Wood Rafter Span Calculator: Understanding the Basics

When it comes to designing and building roofs, calculating the span of wood rafters is a crucial step. But what exactly does this involve? In this article, we'll delve into the world of wood rafter span calculation, exploring the key factors that need to be considered.

Notches and Locations

One of the primary considerations when working with joists is the placement and size of notches. According to industry standards, notches should never exceed 0.125 of the depth of the joist, and should always be located between 0.07 and 0.25 of the span from the support. This ensures that the structure remains stable and secure.

Timber Exposition

It's also important to note that these span tables do not apply to timber which is fully exposed to the elements. In other words, if the joists are not covered or protected in any way, the calculations used here may not be accurate.

Roof Slope and Insulation

The span tables mentioned above only apply to 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 they are not fastened to the boarding above, the depth-to-breadth ratio of the joists should not exceed a ratio of 4:1.

Dead Loads and Self Weight

The dead loads shown at the top of the span table do not include the self-weight of the joists. However, the calculations used to determine the permissible clear spans take into account both the dead loads and the self-weights of the joists.

Standards and Regulations

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 5268-7.2:1989 Structural use of timber - Section 7.2 Joists for flat roofs.

In summary, when using a wood rafter span calculator, it's essential to consider the placement and size of notches, the exposition of the timber, roof slope and insulation, dead loads and self-weight, and adherence to industry standards and regulations. By keeping these factors in mind, you can ensure that your roofing design is safe, secure, and compliant with building codes.

By using a wood rafter span calculator that takes into account these key considerations, you'll be well on your way to designing a roof that meets all the necessary requirements for strength, stability, and safety.

Wood rafter span calculator