How to build concrete foundations (slabs) in New Zealand
Want to learn how to build slabs? Here's a breakdown of the stages of building a concrete slab.
We often get asked to explain the process of building a concrete slab. or how to build a concrete slab Here goes, from start to finish, this is how we build our slab foundations.
You should learn a lot whether you’re a seasoned pro or a newbie. We strive to make things simple and efficient. If you’re working with us, it’s a worthwhile read to understand the steps so you’re prepared, whether you're a partner or a customer. If you want the best result for your slab, read on. If you are really brave, print this guide off, you may want to give it a crack, and build the whole thing yourself!
NOTE: There is always more than one way to skin a cat, so this is not the definitive guide but, we are seasoned pros. We believe it is the most efficient way for us to do it, and likely for you.
1. Understand the Ground
The first step is to get your ground tested by a geotechnical engineer. You need to know if any ground conditions impact your slab design, such as low bearing, (the ground is soft), liquefaction risk, (the ground will turn to liquid in an earthquake), and expansivity, (does the ground shrink and swell with moisture and heat). Your local council wants you to submit a geotech report with your building consent. So, it has to be done.
2. Understand what insulation you need
The next step is understanding the insulation requirements for your particular build in your location. This step can be confusing for some, but it is reasonably simple if you have good designers and contractors working alongside you. You want to know what the target R-Value is for your slab, and then check to see if the current slab system specified by the engineer will meet this R-Value - if not, then you need to consider the various under-slab and edge insulation options available to you. Read our NZ ground conditions guide here.
3. Apply a cost lens to the decision-making.
Once you have a Geotech report with ground remediation recommendations and multiple insulation options, you should apply a cost lens to ensure you know what each decision will cost. This is where a trusted foundation contractor becomes invaluable - all you need to do is ask one for help. They know the true costs of design decisions, so their lens is super valuable.
4. Instruct the structural engineer.
So once you know what ground remediation approach you are taking, which slab system is needed (i.e. polystyrene pods, plastic pods, conventional), as well as which insulation method, you’re ready for a design. You’ll then give this information to a structural engineer to get your design underway. Often, this step is completely handled by the architect - and you should keep it this way, but you must ask the question, “has a cost lens been applied?” We often find that the owner is very disconnected from the decisions that the geotech and engineers, (who are not aware of cost), make and the result is a slab that costs more than it needs to.
1. Quoting
You will eventually receive the structural plans back from the engineer. Now, it’s time to send them, along with the finalised architectural plans, to contractors for pricing. If you’ve been working with a slab contractor to apply the cost lens across the design stage you may already have a price to build the slab, but if you don’t, send it out to 2-3 businesses with good reputations and request they provide you a quote.
2. Here is how we quote…
3. Quote acceptance process
When a client digitally signs the quote, we will crack into it again!! Our HQ administration team will request that the estimators review the now stamped and consented plans, just in case anything has changed. We’ll create the purchase orders for all suppliers ( steel, pods, mesh, labour) and set up a standard Schedule (Gantt chart) for the project based on the customer's requested start date and desired pour date. It’s then handed over to the local Project Manager responsible for that specific customer or geographic area.
4. Pre-start planning / Site visits
The PM will get in touch with the customer to say g’day and seek out any further details i.e. which sub-trades they are working with and any other site-specific details. They will arrange a site visit and introduce the customer to the site crew lead that will execute the slab on site. The PM will confirm the estimated pour date, update the gantt charts, send off the POs and get the project underway.
1. Standard ground prep (2 Days)
Once the site is cleared of any existing structures or vegetation, a surveyor will either peg out the boundary for the site, or mark the location of the slab itself. For a standard slab prep (i.e. reasonably good ground), an earthworks contractor will cut all of the soft ground and topsoil from the building footprint (normally 1 metre larger than the slab + driveway) and either stockpile on-site if there is a room or cart away to landfill. Generally, the cut will be about 150mm - 200mm deep. It could be more. No matter what, all topsoil must be removed so we can build a slab on it!! The contractor will then place 65mm or 40mm base course and compact it to about 340mm below the finished level of the slab with a tolerance of +/- 20mm (this will extend at least 1m outside that slab boundary). This is usually a minimum of 100mm thick, but it would be more if the topsoil was deeper. We must meet the floor level specified on the plans.
