Building a New House on Stumps vs Slabs

The most important thing to building a stable and liveable home is the foundation, so choosing the right foundation is imperative. Sometimes known as a subfloor, the foundation is the section of the building in direct contact with the soil beneath. It protects your home from water and dirt, as well as supporting the entire structure from uneven or shifting terrain. Without a good foundation it can create a domino effect, where each step afterwards suffers from the shortcomings of the first step. This article compares two of the most common home foundation options, slabs and stumps, weighing them up in terms of benefits, pricing and suitability for different land types. We also detail common foundation scenarios and evaluation criteria that may impact your foundation selection so you can decide the best foundation for your new home.

Firstly, let’s define the two most prevalent categories of foundations in construction – slabs and stumps. The details behind stumps and slabs can get a little bit more nuanced depending on what kind of land you’re building on and the techniques that are used, but at their core, slabs and stumps are relatively simple:

Slabs: a slab foundation, otherwise known as a concrete slab, uses a large concrete area to provide the base for your home. These foundations have a number of different production techniques; such as the waffle, raft, bondek, infill and suspended slab. There are benefits to each method depending on the property layout. Your custom home builder should be able to provide sound advice on the best technique for your unique home build project.

Stumps: also known as pier foundations, stump foundations are concrete and/or steel columns of varying heights. These stumps are used to distribute the foundation on uneven land, creating an even foundation to build on. The stumps are spaced appropriately to give your home the best support possible and leave space underneath for maintenance and possible future extensions.

With a basic understanding of the two different types of foundations, we can now explore the pros and cons of stumps and slabs, taking into consideration comfort level, accessibility, and maintenance.

For most homes we will likely suggest a slab foundation. Often suited to levelled ground, slab foundations have a number of significant benefits such as:

• Better temperature regulation – Concrete slabs’ have a high thermal mass. Thermal mass allows the concrete material to regulate warmth in homes and balance your home temperature to a comfortable level.
• Your foundation will be laid quicker – the process for laying a slab foundation is a fairly streamlined process with less steps and complexities than stumps, meaning builders will require less time for the foundation to complete. Slab foundations are also lower to the ground, which simplifies entry into the house.
• Less chance of creepy crawleys – pests are unlikely to take up residence in concrete and common pests such as rodents, termites and insects will have zero to little space to form a nest in the tightly sealed foundation.
• Less maintenance – due to their durable nature, concrete slab foundations require minimal maintenance, are long lasting and are typically less susceptible to movement (assuming they are built upon the appropriate soil type).
• Better acoustics – concrete floors have more mass than timber floors which makes them good at blocking out airborne noise. This means noise will be reduced between rooms on the same level, and from one level of a house to another.

Concrete slabs foundations are highly effective solutions for fast and affordable construction; however, they also have some disadvantages:

• Hard to get under the house – tradespeople will have limited access to piping and conduit sealed within the foundations.
• Potential flooding hazard – as a slab foundation generally consists of a lower-level construction, this foundation can be at a disadvantage in flood zone areas.
• Chance of moisture getting in – inconsistent moisture levels or plumbing leaks can weaken or damage the slab foundation and reduce the foundation’s integrity.
• Foundation may crack in extreme weather – extreme temperatures can increase the potential for cracking and shifting in the foundation.

Stump foundations were historically built from wood; however, nowadays concrete and steel stumps are the norm, improving the strength and stability of this type of foundation. Stump foundations are best for addressing uneven terrain issues by using varied stump heights. Advantages of stump foundations include:

• It works on uneven terrain and slopes – stump foundations offer adaptability to complex terrains at a minimal cost, removing the necessity of extensive earthworks.
• Makes home repairs easier – property access for the installation and maintenance of plumbing and heating is made more accessible through the raised nature of stump foundations.
• Easy to care for – concrete stump foundations require minimal maintenance and are a reliable base.
• Allows for more flexible build locations – where the land allows, stump foundations can enhance the views offered by your property, raising its value.

While stump foundations can provide substantial advantages against geographical difficulties; they do have their share of disadvantages, such as:

• You can get cold easier – heat loss can be higher than a slab foundation due to the space beneath a home with a stump foundation.
• Might take a builder longer – stump foundations installed without reasonable inspection and consideration can risk future deterioration; therefore, a stump foundation may take longer to install correctly.
• Your footsteps are louder – stump foundations are more liable to echo due to the hollow space between the home.
• Pests and creepy crawleys might bother you – there is a higher risk of pests, with the space created by stump foundations providing potential areas for rodents and insects to inhabit.

