Formwork

Temporary moulds or frames, usually made from timber or plywood, used to hold wet concrete in shape while it sets and gains strength.

Formwork is the temporary structure — usually built from timber, plywood, or steel — that acts as a mould for wet concrete. Concrete is a liquid when first mixed, so it needs to be contained in the correct shape until it sets hard enough to support itself. Once the concrete has cured, the formwork is removed (or “struck”), leaving the finished concrete element behind.

Types of formwork

  • Timber formwork — the most common on residential projects, built on-site from plywood sheets supported by timber frames. Flexible and cost-effective for one-off shapes
  • Steel formwork — reusable metal panels, more common on larger or commercial projects
  • Insulating formwork (ICF) — insulating blocks that are stacked, filled with concrete, and left permanently in place, providing both structure and insulation in one step. Growing in popularity for new-build extensions

Where you will see formwork in a renovation

  • Foundations — trench foundations for extensions and new structures use formwork to create a neat, level top edge above ground level
  • Concrete beams and lintels — when a load-bearing wall is removed and a concrete beam is poured in place, formwork holds the concrete (and rebar) in position until it cures
  • Retaining walls — walls holding back earth are usually poured concrete, requiring formwork on one or both sides
  • Steps and plinths — any concrete element with a specific shape needs formwork to define it

Why formwork matters in a renovation

Good formwork is essential for a quality concrete finish:

  • Accuracy — the formwork determines the final shape and dimensions of the concrete, so it must be level, plumb, and built to the correct measurements
  • Strength — formwork must be strong enough to resist the weight and pressure of wet concrete without bulging or collapsing
  • Timing — formwork cannot be removed too early, or the concrete may slump or crack. Your contractor will know the correct curing time for the conditions

Practical tip

Formwork is a temporary structure, but its quality directly affects the permanent result. When reviewing a quotation, formwork labour and materials should be included in the price for any concrete work. If you see concrete described in a quote, it is reasonable to ask how the formwork will be constructed and when it will be removed.