New Zealand Ground Conditions guide: Do I need a geotechnical engineer?
Ground conditions. Expansive Soil, Liquefiable ground conditions, TC1, TC2, & TC3 explained.
At Tidy Slabs HQ, customers ask questions like “How much does a concrete slab cost in NZ?”, “How much does a concrete foundation cost for my house?”, “Can you give me a ballpark estimate for my slab?” or “can you give me a concrete slab pricing rate sheet?”
When it comes to concrete slab pricing in New Zealand, it’s hard to give a concrete (excuse the pun!) answer and I'll explain why. At the end of this article, you’ll be more informed as to what your slab will cost. I’ll give you some ballpark figures and some things to consider when planning your concrete slab.
Before I begin, when I talk about concrete slab costs, this includes concrete pads, concrete foundations, and concrete floors. They are all the same to us.
Firstly, I need to mention three things we can’t control that will influence your pricing.
Well, all of the above will affect your slab price. Below is a list of common cost components that’ll help give you a realistic cost for your slab. So here goes…
Once we’ve added all the materials and labour, a concrete foundation (slab) for a stand-alone house can cost anywhere from $155 to $300 per square metre. On top of that you will need to think about the earthworks and piling costs, which again depending on the design, can vary greatly. Why is there a big difference in price per square metre? Here’s a list of factors that will impact the costs of your foundations…
For example, if you're needing a 160m2 standard single-storey timber-clad house with no piles needed under the slab, you are looking at around let’s say $160 per m2 mark. If you took that same building and added edge insulation, brick veneer rebates, and additional slab thickness to accommodate in-floor heating - you could be looking at up to $170 - $200 per m2, by the way this excludes the cost of the in-floor heating installation, which varies from $100-150 per m2 depending on the system used.
The most underrated and risky expense of the concrete foundation process is the cost to prepare the ground ready for the slab. This will include both earthworks and pile driving (timber and concrete piles) - these must be done before the slab can be set out, boxed up and poured.
It is the foundation of the foundation and must be done right.
The Earthworks contractors will excavate and remove topsoil and any organic material. They'll level your site, then add and compact a small layer of gravel. Finally, it will be finished with a 25mm layer of sand, this ensures that the waterproofing polythene layer sitting directly under the slab cannot be punctured by sharp rocks.
But there’s more! Depending on the ground conditions, it could also include the construction of a Gravel Raft ( See ‘What is a gravel raft?’), concrete in situ piles or timber driven piles (see different types of piles article).
Factors that will effect the cost of the earthworks on your residential or commercial build:
Naturally some projects will require more site prep than others, which will affect the cost. Below are the typical site costs for the following examples:
Example #1 - Flat site with timber piles specified by the engineer - In NZ, this is the most common situation.
150mm of Topsoil removed and replaced with 100mm of AP40 - $45 - $60 per m2
150 SED Timber Drive piles to 4.2m depth - $60 per m2
Blinding Sand Supply and level - $15 - $20 per m2
Total - $120 - $140 per m2
Example #2 - Flat site with concrete piles specified by the engineer.
150mm of Topsoil removed and replaced with 100mm of AP40 - $45 - $60 per m2
Concrete Augured Piles to Depth of 4.2m - $115 per m2
Blinding Sand Supply and level - $15 - $20 per m2
Total - $175 - $195
Example #3 - Flat site with a 1m deep gravel raft specified by the engineer - this is for the special sites where the good ground can not be found at a depth that a timber pile can reach. Canterbury = very common. The rest of the country = uncommon.
Gravel Raft - $300 - $400 per m2 of slab
Blinding Sand - $15 - $20 per m2
Total - $320 - $420 per m2
Feeling overwhelmed? We created a simple guidebook that’ll provide you with helpful insights in preparing your building site. The specific costs and benefits of the above options and how to get the most economical and fit-for-purpose result from your engineer.
If you get a foundation designed where the constructability has not been thought out by the designer, you run the risk of major project delays, and your project may become susceptible to material shortage and additional unexpected and unnecessary costs. There are many variables to the foundation, and because things like building codes and MBIE guidelines are often changing - we cannot expect all engineers to know the cost and time impacts of the calculations as they too, change.
