Marking out and digging post holes in a straight line is one of the most important steps in building a deck — get it right and everything else follows. With a few basic tools, some string lines, and a methodical approach, most homeowners can handle the layout themselves, though the actual construction may involve restricted building work depending on your deck's height and size.
Check Before You Dig: Consents and Underground Services
Before you pick up a spade, there are two critical checks to make.
Building consent: In New Zealand, decks over 1.5 m above ground level generally require a building consent. Some lower decks may be exempt under Schedule 1 of the Building Act, but exemptions have conditions — always confirm with your local council before starting. The MBIE Building Performance website has clear guidance on what requires consent.
Underground services: Contact Dial Before You Dig (0800 248 344) or visit their website to identify any buried pipes, cables, or drains on your property. Striking a gas line or electrical cable is extremely dangerous and potentially fatal.
Tools You'll Need
- Tape measure (at least 5 m, ideally 10 m)
- Builder's line (string line) and line pins or stakes
- Spray paint or chalk powder
- Spirit level or line level
- Batter boards (timber offcuts and pegs)
- Right-angle square or the 3-4-5 triangle method
- Post hole borer (manual or hire a petrol-powered one)
- Spade and crowbar for clearing
Step 1: Establish Your Baseline
Start from your house wall or an existing fixed structure. This is your datum line — the reference everything else is measured from.
Drive two stakes into the ground at each end of where your deck edge will run, parallel to the house. Stretch a taut string line between them at a consistent height. Use a line level or a long spirit level resting on a straight batten to make sure the string is level — this matters because post depths will be calculated from this reference.
Step 2: Square Up Your Layout Using the 3-4-5 Method
To get perfectly square corners without expensive equipment, use the 3-4-5 triangle method (a practical application of Pythagoras' theorem):
- From your baseline, measure 3 m along the house wall and mark it.
- From the same starting point, measure 4 m out along your proposed deck edge.
- The diagonal between those two points should measure exactly 5 m if your corner is square.
- Adjust your string line until it does, then lock it in place.
Repeat for all corners. For larger decks, scale up — 6-8-10 m or 9-12-15 m — for greater accuracy.
Step 3: Mark Post Hole Positions
With your string lines set up square and level, you can now mark individual post positions. Post spacing depends on your bearer and joist sizing — this is typically specified in NZS 3604:2011, the timber-framed buildings standard that most residential decks are designed to. Common post spacings range from 1.2 m to 1.8 m, but always check your plans or consult a builder.
- Drop a plumb bob or use a spirit level held vertically from the string line to transfer each post position accurately to the ground.
- Mark each position with a dot of spray paint or a small peg.
- Double-check all measurements before digging — it's much easier to move a mark than a hole.
Step 4: Set Up Batter Boards
Batter boards are simple timber frames set back from the corners that let you remove and re-attach your string lines without losing your reference points. Nail a horizontal batten between two stakes set about 600 mm back from each corner. Cut a small notch in the batten where the string line sits. This way you can dig freely and re-establish your lines at any time.
Step 5: Digging the Post Holes
Post hole depth in New Zealand depends on your soil type, frost zone, and the load the post carries. As a general guide:
| Deck height | Minimum hole depth (firm ground) |
|---|---|
| Low (under 600 mm) | 450–600 mm |
| Medium (600 mm–1.5 m) | 600–900 mm |
| Higher or soft ground | Engineer's specification required |
Always check with your council or a structural engineer for your specific site — ground conditions in NZ vary enormously, from Auckland's clay to Canterbury's gravels.
- Use a petrol post hole borer (available from Bunnings or Mitre 10 hire centres) for efficiency — hire one rather than buying if this is a one-off job.
- Keep the borer vertical by checking two sides with a spirit level as you go.
- Clear loose material from the bottom of each hole with a spade or post hole shovel.
- In wet or unstable ground, you may need to pour a concrete footing — again, check your plans.
Step 6: Final Check Before Pouring
Once all holes are dug, re-attach your string lines to the batter boards and verify each hole centre aligns with the string. Check spacing between holes with your tape measure. A few minutes here can save hours of correction later.
When to Call a Professional
If your deck is over 1.5 m above ground, connects to the house structure, or requires a building consent, portions of the work may be restricted building work under the Building Act. This must be carried out or supervised by a Licensed Building Practitioner (LBP). Even for simpler decks, if you're unsure about soil conditions, post sizing, or structural requirements, it's well worth getting a qualified deck builder involved early. You can find experienced local deck builders on tradedirectory.co.nz.
Licensed Building Practitioner (LBP)
