How Far Apart Should Roof Joists Be

Most are spaced between 18 26 inches apart.

How far apart should roof joists be.

Once you ve calculated the number of trusses you will need you should purchase the lumber. Builders can adjust their choice of lumber size and spacing depending on circumstances. If the roof is not completely square you can decrease the space between joists but never increase the space. Measure from the ends.

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. A roof joist is always spaced no more than 16 inches apart from another roof joist. The same concept holds true for roof trusses.

In other words you would space a roof joist 15 inches apart from another roof joist but never 17 inches apart. In these cases 2 inch by 10 inch lumber is often used for ceiling joists although trusses may be more practical. Given that 2x8s are actually 1 inches wide it works out to be 14 inches between each joist. 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.

Putting the trusses into place. So the space between rafters can be greater without reducing the maximum span. Normally floor joists are spaced 16 inches apart on center.