How are floor joists supported?
Herein, how are floor joists attached?
Joists run from one beam to another and are usually spaced 16 inches apart, center-to-center. Plywood sheathing attaches to the joists, then hardwood flooring or tile. Since joists carry the load of the floor, they must be securely attached at either end to a ledger board or a beam, and must be level in all planes.
Herein, can I drill through floor joists?
The hole cannot be larger than one-third the depth of the joist, so the maximum hole size for a 2×12 joist (actual size 1-1/2 x 11-1/4 in.) is 3-3/4 in. You can drill the holes anywhere along the length of the joist (first photo). If you have manufactured I-joists, you can drill holes up to 1-1/2 in.
In general terms, joists spaced 16 inches on center can span 1.5 times in feet their depth in inches. A 2x8 up to 12 feet; 2x10 to 15 feet and 2x12 to 18 feet. The larger the deck, the larger the joists. 2x6 joists should only be used on ground-level decks that do not require, and will not provide for, any guards.