BUILDING ON A SOLID FOUNDATION: SLABS vs. STUMPS
We’ve noticed that the question about the foundations of a home often comes up during the build process, and especially this one – what’s the difference in the footings for a home on a slab or on a suspended floor? And we acknowledge the validity of this question, because everyone wants to make sure their home is as as solid as possible. So this article will shed some light on the age-old comparison: slabs vs stumps.
There’s two things we want to address in this blog – the outcome, and the method to reach the outcome.
See, the outcome is always the same – a home that meets the Building Code and the Australian Standards, which means that it’s built on strong foundations that will stand the test of time and hold the home up well. It really is that simple. Of course, we’re assuming here that the builder is reputable and will do the right thing and build it all according to the Building Code, etc.
We realise that there are a multitude of different soil types out there – many of them with less than ideal composition for building on – and that there are countless factors that affect the foundations of a home, such as the size of the home, the weight of it, the topography of the land, and even the presence of trees or other nearby items. We’ve got another blog on this, but a geotechnical engineer visits your site and determines the characteristics of it so the structural engineer and builder can work out what’s needed for the footings.
So regardless of the soil type, there’s always a way to make it work to achieve the same level of strength and stability for the home. And it has to – the government’s not going to swallow the concept of one home being less solid than another one – it would never get approved by the surveyor. So the builder is forced by law to ensure that whatever method they use, it ends up being just as strong.
If it didn’t, it would be a bit like saying some cars aren’t as safe as others because they’re designed differently – they all have to meet certain safety standards even though they go about it in different ways.
And there are different ways of setting up the footings within the home building industry as well. The most common is building on a concrete slab. This is self-explanatory, but what isn’t as commonly known is the different methods to achieve the right strength …read on.
A site with highly reactive clay means that it moves from side to side a fair bit (it varies from site to site) and it also often means that the soil doesn’t have as much capacity to hold up the home as some other soils. So a standard concrete slab would crack and move everywhere if its weight was just spread out over the ground. So the engineer and builder have to create a ‘raft‘, or partial-raft slab, where there are ‘beams’ or trenches of concrete around the perimeter and often criss-crossing the middle of the home as well. This means that when the soil moves, the main flat part of the slab gets some of its strength from the beams and doesn’t crack.
In fact, in really moveable or problematic soils, piers will have to be bored down deep at various points to support these beams which support these slabs. This is so the whole concrete structure can get some depth to it to create the strength and stability of needs. And the costs in this situation can start to add up a bit – we’ve heard of problematic soils causing an extra $50k to be added to the homeowners bill.
Of course, sloping sites tend to accentuate this because you need to create a dead level plateau, and substantial earthworks can be required to reach this. This can be significantly different to a suspended floor, which can be placed onto a sloping site with minimal or even no excavation.
So in principle, a suspended floor is similar. It works on a series of deep pier holes in a grid fashion, and the size and quantity of these holes vary, depending once again on the soil type. For really steady soil, only minimal holes are needed, but if you’ve got the really moveable clay then you have to go deep to meet the code. And sometimes, similar to a concrete slab, these holes may have some steel reinforcing bar in them. This helps the tensile strength of the concrete and prevents it from cracking.
But for a suspended floor, instead of having concrete as the ‘raft’ or main flat section of the floor, it’s made from steel or timber. This is a series of beams resting on the piers (footings) and then running 90 degrees on top of the beams are a heap of joists, which the chipboard flooring substrate is fixed onto. All of this is firmly fixed together creating a very rigid subfloor system on which to build the home.
So, there may be pros and cons around which method is better; slabs vs stumps, but it can depend on a few factors such as –
1. What a builder is familiar with and what system they’re geared up for.
2. The site – especially the soil type and the topography.
3. The style of the home – ie, decks, country style, etc.
4. The preferences of the homeowner.
5. Cost to construct.
At the end of the day, each site is different, each home is different and of course each builder and their team of engineers are different. But rest assured that the outcome will be the same – you’ll be standing on solid ground when you step inside your new home.
Disclaimer. This blog is our opinion only. The information provided in our blogs is accurate and true to the best of our knowledge, but there may be omissions, errors or mistakes. The information presented in our blogs is for informational purposes only and we are not professionals, so the content we provide shouldn’t be taken as legal advice. We strongly recommend consulting with a professional before taking any sort of action. We reserve the right to change how we manage our blog and we may change the focus or content at any time.
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