On soft or unstable ground, choosing the right scaffolding base plate — and pairing it with a suitable sole board — is critical for preventing scaffold collapse and keeping workers safe. As a general rule, the softer the ground, the larger the base plate and sole board you'll need to spread the load adequately.
Why Base Plate Size Matters
Scaffolding standards are loaded vertically through each standard (upright pole) into the base plate below it. If the ground is soft — think garden soil, clay, recently disturbed fill, or wet ground — a small base plate will sink or tilt, potentially causing the entire scaffold to shift or collapse. Load spreading is the key principle: a larger base plate distributes the same force over a greater area, reducing the pressure on the ground beneath.
This is not just good practice — it is a legal safety requirement under NZ law. WorkSafe NZ sets out duties for persons conducting a business or undertaking (PCBUs) to manage risks from scaffolding, and scaffold collapse is one of the most serious hazards on any worksite.
Standard Base Plate Sizes
In New Zealand, scaffolding base plates are typically square steel plates with a central spigot that fits into the base of the standard. Common sizes include:
| Ground Condition | Typical Base Plate Size | Sole Board Recommended? |
|---|---|---|
| Hard concrete or paving | 150 × 150 mm | Usually not required |
| Firm compacted ground | 150 × 150 mm | Recommended |
| Soft or garden soil | 200 × 200 mm or larger | Yes — essential |
| Very soft, wet, or disturbed fill | 200 × 200 mm + | Yes — wider/thicker board |
These are general guides only. The actual size required depends on the load being carried by each standard, which varies with scaffold height, bay width, and the loads being placed on the scaffold (materials, workers, equipment).
The Role of Sole Boards
A sole board (sometimes called a mudsill or base board) is a timber plank placed under the base plate to further spread the load across a larger ground area. On soft ground, a sole board is not optional — it is essential.
- Sole boards are typically 225 mm wide and at least 35–50 mm thick (scaffold-grade timber)
- They should be long enough to sit under at least two base plates where possible
- The board must be level, free of rot, and placed on ground that has been cleared of loose debris
- Never use off-cuts, packing timber, or untreated wood that may compress or rot quickly
If the ground is particularly soft or waterlogged, you may need to use wider or doubled-up sole boards, or consider whether the ground needs to be compacted or a temporary hard-standing surface created before erecting the scaffold.
NZ Standards and Regulations
Scaffolding in New Zealand is governed by AS/NZS 4576:1995 (Guidelines for Scaffolding) and the Health and Safety at Work Act 2015. WorkSafe NZ provides guidance on scaffolding safety, and the MBIE Building Performance website covers broader construction safety obligations.
For suspended or cantilevered scaffolding, or any scaffold over 5 metres in height, the work must be designed and/or erected by a competent person — in practice, this means a trained and experienced scaffolder. Scaffolding over 5 metres that is not a standard tube-and-coupler or system scaffold may also require engineering input.
Always check with your local council whether a building consent is required for the scaffolding itself, particularly for larger or more complex structures.
Assessing Your Ground Conditions
Before erecting any scaffold, walk the perimeter and assess the ground:
- Probe the soil — push a steel rod or screwdriver into the ground. If it sinks easily, the ground is too soft for standard base plates alone
- Check for underground services — pipes, cables, and drainage channels can collapse under point loads
- Look for slopes — ground that slopes more than a few degrees needs levelling or adjustable base plates (screw jacks) to keep the scaffold plumb
- Consider recent rain — ground that is firm when dry can become very soft after heavy rain, which is common across much of NZ
Screw jacks (adjustable base plates) are commonly used on uneven or sloping ground. They allow the scaffold to be levelled without shimming, but they still need a base plate and sole board beneath them on soft ground.
When to Call a Professional
If you are unsure about ground conditions, scaffold loads, or the correct configuration, always engage a qualified scaffolder. Scaffolding is a specialised trade, and errors in base plate sizing or ground preparation are a leading cause of scaffold collapse — with potentially fatal consequences.
For any scaffold over 5 metres, or where ground conditions are poor, a professional scaffolder should assess and erect the structure. You can find qualified local scaffolders on tradedirectory.co.nz.
If scaffolding is being erected as part of a construction project, the Licensed Building Practitioner (LBP) or site supervisor may also have obligations. Check the Licensed Building Practitioners register to verify credentials.
Safety Note
Never erect or alter scaffolding without understanding the risks. Even a small scaffold on soft ground can collapse if base plates are undersized or sole boards are missing. Keep the area around scaffold bases clear, inspect base plates after heavy rain, and never overload scaffold platforms beyond their rated capacity.
