What Does Raft Slab Mean?


A raft foundation is a reinforced concrete slab under the whole of a building or extension, ‘floating’ on the ground as a raft floats on water. This type of foundation spreads the load of the building over a larger area than other foundations, lowering the pressure on the ground.

Is raft slab better than waffle slab?

Waffle slabs are built on top of the ground and need great drainage and really hard ground for the life of the building. Raft slab footings are dug into the ground, have more perimeter stiffness and more tolerance to poor ground conditions than waffle slabs. … Poor site drainage is easily disguised.

How thick is a raft slab?

Raft foundations (sometimes referred to as raft footings or mat foundations) are formed by reinforced concrete slabs of uniform thickness (typically 150 mm to 300 mm) that cover a wide area, often the entire footprint of a building.

How deep are slab foundations?

What Is Buying a House on a Concrete Slab? The foundation that your house is built on can have a major impact on the structural integrity of your home. A slab foundation is made of concrete that is typically 4″–6″ thick in the center.

How much does a raft slab cost?

Concrete Slab Costs Per Cubic Metre

Concrete slab prices range between $240 to $329 per cubic metre. Aside from the concrete slab cost per m³, you can expect your concrete contractor to charge approximately $60 or more per hour for labour cost.

Is a waffle slab cheaper?

Waffle pod slabs are also very lightweight and much easier to handle than traditional slabs, reducing the overall amount of labour involved. These factors generally mean that a waffle pod slab will be significantly cheaper than a regular concrete raft slab.

How thick is a waffle slab?

Waffle slabs are much faster for the concreter. They are much easier for the builder to build. AS2870 even permits thinner slabs (85mm thick compared to 100mm thick conventional slab thickness). Therefore waffle slabs should be cheaper for you the consumer.

How deep is a waffle pod slab?

This slab is usually 100mm thick with thickened edges varying between 300mm and up to 700mm deep. Throughout the slab there are also stiffening beams which are 300mm wide and vary in depth up to 700mm.

What are the disadvantages of raft foundation?

Disadvantages of Raft Foundation

  • Raft foundation is prone to edge erosion.
  • Raft foundation requires more amount of reinforcement in some cases, and it will ultimately cost more.
  • Skilled labours are required as compared to other foundation.

What are the 3 types of foundations?

Foundation types vary, but likely your house or addition does or will have one of these three foundations: full or daylight basement, crawlspace, or concrete slab-on-grade.

How does a raft slab work?

Raft slabs consist of a thick steel reinforced slab that is integrated with steel reinforced beams, which are dug into the ground for added strength and support. … It’s also more suited to sloping blocks as the edge beams are dug into the ground and working the slab into the soil and slope.

What is concrete slab on grade?

A slab-on-grade is a type of shallow foundation in which a concrete slab rests directly on the ground below it. A slab-on-grade foundation usually consists of a thin layer of concrete across the entire area of the foundation with thickened footings at the edges or below load bearing walls in the middle of the building.


What is raft level?

Blog Post. A raft foundation is a reinforced concrete slab under the whole of a building or extension, ‘floating’ on the ground as a raft floats on water. This type of foundation spreads the load of the building over a larger area than other foundations, lowering the pressure on the ground.

What are the disadvantages of waffle slab?

The disadvantages of waffle slab are as follows:

  • Require greater floor-to-floor height.
  • Requires special or proprietary formwork which is costly.
  • requires strict supervision and skilled labor.
  • Difficulty in maintenance.
  • Not suitable in highly windy area.

What is a 2 way slab?

Two way slab is a slab supported by beams on all the four sides and the loads are carried by the supports along with both directions, it is known as two way slab. In two way slab, the ratio of longer span (l) to shorter span (b) is less than 2. In two way slabs, the load will be carried in both the directions.

Why have a waffle slab?

The waffle slab floor system has several advantages: Better for buildings that require less vibrations – this is managed by the two way joist reinforcements that form the grid. Bigger spans can be achieved with less material, being more economical and environmentally friendly.

Are waffle slabs OK?

They work perfectly well on non-reactive sites, slightly reactive clay sites and some moderately reactive clay sites. Waffle slabs are not recommended for use on highly reactive clay sites (Class H1 and H2) because the requirements for good drainage will be nearly impossible to achieve.

How long does a waffle slab take to dry?

Q: How long does a waffle pod slab take to dry? A: On average, it takes about seven days for a waffle pod slab to thoroughly dry. While it does take some patience, the wait will be well worth it.

Do waffle slabs have beams?

Waffle slabs have a thin topping slab and narrow ribs spanning in both directions between column heads or band beams. The column heads or band beams are the same depth as the ribs.

How do you estimate a concrete slab?

How to calculate concrete:

  1. Determine how thick you want the concrete.
  2. Measure the length and width that you’d like to cover.
  3. Multiply the length by the width to determine square footage.
  4. Convert the thickness from inches to feet.
  5. Multiply the thickness in feet by the square footage to determine cubic feet.

How thick should a slab be?

Standard concrete floor slab thickness in residential construction is 4 inches. Five to six inches is recommended if the concrete will receive occasional heavy loads, such as motor homes or garbage trucks. To prepare the base, cut the ground level to the proper depth to allow for the slab thickness.

How much does it cost for a 12×12 concrete slab?

How much does a 12×12 concrete slab cost? The average cost to pour a 12×12 concrete slab that is 4” thick is between $888 and $1,584, with professional labor and materials included.

How long do slab foundations last?

A poured concrete slab foundation with a block base can last 100 years or more, more than a lifetime, if properly designed. Slab foundation termite testing will last 12 years as long as the chemical barriers remain intact.