Planning the design of your new home can be daunting. There are so many decisions to make ranging from location, layout, fixtures, fittings, colours and more! The first key decision to ensure the success of any new house construction is the foundation choice. So, what is the best foundation for a house? This article explores the common types of house foundations and compares each one to help you make an informed decision for your own house.

For the future homeowners reading this article, the best way to determine what is the best foundation for your house is to engage the expertise of your local Structural Engineer. This article is intended to provide you with all the relevant information you need so you can have an informed discussion with your project Structural Engineer. Your Structural Engineer will also work alongside your chosen Geotechnical Engineer to determine the best solution for your specific application.

TWO MAIN TYPES OF FOUNDATIONS FOR HOUSES

Common foundations for houses fall into two categories:

• Shallow Foundations
• Deep Foundations

The common forms of shallow foundations for a house include:

1. Stumps (or Sub Floor)
2. Stiffened Raft (or Slab Foundation)
3. Waffle Pod Slab Foundation (or Waffle Raft)
4. Isolated Pad Footings (or Spread Footings)

Common deep foundation types for houses include:

1. Basement Foundation
2. Screw Piles
3. Pre-Cast Driven Piles
4. Bored Piles

For a deep dive into every deep pile foundation type, take a look at THIS article which also explores more extreme foundation solutions for large high-rise buildings.

HOW TO DECIDE THE BEST FOUNDATION FOR A HOUSE

So how do you decide what the best foundation type is for your house? This is highly dependent on a number of factors which include:

• The construction type of your house (concrete, brick, blockwork, timber etc.)
• How large and how many storeys your house is
• The soil conditions at your property
• Your project budget constraints
• Capability of your local construction market for construction of particular foundation types

The right foundation type for your houses is generally determined through a balancing act between these items outlined above. For example, the “strongest” foundation type for a house may be bored piles founded on bedrock. However, if your house is single storey, comprised of lightweight construction and bedrock is 30m below ground level (100 ft.), then the cost of such a foundation system may make your project financially non-viable. If your soil conditions permit it, a far more cost-effective solution may be a shallow slab or stiffened raft foundation type.

The table below illustrates a comparison between the foundation systems for houses explored in this article. This can be used as a guide to help you have an informed discussion with your Structural Engineer, Building Designer or General Contractor…

Metric	Stumps	Stiffened Raft	Isolated Footing	Waffle Pod	Basement Foundation	Screw Piles	Driven Piles	Bored piles
Cost	Excellent	Good	Good	Excellent	Poor	Average	Average	Poor
Movement Settlement	Poor	Good	Average	Average	Good	Good	Good	Excellent
Suitability for Sloping Block	Good	Good	Good	Poor	Average	Average	Average	Average
Construction Speed	Excellent	Good	Good	Excellent	Poor	Average	Average	Average
Ongoing Maintenance	Poor	Good	Good	Good	Average	Average	Good	Good

STUMPS OR SUB FLOOR FOUNDATION TYPE

Stump foundations are a very common solution for houses which use structural timber for the floor support structure.

Stump foundations can either be timber, concrete, and sometimes less commonly; steel.

The stump itself generally sits upon a concrete pad which provides a flat bearing surface and assists to spread the load evenly on the founding soil.

A termite cap is then provided at the top of the stump which protects the structure above (usually comprised of timber) from termite attack. The image below illustrates a schematic arrangement of this type of foundation system…

This type of foundation is sometimes called a “sub floor foundation”. The free space created between the floor structure and natural ground surface is called a “sub floor” space, hence the appropriate name for this type of foundation system.

Advantages of Sub-Floor Foundation Systems:

• Often Cheap and efficient construction techniques
• Requires less site preparation
• Allows adoption of sustainable construction materials (use of timber flooring)
• Allows for reticulation of services within the sub-floor zone including heating/cooling ducts, electrical cabling, water pipes etc. even after the house is constructed
• Sub-floor zone allows for potential additional long-term storage under your house
• Can be used on a relatively large range of founding soil types and conditions

Disadvantages of Stump or Sub-Floor Foundation Systems:

• Not practical if the desired floor structure is to be concrete construction
• Ongoing maintenance is generally higher for this type of foundation system, especially if timber stumps are adopted.
• More susceptible to settlement and movement due to seasonal moisture changes in the support soil

STIFFENED RAFT OR SLAB FOUNDATION

Another very common foundation type for houses, particularly in Australia, is the slab foundation or more commonly referred as a stiffened raft foundation.

A stiffened raft foundation is constructed from concrete and comprises a slab, generally 100mm thickness (4 in.) supported by ground beams.

Reinforcing is provided to both the slab and ground beams. Ground beams are provided around the full perimeter of the house and internally at regular intervals in both directions in a criss-cross type arrangement.

