Deck Joist and Bearer Spans: A Plain Guide

Deck boards transfer loads to joists. Joists transfer those loads to bearers, and bearers carry them to posts or another support. Each member’s size and spacing affects the next part of the structure. If one spacing is changed, the frame count and the spans between supports change too.

The words “joist spacing” or “bearer spacing” describe distances between members, often measured centre to centre. They are not spans by themselves. Span is the distance a member is designed to bridge between supports. A spacing suitable for one board or timber grade may be unsuitable for another material or load.

Australian deck span limits should come from a current span table, product specification or design that matches the material, grade, member size, support arrangement and loading. The DIYSums decking calculator deliberately contains no joist, bearer or post span tables. Its verified references cover product-specific composite board spacing and fastening examples, not a structural span approval.

Do not treat a familiar spacing value, an example online or a result from a shopping calculator as evidence that the frame is safe. Ask the timber supplier or a qualified builder or engineer to specify the frame and check the current state or territory requirements for the project.

The documented quantity example is a 4 m long by 3 m wide timber deck, 600 mm high. With 140 mm boards, a 5 mm gap and 3.6 m board lengths, the take-off calculates 21 rows and two boards per row, or 42 boards before waste. These are material counts for those inputs, not an endorsement of the frame spacing.

At an entered 400 mm joist spacing, 1,200 mm bearer spacing and 1,200 mm post spacing, the calculator returns 9 joist lines, 5 bearer lines and 20 posts. Four board rows across the width would not be an alternate result: the row count uses board width plus the gap, so 21 is the output for these example measurements. Confirm the spacing with the board supplier and structural design before construction.

Boards normally cross joists, so changing board direction changes which deck dimension determines the number of rows. The sample uses boards laid across the 3 m width and pieces along the 4 m length. Actual layouts may need staggered joins or additional support beneath board ends, which this estimate does not optimise.

For composite boards, follow the selected manufacturer’s spacing and fixing guide. One Australian composite reference used in the tool states a maximum joist spacing of 400 mm for that product family. This is not universal across composite boards. Timber spacing must likewise follow the board manufacturer and the project design.

The example counts concrete using one 300 mm square by 450 mm deep footing per calculated post and a cited bag yield. It estimates 0.81 m³ of concrete, which is 81 bags at the tool’s example yield of 0.01 m³ per bag. This arithmetic does not account for soil movement, drainage, reinforcement, footing design or local approval requirements.

Have the structure checked by a qualified builder or engineer before building, particularly for elevated decks, unusual loads, attached structures, poor ground or barriers and stairs. Use the current supplier details for boards and fixings. The calculator helps organise a shopping list after the design choices have been made.

When you obtain a frame design, ask for the member sizes, material grade, support points, centres, connections and footing details together. A span table applies only within the conditions and assumptions stated in that document. If the deck is attached to a building or close to an excavation, pool or boundary, include those interfaces in the review rather than checking only the freestanding frame.

Keep the quantity take-off separate from the design record. Once a qualified person confirms the layout, enter those spacings into the calculator to estimate boards and members. If the design changes, update the inputs and check that board joins, stairs, barriers and doubled supports are still represented in the purchase list.

For the sample, the estimated 20 support posts and 81 bags are consequences of the example spacings and footing dimensions only. Changing the bearer layout changes post lines, and changing post spacing changes the number of posts along each bearer. The tool multiplies the resulting post count by one assumed square footing volume. It does not calculate timber grades, member deflection or soil bearing capacity, so a different design must be checked independently.

A useful review package includes a dimensioned plan, deck height, board product, timber grade and proposed member centres. The designer can then check each span and connection against the relevant load case. Keep the final material calculation beside that approved layout, and revisit it if the board direction, support arrangement or deck size changes. A take-off alone cannot show whether an altered frame remains structurally adequate.