How to Measure and Mark Out Deck Post Positions Accurately

Getting your deck post positions right from the start saves you a world of trouble later. Inaccurate layout leads to beams that don't line up, joists that run out of square, and a finished deck that looks off — or worse, fails a building inspection. With a few basic tools and a methodical approach, you can mark out post positions accurately and confidently.

What You'll Need

Before you start, gather these tools:

• Builder's tape measure (at least 8 m)
• String line and line pins or stakes
• Line level or water level
• Spray paint or marking pegs
• Batter boards (timber offcuts and stakes)
• Calculator (or a phone)
• Square (speed square or builder's square)

Understand Your Deck Design First

Before marking anything on the ground, have your deck plan clearly sorted. You need to know:

• The overall deck dimensions (length and width)
• Post spacing — in NZ, residential timber decks are commonly designed to NZS 3604, which sets out span tables for bearers, joists, and posts. Post spacing is typically 1.8 m to 2.4 m centres, but always follow your specific design.
• Setback requirements — check with your local council. Many decks under a certain height above ground and within certain size limits may be exempt from building consent under Schedule 1 of the Building Act, but you must confirm this with your council before starting. See MBIE Building Performance for guidance on consents and exemptions.
• Ground conditions — sloping or unstable ground affects footing depth and type.

Set Up Batter Boards

Batter boards are the key to accurate layout. They sit outside the deck footprint so they don't get disturbed when you dig post holes.

1. Drive two stakes into the ground about 600 mm beyond each corner of the deck.
2. Nail a horizontal board across each pair of stakes at roughly the height you want your string lines.
3. Run string lines between the batter boards to represent the outer edges of the deck frame.

This setup lets you remove and replace string lines without losing your reference points.

Square Up Using the 3-4-5 Method

This is the most reliable way to ensure your layout is perfectly square without expensive equipment.

• From one corner, measure 3 m along one string line and mark it.
• Measure 4 m along the adjacent string line and mark it.
• The diagonal between those two marks should be exactly 5 m if the corner is square.
• Adjust your string lines until all corners check out.

For larger decks, scale up: 6-8-10 m or 9-12-15 m work just as well. You can also check by measuring both diagonals of the full rectangle — if they're equal, you're square.

Mark Individual Post Positions

Once your string lines are square and level:

1. Measure along the string lines at your planned post centres and mark each position with a plumb bob dropped to the ground, then a peg or spray paint dot.
2. Double-check every measurement from both ends — small errors compound over a long run.
3. For posts that sit inside the perimeter (internal posts supporting mid-span bearers), use additional string lines stretched across the deck area to locate them precisely.

Accounting for Post Offset

If your posts will sit inside the beam rather than directly under it, account for this offset when marking. Mark the centre of the post, not the edge of the beam.

Check for Level

Use a line level or water level on your string lines to confirm they're all at the same reference height. This is especially important on sloping sites, where post heights will vary. Marking the string line height on each batter board lets you re-establish lines easily if they get knocked.

Locating Services Underground

Before you dig a single hole, contact Dial Before You Dig (0800 248 344 or via their website). Underground power, gas, water, and drainage lines can be present even in residential backyards. Striking a buried service is dangerous and potentially very costly.

Building Consent and NZ Building Code

Many standard residential decks qualify for a Schedule 1 building consent exemption, but this depends on height, size, and proximity to boundaries or other structures. Even exempt work must still comply with the NZ Building Code — particularly clause B1 (Structure) and clause B2 (Durability). Always check with your local council before starting. BRANZ also publishes useful guidance on timber deck construction for NZ conditions.

When to Call a Professional

If your deck is elevated (over 1 m above ground), on a steep or unstable slope, attached to the house structure, or requires a building consent, it's well worth engaging a qualified deck builder. Errors in post layout on complex decks can be expensive to fix once concrete footings are poured. A Licensed Building Practitioner (LBP) must carry out or supervise restricted building work — check the LBP register to find a qualified professional. You can also find experienced local deck builders on tradedirectory.co.nz.

Sources & Further Reading

• MBIE Building Performance — Building Consent Exemptions
• BRANZ — Deck Construction Guidance
• Standards NZ — NZS 3604 Timber-framed Buildings
• Licensed Building Practitioners Register
• WorkSafe NZ — Safe Work on Construction Sites