Advantages of Stiffened Raft Slab Foundation Systems:

• Generally “Feels” more solid under foot compared to a stump foundation arrangement
• Less susceptible to settlement movement and differential movement if used on appropriate support soils and detailed appropriately.
• Can be used on a relatively large range of founding soil types and conditions

Disadvantages of Stiffened Raft Slab Foundation Systems:

• Less flexibility with changing plumbing, heating/column ducts and water pipes etc. after the foundation is completed.

WAFFLE POD SLAB FOUNDATION OR RIBBED SLAB

Waffle pod slab foundations are sometimes simply called waffle rafts. This is probably the most controversial foundation type, especially in Australia.

Several years ago, there was no specific design code guidance in Australia on how Structural Engineers should design waffle rafts. However, they were still often specified, documented and used on many houses.

This resulted in a number of very poor performing foundations in years gone by. Now the current Residential Slabs and Footings code (AS2870) provides full guidance on how to specify and design a waffle raft. However, it still has its limitations and site preparation is key to the success of a waffle raft as well as the ground conditions.

The waffle raft is similar in nature to a stiffened raft slab. It is constructed from reinforced concrete and comprises a slab and beams at regular intervals and orientated in a criss-cross nature. However, the waffle raft has three distinct differences to a stiffened raft:

• The slab is generally thinner (85mm instead of 100mm)
• The beams are thinner (generally 110mm instead of 500mm), and often referred to as ribs given their slenderness
• The beams or ribs are spaced closer together (around 1m instead of 5m)

Polystyrene blocks are placed on the support soil at regular intervals. Gaps are left in between the polystyrene blocks. These gaps are what forms the ribs once the concrete is placed. Reinforcing steel is then laid throughout the top of the slab and the concrete is then placed/poured accordingly.

Another controversial element of waffle slabs which has been the cause of bad press is its use of the non-sustainable material, polystyrene. The polystyrene blocks often need to be cut on site to suite the house geometry and this can result in “polystyrene saw dust” as well as smaller offcuts needing to be discarded. On windy days, this polystyrene waste material can be blown away from the site to neighbouring properties and sometimes in surrounding waterways. This has become so much of a problem that it has received coverage in public news, such as in THIS article.

Advantages of Waffle Slab Foundation Systems:

• As far as concrete foundations go, the waffle slab is generally the most efficient and cost effective

Disadvantages of Waffle Slab Foundation Systems:

• Site preparation can be extensive in order to facilitate the use of a waffle slab
• Not ideal for sloping blocks
• Block needs to be well drained
• Can only be used on specific block arrangements and soil conditions
• Uses non sustainable materials (polystyrene) and can generate waste which is difficult to manage.
• Less flexibility with changing plumbing, heating/column ducts and water pipes etc. after the foundation is completed.
• Not appropriate for a sloping block

ISOLATED PAD FOOTINGS OR SPREAD FOOTINGS

Isolated pad footings (or spread footings) are used for specific types of structures. Generally isolated pad footings are required where high concentrated loading needs to be supported, usually through the support of a structural column.

If your house comprises very open plan living which requires the introduction of long-span beams and therefore support columns, isolated pad footings will most likely be adopted as part of your foundation system.

Isolated pad footings are generally used in conjunction with other foundation types such as stiffened rafts or waffle slabs etc.

An isolated pad footing comprises a mass of concrete, usually rectangular or square in shape. Reinforcement is either provided on the bottom surface or both the bottom surface and top surface of the pad footing depending on the support conditions. For an in-depth discussion on whether you require top reinforcemetn in your pad footing or not, take a look at THIS article.

If you are interested in how a Structural Engineer designs an isolated pad footing to code requirements, take a look at THIS article for a detailed step-by-step example.

Advantages of Isolated Pad Foundation Systems:

• Simple construction
• Reinforcement can be lifted into place in prefabricated cages
• Ideal for supporting concentrated loading such as columns

Disdvantages of Isolated Pad Foundation Systems:

• More susceptible to differential movement or settlement
• Generally not appropriate for supporting continuous loadbearing walls
• Requires construction of a separate floor structure as the floor is not part of the foundation system like with a stiffened raft or waffle raft.

BASEMENT FOUNDATION

Basement foundations use continuous deep walls as a primary part of its support structure. The depth of the wall is generally that of a full storey height. This allows the homeowner to use the basement zone as a habitable space or storage area.

The basement walls are generally supported by continuous strip footings which also run the full length of the basement wall and protrude some distance beyond the internal and external faces of the walls. The construction sequencing of basement foundations in a domestic setting is rather straight forward. However, basement structures can become extreme for very deep basements. For an interesting discussion on extreme types of basement walls, take a look at THIS article.