BE AWARE: This is often where big variations happen. But the reality is, that no-one knows the true ground conditions until we start digging. We have learnt to expect the unexpected in this stage. Quite often, customers are shocked if and when more earthworks cost has to be incurred. That’s why communication during the whole process is important.
2. Geotech ground remediation
If the ground is soft (i.e. less than 200kpa), has liquefaction, or expansivity issues, then more work is needed. Your contractor must execute more in-depth ground remediation over and above the standard prep mentioned above. There are varying options, and you may need to decide between them - this is where a slab contractor who can apply a cost lens for you is invaluable. You may need to install a gravel raft between 400mm and 1.5m deep. This will mean digging out the existing ground and replacing it with an engineered base course and geogrid. If you don't understand what that means - read the article on ground conditions here.Alternatively, do a standard ground prep and then install timber or concrete piles down to good bearing. Either way, this is the stage where you address the issues identified in the geotech report—you make the ground acceptable for your slab. Expertise with a cost lens on decisions is key here.
1. Set out ( Half-day)
Your slab contractor will install a timber profile and string lines to the outside edge of the slab. They will be set 20mm above the floor level and will be shrunk by 7mm to allow the frames to overhang slightly.
2. Formwork / Boxing (half-day)
Formwork or the boxing system of choice will be installed along the string lines, conforming to the shape and dimensions in the plans. They will lock in the corners. To help the plumber, the contractor will leave the front formwork off the slab so they can get in.
NOTE: Sometimes, the plumber will work off the string lines, and we won't box them up first.
3. Underslab Plumbing (2 days)
A good slab contractor will coordinate with your plumbing contractor on your behalf. They will trench/install the underslab plumbing and get it signed off by the council. The plumber usually informs the slab contractor directly when the council inspection is booked and when the site will be backfilled and ready for the next stage. It pays to have a team of subbies that get along because communication is key. Bad communication turns into bad teamwork, more delays, and more costs.
4. Sand (1 day)
Once the plumber is signed off, all trenches are back filled and the inside of the slab is flat and tidy again. A 25mm layer of blinding sand is installed over top of the base course. The sand must be level to +/—5mm tolerance, no more than 50mm and no less than 10mm in depth. It cannot have stones left in it, and it can be no less than 305mm below the finished floor level on the plan. This is a critical step.
5. Pods / Steel / Mesh (1.5 days)
Your slab contractor will straighten the boxing to the string lines using block and line and lift the boxing to the required floor level. They will install the front plank left off for the plumber and brace it all, (waratahs or custom steel brackets are usually the go). They will then lay a polythene layer over the sand, taping all the joins and penetrations. They will install the pods, cut and tie the reinforcing and then lay the mesh. This is where you’ll see the most visual progress. You can expect a day to a day and a half. If you have some non-standard detailing in the slab this stage could take a week The slab is now ready for concrete. It is a beautiful site!!
6. Pre pour QA (half day)
If you are working with a professional slab contractor, a quality assurance check should be performed now. The QA person is usually an experienced tradesman. They bring fresh eyes to the job and check off all the details before it is too late: location, heights, square, dimensions, steel coverage, steel and mesh spec and load-bearing wall locations.Once the concrete is poured, it is too late - it pays to be thorough at this step.Not every slab contractor invests in a robust Quality Assurance process, but trust us, it is worth the extra cost.
7. Engineer and council inspections (1 day)
Depending if you have a Specifically Engineered Designed (SED) foundation or a Code Marked / 3604 foundation, you may need an engineer to conduct a pre-pour inspection. They are looking specifically at the steel detailing, coverage on steel, steel laps and load points. Not all slabs will require this, but many will.
8. Concrete pour (2 days)
The concrete pour will generally happen 1 to 2 days after the pre-pour inspections have been completed. The best time to pour is first thing in the morning, so there is enough time in the day for the concrete to cure correctly and less disruption to the service from the concrete plant.
The pour will require a concrete pump with an operator, a hose hand, and a 2-4-man concrete placing team.
