Deck
calculator

How to use the
deck calculator

Enter the deck's length along the house, how far it comes out, and its height above the ground. Choose your decking, the way the boards run and any balustrade. The calculator picks joist and bearer sizes and spacings from the NZS 3604 span tables, lays out the piles, sizes the footings, draws the framing plan and lists the materials.

Joists, bearers and piles

• Joists sit under the boards and run across them. Thin decking needs closer joists: 19 mm boards go on joists at 450 mm centres at most, 32 mm boards at 600 mm.
• Bearers carry the joists. The distance between bearer lines is the joist span.
• Piles hold up the bearers, on concrete footings sized for the spans they carry.

On an attached deck with boards running parallel to the house, the joists usually hang off a stringer bolted to the house framing, so there's one less bearer line.

Bracing

A deck bolted to the house that comes out no more than 2 m doesn't need subfloor bracing. Wider decks, and freestanding ones, need bracing sized for your wind and earthquake zone. The calculator shows indicative braced or anchor piles, and we confirm the rest for your site.

When you need a barrier

Under clause F4 of the Building Code, you need a barrier wherever someone could fall 1 metre or more. Measure from the deck surface to the ground at the lowest point below the edge.

When you need building consent

Decks under 1.5 m high are usually exempt from building consent, but they still have to meet the Building Code. Higher decks need consent. Check with your council, or ask us, before you start.

This calculator gives an indicative framing layout and materials estimate, based on NZS 3604 for a deck on good ground within the standard's wind and earthquake limits. It isn't a construction drawing or design certificate. Send us your deck and we'll confirm it for your site.