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What are Floor Joists?

How to Construct a Raised Floor with Floor Joists and Joist Headers

By Mark J. Donovan

Floor joists are used on raised floors, or floors that are not built on a slab. Floor joists are structural members that support much of the weight of a home. They are horizontal framing members that span the gap between foundation walls and support beams and which the subfloor sheathing is attached. Floor joists tie into joist headers that run perpendicular to them. In addition, the joist headers act to cap off and band the floor joists together.

If your home includes an unfinished basement you can observe the floor joists from the basement.


Again, they are the long horizontal 2x10s, or 2x12s pieces of lumber that span the basement ceiling and rest on the foundation sill and center support beams.

Floor joists are typically spaced 16 inches on center, however in some cases they may be spaced at 12 or 24 inch centers, depending upon the structural design of the home and building code requirements. Floor joist sizing, in terms of span length and lumber dimensions, is determined by building code and the specific blueprint requirements of the construction project.

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Using undersized floor joists, at a minimum, can lead to excessive deflection in the raised floor, so it is imperative to use the right size lumber for floor joists.

Floor joists never rest directly on the concrete foundation. Instead a layer of pressure treated lumber followed by a non-pressure treated layer of lumber is fastened to the concrete foundation sill. Typically there is an insulation blanket that is sandwiched in between the concrete foundation and the pressure treated lumber. The floor joists are then stood on edge and rested on the wood sill. They are then nailed to the joist headers. In many cases joist hangers are used to more securely fasten the floor joists to the joist headers.


The non-sill end of the floor joists typically rest on a center beam, where they are often spliced with other floor joists coming from the opposite side of the foundation wall. Spliced floor joists should over lap each other even when they rest on a center beam. In some splicing cases, if there is no center beam, the floor joists are butted end-to-end and spliced with plywood gusset plates. Where openings in the raised floor are necessary, e.g. for a stairway opening, floor joists are doubled up at the edges of the opening and perpendicular joist headers are used to cap them.


Once the network of floor joists and joist headers is constructed, subfloor sheathing is then attached to the floor joists to finish the raised floor. The sheets of plywood are placed over the floor joists perpendicular to them. Each course of plywood deck sheathing is staggered from one row to the next to create a strong raised floor.


Fixing an Existing Raised Floor

If an existing raised floor has excessive deflection, the floor can often be stiffened up by sistering additional lumber to the existing floor joists.

Floor joists supported by a center I-beam.

Photos by Mark Donovan

In some cases lumber of the same size as the existing floor joists are used and are simply fastened to the existing floor joists. In other cases, smaller lumber than the existing floor joists are used. Alternatively, additional lumber can be placed on either side of each existing floor joist, effectively sandwiching each one, to make the floor stronger.

For a comprehensive new home construction checklist, see my New Home Construction Bid Sheet. The New Home Construction Bid Sheet provides a request for quote checklist section that you can provide to prospective building contractors. It also includes a comprehensive new home construction cost breakdown table, in Microsoft Excel format, that allows the contractor to include his projected new home construction costs for every phase of the project.


How to Finance your New Home Construction Project -  can help provide funds for your new home improvement project if financing is required.


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