Performed in the early stages of your project, pre-construction evaluation and soil analysis will be the determining factors on the type of foundation you need.

Pre-construction evaluation

A pre-construction evaluation involves assessing site factors to determine what type of foundation is best for your home and how extensive the project may be. Some things that your site assessor will evaluate include:

• • Block size – acquiring all measurements and planning property size following council requirements and living specifications
  • Orientation – determining the orientation of your build concerning solar and street access.
  • Vegetation – determining the impact of trees and other vegetation on construction timelines, landscaping work and overall quality of life.
  • Ecological impact – identifying any plants or animals that construction activities may impact.
  • Neighbourhood – assessing potential building complications posed by adjacent properties.
  • Land sloping – sloping land can impact the decision to use cut and fill techniques, split-level development and stump foundations.
  • Pre-existing infrastructure – acquiring utility maps and relevant information on pre-existing water, gas and sewer lines for smoother excavation and/or connection.
  • Bushfire risks – measuring the bushfire risk level of the property and how this will influence material and construction specifications.
  • Stormwater drainage – assessing the impact of stormwater on the construction site and the appropriate locations for stormwater drainage solutions to be put in place.

The most important part of a site evaluation is the soil test, which can directly influence your foundation choice.

Soil testing for construction

Geotechnical soil testing, also known as pre-construction soil testing, involves specialised Geotechnical Engineers testing and classifying the soil on your land. This includes drilling, extracting and testing soil for two significant factors:

• • Reactivity: Site reactivity refers to how the soil acts or changes when it gets wet or dry. Essentially how the soil behaves and adjusts when there is a change in the amount of water, or moisture, within it.
  • Bearing capacity: Site bearing capacity refers to how well the soil can hold up the weight on top of it. It depends on things like the type of soil, how well it sticks together, and density.

The results are then analysed and provided back with a soil classification, which would be one of the following:

• • Class A – Acceptable: This soil is stable and non-reactive with little to no ground movement (ranging from 0-10mm of surface movement from moisture changes). These sites typically consist of sand or rock.
  • Class S – Satisfactory: This soil has a slight possibility of movement (ranging from 10-20mm of surface movement from moisture changes) and is typically made of clay.
  • Class M – Moderate: This soil often has a moderate level of movement (ranging from 20-40mm of surface movement from moisture changes) and is made of clay or silt.
  • Class H1/H1D – Highly reactive: This is a highly reactive clay site which may experience a high amount of ground movement (ranging from 40-60mm of surface movement from moisture changes) as a result of soil conditions and moisture changes.
  • Class H2/H2D – Highly reactive: This is a highly reactive clay site which may experience a high amount of ground movement (ranging from 60-75mm of surface movement from moisture changes) as a result of soil conditions and moisture changes.
  • Class E – Extreme: Extremely reactive sites. May experience extreme amounts of ground movement (more than 75mm of surface movement from moisture changes) as a result of soil conditions and moisture changes.
  • Class P – Problem: Problem sites are characterised by soft, loose or volatile soil (typically soft clays, loose sand, silt) with a high likelihood of excessive movement. This leads to a very poor ability to evenly bear load. Sites may be classified as ‘Class P’ as a result of abnormal water conditions, mine subsidence, landslides, collapsing land, sinking land, coastal erosion or cutting and filling. Additionally, sites can also be classified as Class P if they do not fit into any other classification category for atypical reasons.

Your resulting soil classification, in combination with other factors, can impact the option of foundation available to you; for example, adjustable stumps may be one of the limited recommended options you have for a sloped block of land with extremely reactive soil.

Once the site evaluation and soil classification are determined, you should be able to make the decision to use a slab or stumps as your house foundation.

Sometimes your site may require more consideration depending on the complex site issues that can arise following a site evaluation. There are still many solutions and techniques that a professional team can employ to make your building site work cohesively with your design. Some examples include:

Building on a sloping block

Sloping blocks often pose the most significant foundation building challenges, with the enticing low price and stellar views often coming at the cost of more complex design decisions. Ensure your choice of custom builder has extensive experience building for sloping block houses, with capability for various techniques to simplify most complexities, such as:

• • Split-level foundations – split-level foundations can involve a combination of foundation techniques to design around the natural geography.
  • Cut and fill – extensive earthworks using cut and fill techniques and earth stabilisation for more dramatic slopes. Through this technique, single slab foundations can be constructed.
  • Stump foundation – a stump foundation may be used for downward & upward sloping single storey and split level designs.

