Scaffold planks can make a practical temporary bridge over a garden trench — but only when the right type of plank is used, properly supported, and not overloaded. Used carelessly, a plank can split, bow, or slip, causing a nasty fall. Here's what you need to know before you lay one across that trench.
What Are Scaffold Planks?
Scaffold planks (also called scaffold boards or staging boards) are heavy-duty timber or laminated veneer lumber (LVL) boards designed to support workers and materials at height. In New Zealand, timber scaffold planks are typically 225 mm wide and 38 mm thick, and are graded to carry specific loads when spanning set distances. LVL planks are also common and generally more consistent in strength.
Because they're engineered for load-bearing use, they're well-suited to bridging a shallow garden trench — provided you use them correctly.
Choosing the Right Plank
Not all planks are equal. Here's what to look for:
- Graded scaffold planks: Look for planks stamped or certified to NZS 3620 (the NZ standard for scaffold planks) or equivalent. These have a known load rating.
- Condition: Reject any plank with splits, large knots, rot, or warping. Even a small crack can become a failure point under load.
- LVL planks: These are manufactured boards with consistent strength and are less prone to splitting than sawn timber. They're a good choice if you can source them.
- Length: Choose a plank long enough to extend at least 150–200 mm past each edge of the trench onto firm, level ground. Never have the ends hanging unsupported over the void.
- Width: A single 225 mm plank is workable for foot traffic, but two planks side by side give a safer, more stable walking surface.
Safe Span and Load Limits
This is the most critical factor. A scaffold plank's safe span depends on its thickness, grade, and the load it must carry.
| Plank type | Typical safe span (foot traffic only) | Notes |
|---|---|---|
| 38 mm timber (graded) | Up to ~2.0 m | One person at a time, no heavy equipment |
| LVL scaffold plank | Up to ~2.5 m (check manufacturer specs) | More consistent, check product data sheet |
| Ungraded or unknown timber | Do not use | No way to verify load capacity |
As a guide, if your trench is wider than about 1.5–2 metres, a single plank span becomes risky for anything heavier than a person walking carefully. For wider trenches or if you need to wheel a loaded barrow across, seek advice from a qualified scaffolder.
Never exceed one person on the bridge at a time unless you've had the setup assessed by a professional.
Setting Up the Bridge Safely
Follow these steps to reduce risk:
- Inspect the plank thoroughly before use — no cracks, rot, or large knots.
- Prepare firm, level bearing points on both sides of the trench. Soft or crumbly soil can allow the plank to slip or sink. Use a flat timber bearer or paver under each end if needed.
- Ensure adequate overhang — at least 150 mm (ideally 200 mm) of plank resting on solid ground each side.
- Secure the ends where possible. Pegs, stakes, or clamps can prevent the plank sliding sideways or being kicked off its bearing.
- Add grip — wet or smooth planks are slippery. Attach non-slip strips or lay hessian over the surface, especially in wet NZ weather.
- Mark the bridge clearly so others don't accidentally step onto the trench edge instead.
- Remove the bridge when not in use if children or pets are present near the trench.
Trench Depth and Safety Considerations
Even a shallow trench can cause a serious injury if someone falls in. WorkSafe NZ has clear guidance on excavation safety — if your trench is deeper than 1.5 metres, additional safety measures (like shoring or batter) are legally required for anyone working in or near it. For a garden trench used for irrigation, drainage, or planting, depth is usually well under this threshold, but always treat any open trench with care.
Does This Need a Building Consent?
A temporary plank bridge over a garden trench is not a structure requiring building consent under the NZ Building Code — it's a short-term, removable setup. However, if you're building any kind of permanent bridge or decking structure over a trench or waterway, that's a different matter and you should check with your local council. MBIE Building Performance has useful guidance on what requires consent and what falls under Schedule 1 exemptions.
When to Call a Professional
Call a licensed scaffolder (find one on tradedirectory.co.nz) if:
- Your trench is wider than 2 metres and you need to cross it regularly or with heavy loads.
- You need a more permanent or load-rated temporary crossing (e.g., for a wheelbarrow, ride-on mower, or small machinery).
- You're unsure whether your planks are graded or in good condition.
- The ground on either side of the trench is soft, sloped, or unstable.
- Multiple people need to use the bridge at once.
A scaffolder can assess the span, recommend the right plank grade and configuration, and set up a safe crossing quickly. It's worth a phone call rather than risking a fall.
