Understanding the Rafter and Ceiling Joist Span Table

When it comes to designing roof systems, understanding the span tables is crucial in determining the allowable clear spans for a rafter or ceiling joist. However, not all span tables are created equal, and it's essential to understand the limitations and calculations used to determine these spans.

In this article, we'll delve into the specifics of the rafter and ceiling joist span table, including its application, exclusions, and calculations.

Application

The span table in question only applies to roof systems with four or more rafters. This means that trussed rafter roofs are excluded from this calculation. It's essential to note that the dead loads shown at the top of the span table do not include the self-weight of the rafters, although the rafter self-weights are included in addition to the dead loads when determining the allowable clear spans.

Calculations

The allowable clear spans have been calculated in accordance with BS 5268-2:2002 Structural use of timber - Part 2: Code of practice for permissible stress design, materials and workmanship and BS 5268-7.5:1990 Structural use of timber - Section 7.5 Domestic rafters.

The span table provides a guide for architects, builders, and engineers to determine the suitable clear spans for a rafter or ceiling joist based on the dead load per square meter (in kN/m²) supported by the rafters, as well as the centre-to-centre spacing of the rafters (in mm).

In conclusion, understanding the rafter and ceiling joist span table is essential in designing roof systems. By knowing the application, exclusions, and calculations used to determine these spans, architects, builders, and engineers can ensure that their designs meet the necessary standards and regulations.

Whether you're a seasoned professional or just starting out in the field of construction, it's crucial to have a solid grasp of span tables and how they apply to different roof systems. By doing so, you'll be able to create safe and functional structures that meet the needs of your clients and users.

Rafter and ceiling joist span table