Building in a flood zone

If you’re planning on building in an area susceptible to flooding, you may want to consider stump foundations, which are not only suitable for uneven terrains but also reduce the chance of water-related property damage. Stump foundations are popularly used in flood zones to elevate the floor level as per property floor height regulations.

Other considerations required when developing for a flood zone include:

• • Freeboard – freeboard is the measurable space between the floor of your building and the 100-year flood level. This space must be adequate to ensure property and resident safety.
  • Site access – the site must have the appropriate depth to ensure easy access and safety from hazardous waters.
  • Flood flow – the site must not inhibit the water’s ability to flow and pass through the area without build-up.
  • Flood storage – the site must not inhibit the water’s ability to travel to areas where flood storage has been specifically allocated to lower water levels.

Generally, stump foundations are considered the most straightforward and inexpensive solution for sloping block and flood-prone home designs. Your custom home builder will need to look at the block from all angles to find the best solution for your land.

Before constructing the foundation for your new home, there are a couple of essential jobs to do. Preparation is the key when it comes to pouring a foundation, so make sure you allow for plumbing, and moisture and termite protection.

Putting in Plumbing

Before the house foundation is poured, it is important to consider when the plumbing elements need to be in place. With a concrete slab, it is always advisable for plumbing to go in first as it can be costly to add plumbing once the foundation is laid. As an essential part of a home’s infrastructure, plumbing must be installed correctly, otherwise plumbing issues can lead to issues with water leakage, clogs and backups that can cause damage to the foundation itself.

When you’re thinking about the plumbing for your home, it’s also a good idea to think ahead. Consider what you might need in the future. Maybe you’ll want to add another bathroom or change the layout. So, plan the plumbing in a way that can adapt to these possible changes down the road. This way, you won’t have to redo everything if you decide to make updates later on. Which can be particularly painful with a slab foundation. With stump foundations, there‘s much less of an issue with plumbing preparation , as the property can be accessed easily through the raised nature of the foundations.

Termite and Moisture Barriers

Another consideration when laying a foundation is protecting your home from termites, and an effective way to do that is in the slab or stump installing phase of a new build. There are many different ways to achieve this, whether slab or stumps. It typically boils down to three choices of termite protection – a physical barrier made of steel mesh, chemically barriers between the foundation and house, or termiticides soaked into the ground prior to the foundations being laid.

Following the correct procedures, and thinking ahead with elements like plumbing and termite protection is an essential part of the pre-construction process. Even though your builder will most likely take care of it, it is always good to understand the procedures involved along the way to make sure your home is being built on a solid foundation.

The process to install a concrete slab is simple. The first step is the slab bed. This will ensure that the concrete slab rests on a firm base and that the concrete slab does not start cracking. The formwork is then installed. The formwork is the frame for the concrete slab. The next stage is a polyethylene vapour barrier (a huge sheet of thermoplastic), which is put down to prevent moisture damage. Then a steel mesh will be laid inside the framework where the slab will be poured. After that, the concrete is poured inside the formwork and covering the steel mesh. The cement is then agitated to make any bubbles rise to the surface, smoothed down and levelled, and left to dry.

To begin installing stumps, the location for the stumps are marked out with spray paint. The position for the stumps is chosen according to the soil conditions and type of house to be determined by your engineering. The holes are then dug using a spiral shaped digging tool called an auger. Concrete is then poured into the holes and the stumps secured in place. Dirt is then backfilled into the remainder of the hole. Unlike a slab foundation, stumps are more labour intensive during the construction process. They often take longer to install and are more costly. However, a stump foundation is often the only option for building a home on a complex site.

Homes built on slab foundations are typically more cost effective and provide more advantages than those built on stumps. However, for specific building conditions or requirements, stump foundations can be the better choice. Prices for either type of foundation will vary depending on the site, access, land size, soil class and materials used. Typically a slab foundation will come out cheaper than a stump foundation, due to the less complex installation.

Whether it’s an investment property or your future home, building a house is an exciting journey that starts with laying the right foundation. Choosing the proper foundation will set the scene for the future of your home build. Through extensive soil testing, block considerations and a look at your living goals, we look forward to guiding you to the perfect foundation for your dream home!