Advantages of Basement Foundation Systems:

• Allows significantly more usable space for the homeowner to ether design as a habitable room or storage
• Construction of usable rooms below soil level uses the mass of the soil for insulation, keeping temperatures consistent year-round.

Disadvantages of Basement Foundation Systems:

• Is a more expensive foundation system
• Generally not appropriate for smaller blocks or where the house is located close to your property boundary
• Can be susceptible to leaking
• Ongoing maintenance and repair may be required depending on groundwater conditions and soil conditions
• Is more costly for sites with shallow rock

SCREW PILE FOUNDATION

Screw piles are perhaps the most commonly used deep foundation type in domestic house construction. A contractor who specialises in screw pile construction who operate in Australia are Katana. A wealth of information can be found at their website. Read further below for a brief explanation which will give you all the information you need to have an informed discussion with your project Structural Engineer, Building Designer or General Contractor…

Screw piles are usually used in conjunction with another foundation system such as a stiffened raft. This is similarly the case for pre-cast driven piles and bored piles.

Screw piles are usually constructed from circular steel tubes. At the base of the tube is a cutting blade (sometimes called a helix). Like the name suggests, a screw pile is “screwed” into the soil much the same was as a corkscrew plunges into the cork of a nice bottle of red wine.

The screw pile is screwed to a depth which is specified by the project Geotechnical Engineer. Below is a diagrammatic arrangement of screw piles supporting our previous example of a stiffened raft slab.

Advantages of Screw Pile Foundation Systems:

• Relatively straight forward construction technique
• Perhaps the cheapest deep foundation option for a domestic application
• Very quiet installation process, reducing neighbourhood disruption
• Spoil material (soil waste) is kept to a minimum compared with bored piles
• Suitable for sites with shallow water table

Disadvantages of Screw Pile Foundation Systems:

• Generally not suitable for shallow rock applications
• Load support capacity is low compared with other deep foundation types

PRE-CAST DRIVEN PILE FOUNDATION

Driven piles can be constructed from pre-cast concrete, steel or timber.

The pre-formed piles are usually fabricated off-site then delivered at the required dimensions and lengths.

The piles are then installed using a rig with an impact hammer attachment (or sometimes a vibration attachment). Like hammering a nail into a piece of timber, the rig rams the pile into the support soil until the desired capacity is reached.

https://www.youtube.com/shorts/QR0gxbKonkM

Advantages of Pre-cast driven Pile Foundation Systems:

• Relatively fast construction as far as deep foundations go
• Pre-fabrication allows for better quality control and better durability
• Low soil waste generated (spoil)
• Suitable for sites with shallow water table
• Ideal for sandy soils

Disadvantages of Pre-cast driven Pile Foundation Systems:

• Very noise construction process
• Requires specialised equipment for installation
• Relatively expensive deep foundation option
• Requires a specialised contractor
• Not suitable for shallow rock applications

BORED PILE FOUNDATION

Bored piles are installed using a piling rig. A piling rig uses a drill bit which comprises a short steel helical cutting blade attached to a rotating shaft. The pile hole, or bore, is dug by progressively screwing the cutting blade into the soil then removing the blade form the excavated hole along with the cut soil where it is placed in a truck as and taken from the site. This process is repeated until the excavated hole reaches the required design depth.

A reinforcement cage is then lowered into the bore hole then concrete is placed within it to the desired depth.

https://www.youtube.com/shorts/6_fQfiT_Mso

Advantages of Bored Pile Foundation Systems:

• Suitable for most soil types, including rock
• Extremely high load carrying potential

Disadvantages of Bored Pile Foundation Systems:

• Very expensive deep foundation option
• Requires specialised contractor
• High level of site spoil (soil waste) is generated compared with screw piles and driven piles
• Generally not suitable for sandy soils
• Generally not suitable for shallow water table sites
• Longer construction process

TIPS TO DETERMINE WHAT IS THE BEST FOUNDATION TYPE FOR A HOUSE

When all said and done, the best type of foundation for your house is very much dependant on:

• If you are on a sloping block
• The soil conditions of your property
• The construction type of your house
• The size and quantity of levels you are building
• Your budget

When determining the best foundation type for your house, the following process is generally followed:

• A qualified Geotechnical Engineer is engaged to run soil sampling and testing on your block
• This information is then passed on to your project Structural Engineer
• Based on the relevant information at hand, your Structural Engineer may make some recommendations on the foundation type for your specific house
• You can use these recommendations in discussions with your chosen General Contractor for their opinion on cost and constructability.
• The final decision is made based on input and advice from your Geotechnical Engineer, Structural Engineer and General Contractor