Which building calculations does a New Zealand job need?

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New Zealand building maths is in millimetres, checked against the Building Code's Acceptable Solutions. A main private stair has risers of 190 mm at most and goings of 280 mm or more (D1/AS1), balustrade gaps must stop a 100 mm sphere (F4/AS1), and BRANZ puts deck joists under 19 mm boards at 450 mm centres at most.

Which rules apply to a New Zealand building job?

The New Zealand Building Code sets what the work must achieve, clause by clause: B1 Structure, D1 Access routes, E1 Surface water, E2 External moisture, F4 Safety from falling. Each clause has Acceptable Solutions, such as D1/AS1 for stairs and F4/AS1 for barriers, and following one is a way to comply, not the only way. Your building consent authority decides what it accepts for your consent.

Timber framing and decks follow NZS 3604:2011 Timber-framed buildings. MBIE has sponsored it as a free download since 2019, so read the table for your floor, wall or deck in your own copy. This page quotes no NZS 3604 table; the deck figures below come from BRANZ and are quoted with their source.

How many joists, boards and balusters for a deck in NZ?

Start with the boards, because they set the joist spacing. BRANZ says joists under 19 mm timber deck boards must be at 450 mm centres at most, and under 32 mm boards at 600 mm. A deck with a cantilevered balustrade changes that: BRANZ's worked deck cites NZS 3604 clause 7.4.1.3 for joists at 400 mm centres at most and at least 190 mm deep.

Worked example: a 4.8 × 3.6 m deck with joists across a 4800 mm bearer, 45 mm joists, 90 mm boards.

  1. Joists at 450 mm (19 mm boards): 4800 ÷ 450 = 10.67, up to 11 spaces, + 1 = 12 joists.
  2. Joists at 400 mm (cantilevered balustrade): 4800 ÷ 400 = 12, + 1 = 13 joists.
  3. Marks at 400 mm, hooked off the end of the bearer with the first joist from 0 to 45 mm: each mark is 45 + n × 400, so 445, 845, 1245 and on to 4445 mm. The closing joist sits from 4755 to 4800 mm.
  4. Boards across the 3600 mm width, with a 4 mm gap (an example: use your board maker's figure): 3600 ÷ (90 + 4) = 38.3, so 39 boards of 4.8 m, which is 39 × 4.8 = 187.2 lineal metres, or 205.9 with 10 percent.

The boards come from the app's Decking calculator: Deck Length 4800, Deck Width 3600, Board Width 90 and Board Spacing 4 give Boards Across 39 and 187.2 lineal metres. More on the joist side in joist spacing in New Zealand, and on counting boards in how many decking boards.

The balustrade follows F4/AS1: in housing, no opening anywhere over the full height of the barrier may let a 100 mm sphere through, and on a stair the triangle under the bottom rail must stop a 150 mm sphere. For a detached house, Table 1 gives a barrier height of 1000 mm on decks and balconies and 900 mm on stairs. Between two posts 1600 mm apart with 42 mm balusters: ⌈1600 ÷ 100⌉ − 1 = 15 balusters, a clear gap of (1600 − 15 × 42) ÷ 16 = 60.6 mm, and 102.6 mm on centres. In the app's Baluster Spacing, set Max Spacing to 100: it starts at 125 mm, the Australian figure. The rule is explained in the maximum gap between balusters in NZ.

How much concrete for the deck piles?

π × radius² × depth for each hole, times the number of holes, plus 10 percent. Hole sizes come from your plans or NZS 3604, so take these as example inputs: nine holes 300 mm across and 600 mm deep.

  • Per hole: π × 0.15² × 0.6 = 0.0424 m³.
  • Nine holes: 0.3817 m³, or 0.4199 m³ with 10 percent.
  • Dricon HandiCrete 25 kg yields about 0.010 m³ a bag, so 0.4199 ÷ 0.010 = 42 bags; the 40 kg bag yields about 0.016 m³, so 0.4199 ÷ 0.016 = 26.2, so 27 bags.

The posts take up some of that volume, so you'll have a little over. The same sum for any hole is in concrete for post holes.

What falls does a NZ job need?

