Rafter Spans Table: A Crucial Aspect of Trussed Rafter Roofs

When it comes to trussed rafter roofs, ceiling joists play a vital role in providing structural integrity. According to the International Residential Code (IRC), ceiling joists must be continuous or securely joined where they meet over interior partitions and are nailed to adjacent rafters to create a continuous tie across the building. This is particularly important when the joists are parallel to the rafters.

In traditional purlin and rafter roof construction, this requirement may not apply, but it's essential for trussed rafter roofs. Ceiling joists and rafters must be nailed together in accordance with Table R802.5.1(9), while the rafter should be nailed to the top wall plate in compliance with Table R602.3(1).

To ensure a secure connection, truss clips connecting the rafter and wall plate should be fixed according to the recommended method. This includes vertical strapping at eaves pitched roofs, where straps 1m long are fixed as shown in the diagram opposite, spaced no more than 2m apart.

Gable walls also require strapping to the roof at a maximum of 2m centres using 30 x 5m galvanised mild steel straps. Effective strapping involves fixing each strap with noggins between the rafters to support the metal strap along its entire length.

The diagram illustrating the typical arrangement of a rafter and purlin roof shows the dimensions referred to in the tables above, highlighting the importance of roof pitch, strength class C24, and purlin size. With a roof pitch ranging from 30 to 45 degrees, it's essential to ensure that all components are properly secured to prevent structural failures.

In summary, the rafter spans table is a critical aspect of trussed rafter roofs, requiring careful attention to detail in the connection between ceiling joists and rafters. By following the guidelines outlined in this article, homeowners and builders can ensure the safety and integrity of their roof structures.

Rafter spans table