Working out post heights for a deck on a sloped section is one of those jobs that looks tricky but becomes straightforward once you understand the method. The key is establishing a consistent datum level — a reference height from which all your posts are measured — and then using a string line or laser level to find how far each post position drops below that datum.
Understand the Basics First
Before you start measuring, it helps to understand what you're solving for. On a flat section, every post is the same height. On a slope, each post must be a different length so that the deck frame (bearers and joists) sits perfectly level regardless of the ground beneath it. The steeper the slope, the greater the height difference between your highest and lowest posts.
Also keep in mind that deck height affects consent requirements. Under the NZ Building Code and Schedule 1 exemptions, decks that are more than 1.5 metres above the ground at any point generally require a building consent (though the exact threshold and conditions should be confirmed with your local council and MBIE guidance, as exemption criteria can be specific). Always confirm this with your local council before you start — rules can vary and your council is the final authority.
What You'll Need
- Tape measure (5 m minimum)
- String line and line level (or a laser level)
- Timber stakes or temporary pegs
- Calculator
- Pencil and notepad
- Safety glasses and gloves
Step 1 — Establish Your Datum Point
Your datum point is the finished deck height at the house end (or highest point). This is usually determined by the door threshold or the existing floor level. Measure down from the threshold to the ground at the house wall — this gives you the height of your first (shortest) post, minus the thickness of your decking boards and framing.
As a guide, allow for:
- Decking board thickness (typically 45 mm for H3.2 treated timber)
- Joist depth (commonly 190 mm or 240 mm depending on span)
- Bearer depth (if used)
- Post cap or connection hardware
Step 2 — Run a String Line
Drive a temporary stake at each post position. Run a string line from your datum point out to each stake, using a line level or water level to keep the string perfectly horizontal. The distance from the string down to the ground at each stake position tells you how much the ground has dropped at that point.
If you have access to a laser level, this makes the job much faster and more accurate — you can shoot a level beam across the whole deck area and measure down at each post location.
Step 3 — Calculate Each Post Height
For each post position, the formula is simple:
Post height = Ground drop at that position + (Datum height above ground at house)
Then subtract the combined depth of your bearer, joist, and decking to get the actual post length you need to cut.
A simple table helps keep track:
| Post | Ground drop (mm) | Datum height (mm) | Total to top of post (mm) | Less framing depth (mm) | Post cut length (mm) |
|---|---|---|---|---|---|
| P1 | 0 | 400 | 400 | 280 | 120 |
| P2 | 150 | 400 | 550 | 280 | 270 |
| P3 | 310 | 400 | 710 | 280 | 430 |
(Numbers above are examples only — your measurements will differ.)
Step 4 — Account for Post Footing Depth
Don't forget that your posts sit on top of concrete footings, not directly on the ground. The footing top should be at or slightly above finished ground level to prevent moisture wicking. If your footing sits 50 mm proud of the ground, subtract that from your post cut length. BRANZ has good guidance on footing sizing and embedment for residential decks.
For decks built to NZS 3604, footing sizes are determined by the load area and ground conditions — check the standard or ask your council building officer if you're unsure.
Step 5 — Check for Bracing Requirements
Tall posts (generally over 1.8 m) may need lateral bracing or knee bracing to prevent racking. The taller your posts get due to the slope, the more important this becomes. NZS 3604 sets out bracing requirements for timber-framed structures — your local Licensed Building Practitioner (LBP) can advise if your design needs engineering input.
Timber Treatment and Post Selection
All posts in ground contact or within 150 mm of the ground must be H5 treated timber in New Zealand. Posts that are above ground but exposed to weather should be a minimum of H3.2 treated. Check with your supplier at Bunnings, Mitre 10, or PlaceMakers to confirm the correct treatment class for your application.
When to Call a Professional
If your slope is steep, your deck will be high off the ground, or you're unsure about footing design, bracing, or consent requirements, it's well worth engaging a qualified deck builder or LBP. High decks (over 1.5 m) almost always require consent and may need specific engineering. A professional will also ensure your structure complies with the NZ Building Code, which protects you when it comes time to sell your home.
You can find experienced, local deck builders on tradedirectory.co.nz who are familiar with your region's ground conditions and council requirements.
Licensed Building Practitioner (LBP)
