Floor Construction
Floor construction
Floor framing questions test span logic, bearing, and the limits on cutting wood. You do not need to memorize every span table if you understand that span depends on member, species, grade, spacing, and load.
Concepts
- Joists must be sized from approved span tables and bear properly at each end; bearing length is limited but required.
- Notches and holes have strict limits: keep notches small and out of the middle third of the span, and keep holes small and centered.
- Bridging or blocking is used where required to prevent rotation and to support concentrated loads.
- Subfloor panels need the correct span rating, thickness, and fastener schedule, with panel edges supported.
- Cantilevers, ledgers, and hangers must be designed and properly fastened; joist hangers need the specified nails, not screws.
- Manufactured floor trusses must not be cut, notched, or field-modified without the truss designer.
- Crawl spaces need access, ventilation, and a ground cover; framing over garages needs separation and fire protection.
Traps
- A large hole in the middle of a joist is not automatically acceptable.
- Hangers installed with the wrong fasteners fail the load path.
- Cutting a truss chord to make room for a drain is always wrong.
- Confusing 'bearing' with 'connection.'
Worked example A plumber wants to bore a 2-inch hole through a 2x10 floor joist. The hole is under one-third of the depth (about 3 inches), but its location and spacing still matter, and no hole may be closer to the edge than allowed. If the hole falls in a cantilever or a concentrated-load zone, it needs engineering-not a rule of thumb.
Rule of thumb Small holes near the center, small notches near the ends-and never modify an engineered truss.