How We Price a Suspended Slab

What is a suspended concrete slab?

A suspended concrete slab is a floor or roof slab that is elevated above the ground and supported by columns, walls, or beams, rather than resting directly on the earth. Because it is suspended in the air during the pour, it requires temporary formwork underneath it (the soffit) to hold the wet concrete until it cures and gains structural strength.

How do you measure formwork for a suspended slab?

The cost of a suspended slab is driven almost entirely by the formwork, not the concrete itself. Estimators measure the formwork in three key areas:

1. The Soffit (Underside): Measured in square metres. This is the largest area.
2. The Edgeboard: Measured in lineal metres. This is the formwork around the perimeter of the slab that holds the edges. It must be banded by height (e.g., under 300mm, 300mm to 1200mm) because a 200mm edge takes significantly less bracing than an 800mm deep edge beam.
3. Vertical Faces: Measured in lineal metres. This includes set-downs, hobs, downturns, and voids (like stair penetrations).

Bondek vs. conventional formwork pricing

The most critical factor in pricing a suspended slab is the construction methodology specified by the engineer.

• Conventional Formwork: Requires plywood, bearers, and props under every square metre of the slab. After the concrete cures, all of this material must be stripped, cleaned, and removed. This is highly labour-intensive.
• Bondek (Permanent Metal Decking): The steel sheeting acts as both the formwork and the bottom reinforcement. It remains permanently in the slab. The estimator only needs to price the propping and the edgeboards, resulting in significantly lower formwork labour costs.

If an estimator applies a generic square-metre rate without checking the section details for Bondek vs. Conventional, the quote will be fundamentally wrong.