2010
lisa leverton
Posted on Jul 20, 2010
Lorena Williams
Posted on Jul 19, 2010
Lorena Williams
Posted on Jul 19, 2010