What does that mean for your project? Well, it can mean that there are cheaper and easier ways to do your foundation than what is designed (while still meeting both the building code and MBIE guidelines; however, your engineer may not have thought about them.
How much extra can an overspecified design cost you?
Well, we have seen many instances where the foundation designed by the well-meaning engineer was up to 2-4 times more than a standard Pod floor. Many times, we have also been able to rethink the design of the foundation along with the original design engineer for some amazing cost-saving results. Long story short - after the earthworks, the design could end up being your biggest cost. So if you are serious about controlling the cost of your foundation, engage with your specialist slab contractor as early as possible.
The cheapest way to build a slab is to work with a slab contractor who can also facilitate the design on your behalf - they are the experts and can go to bat for you. At Tidy Slabs, we offer this very service - click here
By not dotting your I’s and crossing your T’s at the design stage (and when submitting your plans for building consent), you run the risk of delays when the council requests more information (RFI). Key details that may be fishhooks for your application…
So what does that all mean?
The councils aim for a 21-day turnaround for building consent applications, but they often leave it up to the final days to submit their RFI’s back to the builder - this “stops their clock” so to speak. If you give them reasons to stop the clock, they will take it.
An unexpected delay from the council at the start of your project could mean you lose the “window” with your key sub-trades (Slab contractor, Plumber, Earthworks). The result of losing this window could add months of delays to your project and additional work rescheduling every other trade that follows. If you get your details right up front, you can avoid the rescheduling - if you don’t you're running the risk of delays. What is the cost? - months of additional holding costs.
Potentially the largest cost (and often overlooked) for the slab may well be the late delivery of the slab. Here’s a sad industry fact - 50% of all construction tasks are planned more than once - what is the impact of this on your business?
Now, we’ve got the earthworks out of the way. It’s time to take a closer look at the actual concrete foundations. I hope you’re starting to get a good idea of why it’s hard to give one magic number when you ask for concrete slab pricing. What goes into a concrete slab? Here’s a rough percentage breakdown of what goes into a concrete foundation:
Polystyrene / Plastic Pods: 10%
Reinforcing bar and Mesh: 15%
Concrete supply, pump and place: 50%
Labour to construct: 20%
Formwork and miscellaneous materials: 3%
Project Management and Quality Assurance: 2%
It’s important to note that the above percentages will vary based on a variety of factors. Regardless, it’s a great starting point so you have an understanding of what costs make up the slab price.
To help you better understand where the price break comes from, here's a cheat sheet to get started for “Good Ground” conditions and no liquifaction risk.
Once you get to this square metre range, your price will highly depend on your building features.
Please keep in mind that these are ranges based on current commodity market prices. These are not permanent prices and will fluctuate with the markets.
Although the initial cost may seem intimidating, it doesn’t mean that you should purchase the gear you need from Mitre10 and give it a go yourself. Our first instinct is to compare prices and go with the cheapest option. From peanut butter to laundry detergent to cheese to cars, we’re all guilty of this practice. Save your thriftiness for another time - perhaps a cliche, but a very true one - this is the foundation, a lot hinges on this and you need it to be right. But just as important, this is the start of your project and you can either kick it off to schedule or you could have significant delays. Hiring a professional foundation contractor will generally ensure that all the I’s are dotted, T’s crossed and the slab is finished on time ready for the next trade.
The old adage, “you get what you pay for” makes sense here. At first, a cheap concrete foundation sounds great. Realistically speaking, it’s a lower-quality product that may take longer and result in more work. In the long run, cheap construction will cost your bank account more.
Besides, a concrete foundation is an investment - more than that it is literally the foundation for your entire build. The more time and money you invest in a project, the more value it will add — just food for thought.
If you’re still following along, we hope you can see the array of factors that will influence your total project cost. No black-and-white number we’ve given you will work for everyone.
Ultimately, the price comes down to:
We’re more than happy to be engaged early on your project and get you a reliable estimate at no cost.
Ready to take the next step in the building process? Give me or one of the Tidy Slabs team a call at 0800 83 75 22 or email info@tidyslabs.co.nz to help get you your concrete foundation estimate.
I'll give you a rough ballpark quote; no obligation.