Deck joist span tables

Where do deck joist span tables come from?

For timber, from AS 1684 (Residential Timber-framed Construction), which WoodSolutions’ Technical Design Guide 21 uses. The guide sends you to Tables 49 and 50 for decks more than 1,000 mm above ground and Tables 5 and 6 for decks at or below 1,000 mm. We don’t reprint them, because the correct row depends on species, stress grade, spacing and load, and a mistake is structural.

For aluminium, each maker publishes its own tables, and they are valid only for that maker’s system.

What do the aluminium tables say?

We show a few published values so you can see how different the systems are, with their conditions. They aren’t a design and don’t carry over to other products.

Futurewood’s June 2024 tables, prepared by Coulthard Shim to AS 1170.0, AS 1170.1 and AS 1664.1 for 6063-T6 aluminium, for use only with Futurewood’s own boards and accessories:

The residential load is 2.0 kPa or 1.8 kN, with a deflection limit of span over 300, and the table assumes decking no heavier than 27 kg per square metre. Futurewood’s ultra-low grid uses joists and pedestals at 450 mm and is designed for 2 kN per square metre or a 1.4 kN applied load, although other Futurewood copy says 1.8 kN, so use the June 2024 tables.

NewTechWood doesn’t publish a joist table for StructurAL. It limits the spacing between pedestals under the joists to 75 cm with a 20 cm overhang, and its NIVO pedestal chart lists the maximum space between pedestals against joist spacing. For a 60 by 40 by 2 mm aluminium joist the chart reads 750 mm at 350 mm joist centres and 650 mm at 550 mm.

ClickDeck publishes residential and commercial tables for its 28, 55, 110, 150 and 200 mm profiles, at a 2.5 kPa residential load with a 1.8 kN point load. We link to the PDF rather than reprint the numbers, because each cell prints two values with an asterisk, and you should read the legend yourself.

How do spans change with load and spacing?

Heavier loads and wider spacing shorten the span. Futurewood’s commercial table cuts the 50 by 45 mm joist from 1,100 to 750 mm. Futurewood also notes that 400 mm joist spacing is stronger than 450 mm, and 300 mm stronger again.

What are the limits of a table?

Every table has conditions: the load class, the spacing, how many supports, whether it is a single or continuous span, and which boards and fixings go on top. The tables we’ve read say they are for general information. WoodSolutions says anything beyond light domestic decks needs a structural engineer, and so do tiled decks and spas.

For counting joists and supports in your own project, use the frame estimator. It takes spacings you copy from these documents and never works out a span itself.