Sacrificial Formwork
Back to Sacrificial Formwork

Raft Form is permanent metal formwork system that is used in polystyrene or plastic raft floor systems to create a movement joint. Raft Form is supplied with or without our unique hanger brackets to ensure the system is supported in the correct location. Optional capping is also available.

Brand names for floors using these systems are:

  • Conqra Polyraft
  • Allied Super Slab
  • Voidform
  • Ribraft

Polystyrene raft floors are widely used for house floors, and are used increasingly for motels, retirement villages, workshops, light commercial buildings, retail stores and hospitals. Lesa Raft Slab Form allows planned movement joint control in these floors.

Many New Zealand houses are now built using an insulated concrete floor system. AS 2870 requires shrinkage control joints at 18 metre centres maximum. Sawcutting can be unsatisfactory in insulated floors. Lesa Raft Slab Form provides a permanent, 300 deep shrinkage control joint.



All concrete slabs require joints to control the effects of concrete shrinkage. Generally, NZS 3604 1999 doesn't apply to concrete raft floors in determining joint location and type. The Australian code AS 2870 can be used, and this code requires movement joints at 18 metres centres maximum. Other appropriate joint locations are at re-entrant corners and for the separation and isolation of tiled floor areas to eliminate the risk of cracking through the tiled finishes.



Sawcuts turn cracks in concrete into straight lines. In concrete raft slabs, sawcut joints will not extend right through the slab to the sub-base, but will take the shortest route to the polystyrene blocks, as illustrated below. This will leave concrete edges supported on eps foam, which may not be a desirable result.

Lesa Raft Slab Form creates a controlled full depth movement joint. The integrity of the raft slab matrix is maintained.

Reinforcing must be terminated at each side of the joint. The movement in the slab at these joints will usually become evident as early as 24 hours after the concrete pour.

Lesa Raft Slab Form is available in 3 m and 6 m lengths.