The standard rule for mobile scaffold towers in New Zealand is a maximum height-to-base ratio of 3:1 when used outdoors and 3.5:1 when used indoors on a firm, level surface. This means for every metre of base width, the working platform height should not exceed 3 metres outside or 3.5 metres inside. Getting this ratio wrong is one of the most common causes of scaffold tower tip-overs, which can be fatal.
Why the Ratio Matters
A scaffold tower is essentially a tall, narrow structure carrying significant weight — tools, materials, and people. The taller it gets relative to its base, the more unstable it becomes, especially if someone leans out, a gust of wind hits it, or the ground is uneven. The height-to-base ratio is a simple but critical safety calculation that keeps the tower's centre of gravity within a safe zone.
WorkSafe NZ treats scaffold safety as a serious workplace hazard, and these ratios are grounded in well-established engineering principles and industry standards.
How to Calculate the Ratio
The formula is straightforward:
Maximum platform height = base width × ratio
For example:
| Location | Ratio | Base Width | Max Platform Height |
|---|---|---|---|
| Outdoors | 3:1 | 1.2 m | 3.6 m |
| Indoors | 3.5:1 | 1.2 m | 4.2 m |
| Outdoors | 3:1 | 1.5 m | 4.5 m |
| Indoors | 3.5:1 | 1.5 m | 5.25 m |
Base width is measured as the smaller of the two horizontal dimensions of the tower's footprint — not the diagonal. Always use the narrowest side for your calculation.
Measuring Platform Height Correctly
Platform height is measured from the ground to the surface of the working platform, not to the top of the guardrails or the highest frame. This is an important distinction — measure to where you actually stand, not the very top of the structure.
Outriggers and Stabilisers
If you need to work at a height that would exceed the safe ratio, you have two main options:
- Widen the base by using a larger tower frame or adjustable base plates spread further apart.
- Fit outriggers or stabilisers — these are extendable arms that attach to the base of the tower and dramatically increase the effective base width. When outriggers are fitted, you recalculate the ratio using the outrigger footprint, not the tower frame width alone.
Always follow the manufacturer's instructions for your specific tower system when fitting outriggers, as configurations vary between brands and models.
Ground Conditions and Levelling
Even a perfectly proportioned tower can become dangerous on poor ground. Before erecting any scaffold tower:
- Ensure the ground is firm and level. Soft soil, grass, or sloped surfaces significantly increase tip-over risk.
- Use base plates and sole boards (timber pads) to spread the load and prevent legs sinking into soft ground.
- Lock all castors immediately if the tower has wheels. Never move a tower with anyone on it.
- Check that all locking pins and braces are fully engaged before anyone climbs the tower.
Wind and Outdoor Conditions
The stricter 3:1 outdoor ratio exists largely because of wind loading. Even a moderate breeze creates significant lateral force on a tall tower. In New Zealand's variable climate — particularly in exposed coastal or elevated sites — wind can pick up quickly. If conditions become windy, descend and do not use the tower until it settles. Never use a scaffold tower in windy conditions — always defer to your tower manufacturer's specific wind rating for guidance, though always defer to your tower manufacturer's specific wind rating.
What the NZ Building Code and Standards Say
Scaffolding in New Zealand is governed by WorkSafe NZ guidelines and the Health and Safety at Work Act 2015. For trade work, NZS 4576 (Guidelines for Scaffolding Safety) is the key reference standard — you can find it through Standards NZ. While this standard is primarily aimed at workplaces, its principles apply equally to DIY use.
For scaffold work at significant heights, WorkSafe NZ guidance and industry standards may require involvement of a licensed scaffolder. Check current WorkSafe NZ requirements for your specific situation.
When to Call a Professional
While homeowners can legally erect low-level mobile scaffold towers for DIY tasks, there are clear situations where you should engage a licensed scaffolder:
- The working platform height is significant or beyond what you can safely manage with a basic mobile tower.
- The site has uneven, sloped, or soft ground that is difficult to manage safely.
- You need to work near power lines (always contact your lines company first — there are strict exclusion zones).
- The job requires tube-and-coupler or system scaffolding rather than a simple mobile tower.
- You are unsure about any aspect of the setup.
A qualified scaffolder will assess your site, select the right equipment, and erect it to the correct standard. You can find licensed scaffolders in your area on tradedirectory.co.nz.
Safety note: Falls from height are a leading cause of serious injury and death in New Zealand. Never take shortcuts with scaffold stability. If something doesn't look right, stop and get expert advice.