The Acceptable Solutions give falls for drains, decks and showers, and none for gutters. The figures:

WhatFallSource
Membrane-covered deckAt least 1.5°E2/AS1, 8.5.1.2
Surface water drain, 100 mm1 in 120 (85 mm: 1 in 90; 150 mm: 1 in 200)E1/AS1, Table 2
Roof guttersNo figure: they "shall fall to an outlet"E1/AS1, 5.3.1
Shower with no upstand, or no wall, screen, door or curtainNo less than 1:50 to the waste, within 1500 mm of itE3/AS1, 3.3.2.3

Worked examples. A membrane deck 3600 mm from the house to its outlet edge at 1.5°: 3600 × tan 1.5° = 3600 × 0.02619 = 94.3 mm of fall. A ratio of 1 in 40 is 1.43°, a little flatter, so set out from the degrees. A 100 mm stormwater drain 15 m long at 1 in 120: 15 000 ÷ 120 = 125 mm. For the gutter, the maker's instructions and your plumber set the figure; the gutter fall guide explains why the Australian 1:500 is not New Zealand's.

Free slope and fall calculator: choose Rise over a length, set Written as to Degrees, and enter 1.5 with a length of 3600 mm for the 94.3 mm deck fall; switch to 1 in X, 120 and 15000 mm for the drain.

Does my stair pass D1/AS1?

Check the riser, the going and the pitch against D1/AS1 Table 6. For a main private stairway: riser 190 mm or less, going 280 mm or more, pitch 37° or less. A secondary private stairway, serving only bedrooms and bathrooms, may have 200 mm risers, 250 mm goings and a 41° pitch.

Worked example: a 2800 mm floor-to-floor height on a main stair.

  1. Fewest risers: 2800 ÷ 190 = 14.7, up to 15 risers of 2800 ÷ 15 = 186.7 mm.
  2. Shortest going: 14 treads × 280 mm = 3920 mm.
  3. Pitch of the nosing line: arctan(186.7 ÷ 280) = 33.7°, under 37°.
  4. For a gentler stair, the comfort rule gives (600 − 280) ÷ 2 = 160 mm as the target riser: 2800 ÷ 160 = 17.5, rounded to 18 risers of 155.6 mm and 17 treads, 4760 mm of going.

Two things follow. A stair set out with Australian numbers, 16 risers of 175 mm on a 240 mm going, fails here on the going alone. And with risers no taller than 190 mm and goings no shorter than 280 mm, the pitch can't pass arctan(190 ÷ 280) = 34.2°, so on a main stair the riser and going limits bind before the 37° does (worked by hand from the two figures).

In the app: Home ▸ Stairs ▸ Rise + Run, Total Rise 2800, Total Run 3920, Riser Limited ticked with Desired Riser Height 190 gives 15 risers of 186.7 mm and 14 treads with a 280.0 mm going. Its amber warnings use fixed ranges (a riser over 190 mm or under 115 mm, a going under 150 mm or over 450 mm), not D1/AS1, so check Table 6 yourself or pick New Zealand in the web calculator below.

Free stair calculator: set Check against to New Zealand (D1/AS1) and it tests the riser, the going and the pitch of the nosing line.

What stud spacing does NZS 3604 give?

It depends on the stud size, the wall height and what the wall carries, so read the table for your wall in NZS 3604 or ask your designer; no single figure fits every wall, and this page doesn't quote one. Dwangs and the lining maker's own maximum centres come on top. Once you have the spacing, the count is the same as for joists: a 4800 mm wall at 600 mm centres takes 4800 ÷ 600 = 8, + 1 = 9 studs. More in stud spacing under NZS 3604.

Which MeisterCalc calculators suit NZ work?

The app was made metric first for Australia, New Zealand and the UK. It works in millimetres, runs on Android 7.0 or later and does every calculation on the phone, so it works where there's no signal. It has no NZ presets: you type the figure from the code or the standard, as in the Max Spacing example above. Stairs, Concrete and Check Square are free. Running Measurements, Stud Counter, Decking, Baluster Spacing, Pitch, Gravel and Dumpy Level are among the other 13, which unlock with a weekly subscription that starts with a 3-day free trial, or a one-time lifetime unlock; Google Play shows the prices in your currency. It doesn't do stair landings, rafter cuts or roof area. Quick sums in a browser are in all the construction calculators; other countries and trades are in calculators by trade and country.

GET IT ONGoogle PlayJOIN WAITLISTiOS App

Codes and sources

Checked on 1 November 2026. The Building Code, your consented plans and your building consent authority decide what a job needs; the Acceptable Solutions are one way to comply.

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