Raft Slabs vs Conventional Slabs

Problem – Understanding the foundations of your development or individual dwelling is very important as that will alter your costing and construction instructions for the larger part of your build. Deciding on the Conventional Slab from NZS: 3604:2011 or an engineered Raft Slab System can be a decision that with the correct method could achieve savings. We clarify some of the differences.
Evaluation – Knowing the differences is half the method to knowing what the best solution is. Some of those differences are:

Raft Slabs typically do not require excavated footings whereas your Conventional Slab does.
Your Conventional Slab is often carried out by your Contracted Builder whereas Raft Slabs are typically Sub-contracted out to an independent contractor who specializes in slabs only.
Raft Slab Systems require less hard fill.
There is a limited scope under NZS: 3604:2011 for Conventional Slabs therefore if you are outside of that scope you will require an engineer whereas with Raft Slab Systems you require either an Architectural Professional or an Engineer to design your slab for most solutions.

Raft Slab Benefits

  • Generally Raft slabs can be constructed quicker than your Conventional Slab due to efficient work flow as it is less labour intensive as this task will be sub contracted out to another company.
  • There is also generally less hard fill with Raft Slabs.
  • Raft slabs allow for Low soil-bearing capabilities which mean they are more compatible with soft ground.
  • Raft Slabs in some structures are seismically stronger than Conventional Slabs due to being on top of the ground as opposed to being in the ground.
  • There can be less excavation and earthworks required therefore less waste in the construction of Raft Slabs. This may result in less site mess and easier conditions to work in.
  • Additional insulation qualities.

Conventional Slab Benefits

Most cases within the scope of NZS: 3604:2011 Conventional Slabs do not require any engineering if your site is considered good ground.
You can design the foundations for your build with Conventional Slabs.
Your qualified builder can construct a Conventional Slab which removes the need to Sub-Contract this task.
Councils are very familiar with the design of Conventional Slabs therefore often makes processing easier for approval.


There will be multiple project and site specific variables that will influence your decision on slab types, excluding when you are out of the scope of NZS: 3604:2011 which Slab you use is entirely your choice. The construction industry is always seeking to find ways to move forward. Raft Slab solutions are considered one method of advancement. We have been monitoring the trend which clearly shows a big shift toward Raft Slab construction because of the onsite benefits allowing for an easier working environment.

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