How Far Apart Should Roof Joists Be

With some structures floor joists are required to be 12 or 24 inches apart on center.

How far apart should roof joists be.

Measure from the ends. A roof joist is always spaced no more than 16 inches apart from another roof joist. Putting the trusses into place. So the space between rafters can be greater without reducing the maximum span.

Given that 2x8s are actually 1 inches wide it works out to be 14 inches between each joist. The same concept holds true for roof trusses. Most are spaced between 18 26 inches apart. 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.

In other words you would space a roof joist 15 inches apart from another roof joist but never 17 inches apart. 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. If the roof is not completely square you can decrease the space between joists but never increase the space. 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.

This means from the center of one upright joist to the center of the next. When you are ready to get to work the first thing you should do is set the trusses approximately where they should go. Normally floor joists are spaced 16 inches apart on center.