Repairing and Replacing

The Step-by-Step Guide to Installing Floor Joists Like a Pro

Joists form the framework of a floor and are typically installed by a framing carpenter during construction. Correct installation of these supports is critical in providing stability to the building structure and its contents.
Installation begins by selecting the correct lumber to use. Not only must it meet code requirements, but it should be strong enough to support the weight of a floor system. Selecting wood with high or medium bending strength is ideal; stronger wood allows it to span farther without needing additional support.

The bending strength of wood will determine its capacity for bearing weight and how much it deflects. Some species, such as hemlock, are more flexible than others.


Before installing your joists, you must create a plan. This step must be done correctly for optimal stability. Otherwise, your floor could become unbalanced and more vulnerable to sagging and other damage.

Professional or amateur, the most efficient way to lay out your floor joists is with a framing layout. You can purchase one from a home improvement store or draw one yourself. Once you have your plan, use a tape measure and pencil to mark each joist on the plan.

Once you’ve selected your layout, cut lengths of 2 by four pressure-treated lumber to each chalked line in Step 4. Space each length approximately 32 inches apart and 8 to 12 inches from either end and secure them to the foundation with concrete nails.
Framers often utilize joist hangers, which are U-shaped hardware pieces nailed in to accept and secure flooring joists. These are most often employed on floors with open spans or where the joists will extend into walls for more than eight feet.
If you’re unsure how to lay out your joists, calling in an architect or engineer for assistance can be beneficial. They will know precisely how many joists need support and where they should go.

Your blueprint may indicate where blocking is necessary, such as when floor joists overlap a centre beam or every eight feet on 2×10 or taller joists. If your floor is below concrete basement or crawl space walls, you must add this blocking.
Once the blocking is in place, attach the joists to the rim joists with one 3/4-inch screw or 8d ring-shank nails at each end. Finally, fasten them onto the spaced joist support plates you installed in Step 5.

Once all your joists are attached to the rim joists, you can finish off your floor by adding sub-floor sheeting or Oriented Strand Board (OSB) that acts as a base layer. This gives your home an elegant appearance and reduces the likelihood that it will sag or warp over time.