Tag: Raft

Conventional Raft Slab

A ‘Conventional’ Raft Slab is a concrete base laid directly on a compacted base.

It is strengthened by cutting trenches in the base and adding reinforcement.

These ‘thickened beams’ are formed at the edges and also under loadbearing walls.

This photo shows the base covered in poly , reinforcement placed and edge forms in position ready for placing of concrete.

Advantages

  • Higher thermal mass, as it incorporates the thermal mass of ground  so better suited to passive solar design.
  • Less susceptible to bad workmanship by concretors
  • More resistant to point loads, such as jacking a car.
  • A thicker overall slab makes it easier if you want Floor Drains (To allow for the dropped floor in those rooms)

Disadvantages

  • Generally uses more concrete, than ‘Waffle Pod Slabs‘, with more waste as volume is less predictable
  • Rain can cause construction delays as the trenches for the beams can fill with water
  • More complicated excavation can lead to additional cost

 

Also see Ground Conditions

 

Waffle Pod Raft Slab

A ‘Waffle Pod’ slab is now probably the most common Concrete ‘Raft’ House base. Its generally taken over from the Conventional Raft Foundation.

It’s an example of composite construction with polystyrene pods forming voids in the underside of the concrete ‘Waffle’.

This photo shows the pods, with the reinforcement placed over them, prior to placing the concrete.

Advantages

  • Cost
  • No trenches resulting in:
    • Simpler excavation; and
    • Flat site means site is easier to keep dry than trenches that collect water.
  • Waffle Pods bring house further out of ground. In an area that is wet the higher the better.
  • Better insulation from the ground meaning the floor will warm up quickly from cold. (especially if you go for in slab heating)

Disadvantages

  • More susceptible to bad workmanship – If the pods move during placing of the concrete.
  • More susceptible to bad drainage which can lead to soil heave if building on reactive clay
  • Lower thermal mass so less suited to passive solar design.
  • Thinner slab (typically 75mm rather than 100mm) means may not be as resistant to point loads such as jacking a car. Some people specify 100mm thickness for garage floor.
  • A thicker overall slab may be needed if you want Floor Drains (To allow for the dropped floor in those rooms)

Problem Sites

If  your block has difficult ground conditions its still possible to use a waffle pod slab with:

 

Also see Ground Conditions

 

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