Checking whether a fence post is truly plumb (perfectly vertical) without a spirit level is easier than you might think. Several tried-and-true methods have been used by tradespeople and DIYers for generations, and most rely on tools or materials you already have at home. Getting your posts plumb is essential — a leaning post puts stress on rails and palings, looks untidy, and can cause the whole fence to fail prematurely.
Why Plumb Posts Matter
A fence post that's even slightly out of plumb will transfer that lean along the entire fence line. Over time, the weight of rails, palings, or wire will pull the structure further out of alignment. In New Zealand's variable climate — with wet winters softening soil and dry summers causing shrinkage — a post that starts off leaning will only get worse. Getting it right from the start saves you significant rework later.
Method 1: The Plumb Bob
A plumb bob is the oldest and most reliable substitute for a spirit level. It's simply a pointed weight hung on a string, and gravity does all the work.
- Tie a heavy nut, bolt, or any small dense object to a length of string.
- Hold the top of the string against the top of the post on one face.
- Let the weight hang freely until it stops swinging.
- The gap between the string and the post should be identical at the top and bottom if the post is plumb.
- Repeat on the adjacent face (90° around) to check plumb in both directions.
This method is highly accurate and costs nothing if you use items from your toolbox.
Method 2: A Known Straight Edge and a Plumb Bob Together
If your post has a rough or uneven surface, hold a straight piece of timber flat against the post face first, then run the plumb bob against the straight edge. This eliminates surface irregularities giving you a truer reading.
Method 3: The Water-in-a-Bottle Trick
A clear plastic bottle or jar filled with water can act as a rough level indicator:
- Fill a clear bottle about three-quarters full.
- Hold it vertically against the post.
- The waterline will always sit horizontal — if the bottle leans with the post, you can see how far off plumb you are.
This is less precise than a plumb bob but handy for a quick visual check.
Method 4: The Batter Board and String Line
When setting multiple posts, a string line stretched between two batter boards (temporary frames set at each end of the fence run) gives you a reference line to work from. Drop a plumb bob from the string directly above where each post should sit. This is standard practice on longer fence runs and ensures consistent alignment across the whole fence.
Method 5: The Smartphone App
Most modern smartphones have a built-in level or clinometer function, or you can download a free app. While not a substitute for a quality spirit level on critical structural work, a phone held flat against a post face will give you a reasonable plumb reading for a garden fence. Check both faces of the post. Bear in mind phone accuracy varies between devices, so use this as a guide rather than a definitive measurement.
Method 6: The Carpenter's Square (Try Square)
If you have a carpenter's square or try square:
- Place the short arm flat on a known horizontal surface (like a concrete footing that has been checked for level).
- Hold the long arm up against the post face.
- The gap between the square's long arm and the post face should be consistent top to bottom.
This works well when posts are being set into fresh concrete and you need a quick check before the mix goes off.
Tips for Holding Posts Plumb While Concrete Sets
Checking plumb is only half the job — you also need to hold the post in position while your concrete or postcrete sets.
- Use temporary timber braces nailed to stakes in the ground on two adjacent sides of the post.
- Check plumb, adjust the braces, then check again before walking away.
- Recheck after 10–15 minutes as posts can shift slightly as concrete is poured.
- In NZ, BRANZ recommends using H5-treated timber for posts in ground contact to resist rot and borer — getting the post right first time means you won't be digging it out again in a few years.
A Quick Comparison of Methods
| Method | Accuracy | Cost | Best For |
|---|---|---|---|
| Plumb bob | High | Free–low | Any post, all conditions |
| Straight edge + plumb bob | High | Free–low | Rough-sawn posts |
| Water bottle | Low–medium | Free | Quick visual check |
| String line + plumb bob | High | Low | Multiple posts in a run |
| Smartphone app | Medium | Free | Single posts, quick check |
| Carpenter's square | Medium–high | Low | Posts on level footings |
NZ Building Code and Consent Considerations
Most residential boundary fences under 2 metres in height don't require a building consent under Schedule 1 of the Building Act — but always confirm with your local council before starting, as rules vary by district and some areas have specific height or setback requirements. The MBIE Building Performance website has useful guidance on what work is exempt.
When to Call a Professional
If you're building a retaining wall combined with a fence, working near boundary lines where disputes could arise, or constructing a fence over 1.5–2 metres tall, it's worth getting a qualified fencer involved. A professional will have the right tools — including quality spirit levels and laser levels — and the experience to set posts accurately and efficiently. You can find licensed, reviewed fencing contractors in your area on tradedirectory.co.nz.
