Floor Joist — framing member that supports floors at home

A floor joist is a horizontal framing member that supports floor loads and transfers them to beams, walls, or foundations.

Floor Joist diagram — labeled parts, dimensions, and installation context

What It Is

A floor joist is one of the repeated structural members that span beneath a floor surface. Joists support the subfloor and finished floor and carry people, furniture, and wall loads to the structure below.

Types

Common joist types include dimensional lumber joists, engineered I-joists, and open-web floor trusses. Each type has different span capacity and drilling rules.

Where It Is Used

Floor joists are used above basements, crawlspaces, and between stories of a house. Their spacing and span depend on the framing design and expected loads.

How to Identify One

You can usually see floor joists from a basement or crawlspace. They appear as evenly spaced horizontal members running between beams or bearing walls.

Replacement

Replacement is needed when joists rot, split badly, are insect-damaged, or were cut excessively for plumbing or mechanical work. Sistering may be enough for limited damage.

Floor Joist — FAQ

What does a floor joist do?

It supports floor loads and transfers them to beams, walls, or foundations. In a home, that helps the surrounding system work the way it should.

How can I tell if a floor joist needs attention?

Sagging floors, excessive bounce, cracked finishes, squeaks, and visible rot underneath all deserve attention. Moisture and plumbing leaks are common causes.

Can I replace a floor joist myself?

Minor blocking or simple sistering can be a capable DIY project if the damage is limited and access is good. Structural repairs affecting multiple joists should be planned carefully.

How much does floor joist repair or replacement cost?

Costs range from modest localized reinforcement to significant structural repair if access is tight or finishes must be removed. Moisture damage often makes the project larger than expected.