Understanding Maximum Rafter Span Tables for Flat Roofs

When it comes to designing flat roofs, understanding maximum rafter span tables is crucial. These tables provide essential information about the permissible clear spans of joists, taking into account various factors such as dead loads, spacing, and ratio of depth to breadth.

The first thing to note is that these span tables do not apply to timber that is fully exposed to the elements. They are specifically designed for flat roofs and those with a slope of up to 10 degrees. Additionally, if insulation is placed on top of the flat roof joists and the joists are not fastened to the boarding above, the depth-to-breadth ratio of the joists should not exceed a ratio of 4:1.

When calculating permissible clear spans, it's important to consider the self-weight of the joists in addition to the dead loads. The dead loads shown at the top of the span table do not include the self-weight of the joists. However, the allowable clear spans have been calculated in accordance with BS 5268-2:2002 and BS 5268-7.2:1989.

To ensure safe and efficient design, it's essential to follow these guidelines when selecting joists for flat roofs. For example, if boarding is placed on top of flat roof joists with a depth-to-width ratio greater than 4, the joists must be fastened securely to prevent any potential issues.

By considering these factors and consulting the maximum rafter span tables, builders and designers can create flat roofs that are both functional and safe. Whether you're working on a small or large-scale project, understanding these guidelines is crucial for achieving success in your endeavors.

Maximum rafter span table