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.  

[JesseS6]

What design software did you use for the final picture?

[JacobZ]

Hi @JesseS6,

I use SketchUp for all of my designing and 3D rendering.

It's a fantastic tool for designing decks and other projects with great accuracy.

Jacob

[JesseS6]

Hi @JacobZ ,

What does the "continuous span" mean? where does it become relevant?

Thanks,

Jesses6

[EricL]

Hello @JesseS6 

In span tables, "continuous span" refers to a structural element (such as a beam, joist, or rafter) that extends over multiple supports without being interrupted by joints. This means the member is not simply supported at two ends but instead spans over at least three supports, allowing it to distribute loads more efficiently.
 

A continuous span becomes relevant in span tables because it allows for longer allowable spans compared to a "single span" (which is supported at only two points). This is due to the way bending moments and shear forces are distributed—continuous spans experience lower maximum bending stresses at mid-span compared to single spans, as some of the load is transferred to adjacent spans.

In practical terms, continuous spans are common in flooring systems, roofing structures, and other building frameworks where structural elements extend across multiple supports, reducing deflection and improving overall strength. If you're using span tables to determine the correct timber or steel size for a project, checking whether the span is single or continuous is crucial for selecting the right material and dimensions.

If you have any other questions we can help with, please let us know.

Eric

[Ben89]

Hi @JacobZ, great info on the span table above. I have a quick question! I am planning to build a deck using 450mm spacing and joist size of 90x45mm

According to the span table, for a single span, maximum allowable for single span is 1400mm spacing between joist supports and for continuous is 1800mm spacing between joist supports. 

The length of decking I'm building is about 3.5m long, does that mean that i would need to cut two 1.7m joist each with 3 joist supports? or is there a way i will be able to maintain the 3.5m long 90x45mm joist just with additional joist supports? (if i do what would the distances be required between each joist support?)

Cheers,

Bernard

[EricL]

Hello @Ben89 

Welcome to the Bunnings Workshop community. It's sensational to have you join us, and thanks for sharing your question about your decking plans.

Your analysis is correct, in order to keep the full length of 3.5m without cutting it, you just need to add more support. To keep it simple, the maximum gap between supports is 1400mm for 90x45. In essence the distance between supports can be less as long as you don't go over 1400. For example, if I wanted 4 supports for 3500mm I would divide the space by three. This will give you an answer of 1166.66mm rounded up would be 1167mm.

A support would therefore be placed at the very start of the joist counted as one and the next support placed at 1167mm (No.2). The next support at 2334mm (No.3) and the last support at the end of the joist at 3500mm (No.4). This would give you 4 supports and all of them within the limits of the span table. Please note that this is based on a single level decking frame, I suggest having a quick look at this featured project - Low-level composite deck over concrete and soil by @oninpena. 

If you need further assistance, please let us know.

Eric

[Ben89]

Hi @EricL thank you so much for your response! 

I have a follow up question, I saw in another post that it is possible to have an overhang for the joist, with that do you think it is possible for me to have the following support locations for my 3.5m joist.

first support will be at 300mm mark, second support at the 1900mm mark, third support at the 3300mm mark. which means it will have 300mm overhang on each joist. 

Also do you know what will be the allowed overhang distance for a 90x45mm joist?

Cheers,

Bernard

[EricL]

Hello @Ben89 

I believe 150mm is the maximum overhang you can place at the start of the first footing for 90x45. This keeps the overhang strong and prevents springiness to the overhang structure. Keep in mind that you are still bound by the rules of the span table and placing the next support at 1900mm will technically be outside the boundary of the 1400mm limit. This could have the effect of having a springy decking frame, I prefer to over engineer and provide full support for the frame. Therefore, with a 3500 length less the two overhangs of 150mm will leave you with 3200mm. When divided by three you get 1066.66mm rounded up 1067mm. 

A support would therefore be placed at the very start of the joist less the 150mm overhang counted as one and the next support placed at 1067mm (No.2). The next support at 2134mm (No.3) and the last support at the end of the joist at 3200mm (No.4) plus the 150mm overhang. This would give you 4 supports and all of them within the limits of the span table. This would be a rock-solid footing as the supports are closer to one another.  

Let me tag my teammate @MitchellMc for his thoughts in regards to the overhang of your deck. 

Eric

[MitchellMc]

Hi @Ben89,

Floor joists can typically cantilever up to 25% of their allowable single span. So a continuous 90 x 45mm joist has a max span of 1800mm, meaning it can cantilever at 450mm. However, that is the max span, and I would never advise working at it. It's always best to fall well within the max span to ensure a solid deck. I generally recommend that 90 x 45mm be supported every 1000mm, so that would give you a cantilever of 250mm.

If it suits, I would keep cantilevers to a minimum and go with Eric's suggestion of 150mm.

Please let me know if you have any questions.

Mitchell

[Ben89]

Thank you guys so much! been really useful information! 

[johnmic]

Good morning, I am having a bit of trouble understanding the span tables.

I am planning on building a rectangular deck 10 metres long by 6 meters wide.

My initial thought is for the bearers to run the length of the proposed deck (10 metres) and joist the width.

At this stage, I have minimal space under the proposed decking, but I will endeavor to increase that space.

What would be a recommendation for bearers and joist size material, please?

[MitchellMc]

Welcome to the Bunnings Workshop community @johnmic. It's terrific to have you join us, and many thanks for your question about span tables.

When it comes to span tables and selecting the right size timber, it really all starts with how regularly the timber is supported. There isn’t one set size of bearer or joist that must be used for a deck of your dimensions, as the sizing depends entirely on the spacing of your posts or footings. In theory, you could even construct the whole deck frame from 90 x 45mm timber if it’s supported often enough, say every metre or so, because the timber never has to span very far without help. Where the larger sizes of timber come into play is when you want to reduce the number of supports and achieve bigger spans. Once you go beyond about 1000mm between supports, you need to increase the calibre of the timber, stepping up to something like 140 x 45mm, 190 x 45mm, or even LVLs, depending on how far you want the timber to carry the load on its own. Essentially, it’s a trade-off: smaller timber sizes mean more posts and footings, while larger timber sizes let you use fewer supports but at a higher cost and weight. 

The span tables show you which combinations of size and spacing are safe, so if you can share how often you plan to support your bearers, we can suggest timber dimensions that will suit your deck best. For your sized deck, you might run bearers of 140 x 45mm, supported every 1500mm, with 90 x 45mm joists on top of those.

Please let me know if you have any questions.

Mitchell
 

[Minyarra]

from the image above, it looks like the bearer is spanning 2600mm. Is that correct?

[MitchellMc]

Hi @Minyarra,

In the image above, the 140 x 45mm timbers spanning 2600mm are considered joists as they hold the weight of the decking. The two 4200mm 140 x 45mm timbers running perpendicular to the joists are bearers as they bear the weight of the 11 joists.

Please let me know if you have any questions.

Mitchell