How to understand deck span tables

[JacobZ]

Span tables are used to help determine the maximum distance between decking structural components.

Once you understand the key components of a deck, the next step is learning how to use span tables to ensure the correct sizing and spacing of your materials without compromising safety.

This simple guide shows you how to read a span table so you can design a safe and durable deck.

What is a span table?

A span table is a reference guide that provides allowable span distances for various types of timber and their sizes. The tables are used to determine the maximum distance between supports for structural members such as bearers and joists.

Span tables take into account factors like:

• Structural rating - timber is graded for strength and stiffness, which affects the allowable span

• Timber size – the dimensions of the timber

• Allowable spans – the maximum distances between supports for each timber size and grade

• Spacing centres - the spacing between the centres of adjacent timbers, which influences the load distribution.

Types of span tables

When building a deck, you typically refer to two types of span tables:

• Bearer span table - shows the maximum span distance for bearers, the horizontal supports that sit on posts or pedestals. It tells you how far a bearer can span between supports (posts or pedestals) while supporting the joists above.

Bearer span table

• Joist span table - shows the maximum span distance for joists, the horizontal components that support the decking boards. It helps you determine how far apart your bearers can be spaced, ensuring that your joists are properly supported.

Joist span table

How to read a span table

There are a few steps to follow when reading a span table. The first is to ensure you have the correct table for either bearers or joists.

Then check the timber material. Look for the timber grade, such as MGP10, F7 hardwood, or LVL (Laminated Veneer Lumber).

Next find the timber size. Look for the size of timber you’re using, such as 140x45mm.

Now check the span distance that timber is capable of. For example, the distance between two posts for a bearer or between two bearers for a joist.

Finally, ensure your selected timber will be supported not further apart than the allowable span in the table.

Deck build example 

Suppose you’re planning a 4.2m x 2.6m deck using 140x45mm H3 MGP10 timber with joists spaced at 450mm.

For the bearer, you would check the bearer span table for the maximum span for 140x45mm timber. The maximum span is 1.4m between supports. This means you would need 4 posts along the length of your 4.2m bearers.

For the joists, refer to the joist span table. The maximum span for 140x45mm timber is 2.6m between supports from the bearers. Your bearers should be no more than 2.6m apart to safely support the joists.

Example deck

 

Expert advice for building a deck

Consulting span tables is important for designing a safe and structurally sound deck. You should also always check with your local council before starting work and consider seeking professional advice if you’re unsure about your design.

The comprehensive step-by-step How to build a deck series by the Bunnings team is an invaluable resource when starting a new deck build. If you have questions about your project along the way, Ask a question and our community members will be happy to help.