Deck Joist Spacing NZ

Two grids sit on top of each other

The structural frame creates one grid: piles or posts support bearers, and bearers support joists. The decking creates a second: every board end, butt joint, picture-frame edge and change of direction needs appropriate support. A joist may be structurally adequate across its span yet still be too widely spaced for a thin decking profile.

BRANZ’s public slatted-decking guidance notes that product requirements vary and that composite-board joist centres commonly come from manufacturer instructions. Its residential deck example is not a licence to apply one number to every species, thickness or proprietary system.

Board thickness and profile change the feel underfoot

A 90 × 19 mm board and a 90 × 32 mm board are different products even when the face width is similar. Thicker boards are generally capable of spanning farther between supports, but the permitted centres still depend on the product, grade, moisture condition and fixing pattern. Radiata pine supplied treated wet can shrink as it seasons; hardwood may require predrilling and a specific gap.

• Confirm whether the listed size is finished or nominal.
• Check the face intended to be installed upward; grip and smooth profiles may have different manufacturer directions.
• Ask whether random lengths will create butt joints that need doubled joists or blocking.
• Do not assume a wider board is automatically suitable for wider joist centres.
• If the decking product changes after framing, recheck the joist layout before fixing boards.

Edges, borders and stairs create extra joist lines

A simple rectangle rarely stays simple once a picture-frame border, breaker board, hatch, stair landing or curved edge is added. A border running parallel to the normal joists cannot float beyond them; it needs dedicated support. Board ends should land on enough timber for the required edge distance and fixings, often leading to paired joists or blocking.

Stairs and landings also need their own structural and board-support design. Do not continue the field spacing through a stair opening by eye. Mark every board direction and joint on the framing plan before counting joists.

1. Draw the deck perimeter and all openings.
2. Choose the exact decking family and obtain its installation guidance.
3. Mark board direction, set lengths or expected random-length joints.
4. Add support for borders, breaker boards and every board end.
5. Overlay the structural joist design and resolve any conflict before ordering.

A 3.6 m width at 400 mm planning centres

Suppose a documented board system and the structural design both allow joists at no more than 400 mm centres across a 3.6 m deck width. Divide 3.6 by 0.4: nine spaces require ten joist lines. That count excludes boundary members, doubled supports, picture framing and stair details.

If the width were 3.7 m, nine spaces would be about 411 mm and exceed the assumed maximum. Use ten spaces and eleven lines at 370 mm, or adjust the detailed edge layout. Round spaces up, not centres outward.

Specify the system, then shop the members

The Timber Dealers catalogue has an H3.2 SG8 140 × 45 deck-joist family and separate H3.2 bearer products. These are research/order families, not pre-approved answers for a particular span. Keep grade, treatment, finished section and length attached to every SKU.

Composite and modified boards need their own manual

A proprietary composite, thermally modified or hidden-fix board may limit joist centres differently at diagonal laying, stairs or high-temperature exposure. It may also require doubled supports at butt joints and special edge clips. Download the current manual for the exact profile and keep its revision with the framing plan.

If boards run at 45 degrees, the distance travelled between joists increases even though joist centrelines do not change. Do not reuse a square-layout count. Ask the board supplier for the permitted diagonal layout and recalculate support lines, waste and fixing quantity.