How Far Apart Should Roof Joists Be

Putting the trusses into place.

How far apart should roof joists be.

In these cases 2 inch by 10 inch lumber is often used for ceiling joists although trusses may be more practical. A roof joist is always spaced no more than 16 inches apart from another roof joist. Builders can adjust their choice of lumber size and spacing depending on circumstances. The same concept holds true for roof trusses.

Normally floor joists are spaced 16 inches apart on center. With some structures floor joists are required to be 12 or 24 inches apart on center. Once you ve calculated the number of trusses you will need you should purchase the lumber. Most are spaced between 18 26 inches apart.

This sample table gives minimum floor joist sizes for joists spaced at 16 inches and 24 inches on center o c for 2 grade lumber with 10 pounds per square foot of dead load and 40 pounds of live load which is typical of normal residential construction. Spans for joists greater than 20 feet that are parallel to the rafters require a 12 inch joist spacing when using 2 inch by 10 inch 2 douglas and hemlock fir or southern pine or 16 inch spacing when using 2 inch by 10 inch 2 douglas fir or southern pine. This means from the center of one upright joist to the center of the next. Common spacing for trusses on this type of building is often between 2 and 4 feet but trusses built with advanced designs may allow spacing as great as 8 or 12 feet.

Measure from the ends. Given that 2x8s are actually 1 inches wide it works out to be 14 inches between each joist. When you are ready to get to work the first thing you should do is set the trusses approximately where they should go.