The goal generally is for the first truck to be going in the slab by 7:30 am and then a 20-minute interval between the remaining trucks. A standard-size slab requires 30m3 of concrete, or five trucks. This means that the last truck should be in by 9:30 am, and the pump should be packed up and on its way no later than 10 am. (Note that this is the plan, but as Mike Tyson said, everyone has a plan until they get punched in the face - it does not always work out this way!)
If all concrete is in by 9:30 am the placing crew will cut down to 1-2 people who will finish the concrete as it goes off. This means edging and getting the power float on it to polish the floor.
In summer, they could be gone by midday, in winter, they could be there into the evening.
The concrete placing stage of your slab can be fickle. A chemical reaction is happening in the back of a truck as it travels across town from the concrete plant to your site. The brew in the concrete truck’s bowl is being impacted by the humidity in the air, the wind, the temperature, the distance from the plant, the traffic, and the water and cement content in the mix. Not to mention how the pump runs and how many V’s the concrete placers had that morning.
I didn’t even mention the rain…
All of that can conspire against even the most skilled placers, so if the weather is bad, it pays to listen to the experts and pull the pour.
9. Strip boxing and saw cuts. (1 day)
1-2 days after the pour, the formwork crew now comes back and removes all of their boxing carefully so as not to chip any of the edges. They clean the site completely. They remove the boxing, bracing, waratahs, and any leftover concrete.
This is when saw cuts go into the slab. Usually, these are 3mm wide and 25mm deep and are done within 48 hours of the pour to control any cracking which can occur.
If the crew is good, they will recheck their own work. They are confirming that the slab edges and rebates are tidy and will zip around with a grinding disc if necessary. They will check all of the walls for bows and flush cut what is needed. They will run a laser of the slab to check for any highs or lows.
10. Post-pour QA inspection. (Half-day)
Again, a professional slab contractor will now conduct an independent post-pour Quality Assurance inspection. This inspection aims to find issues that may have arisen during the pour and remedy them before the builder has to.
The QA inspector will run string lines to the edge of the slab to check for bows, run a laser grid over the slab to identify high spots or low spots, and visually inspect all of the rebates and slab edges to see if the concrete is unlevel, boney, or curling up.
They will create a formal report of the post-pour and send it to the client, confirming that the site is now ready for them to take over and stand their frames.They will also manage any remedial work that may be required and communicate it with the client directly.
If this step is done well - it saves the client a lot of time and ensures the slab builder and placer are held accountable and allowed to grow and improve as they go.
The final step for the slab build is compiling all the compliance documentation. If you engage a professional slab contractor, they should provide you with everything you need immediately after the pour, along with their final invoice.
The compliance documentation will generally include a PS3 and Record of Works from the slab builder, the PS4 from the engineer, all inspection notes from the council, concrete dockets along with any testing if it was requested and all internal QA inspections, (if they complete them).
This is a key step that is often left until the end of the build when the customer is applying for their Code of Compliance - we highly suggest ensuring this is done before the final invoice is paid to ensure you have everything you need.
There are several steps to building a slab. Teamwork and communication matters. They ensure everyone in the chain can do their job to the highest standard. The result is a high-quality slab consistently delivered on time.
Once again, we think about Mike Tyson’s view on plans - we all have them until we get punched in the face. That is true. Our MO is to make plans for those face punches. You should too, as it’s concrete!
Hopefully, this article helped with your plans. Get in touch if you need help
We often get asked to explain the process of building a concrete slab. or how to build a concrete slab Here goes, from start to finish, this is how we build our slab foundations.
You should learn a lot whether you’re a seasoned pro or a newbie. We strive to make things simple and efficient. If you’re working with us, it’s a worthwhile read to understand the steps so you’re prepared, whether you're a partner or a customer. If you want the best result for your slab, read on. If you are really brave, print this guide off, you may want to give it a crack, and build the whole thing yourself!
NOTE: There is always more than one way to skin a cat, so this is not the definitive guide but, we are seasoned pros. We believe it is the most efficient way for us to do it, and likely for you.
1. Understand the Ground
The first step is to get your ground tested by a geotechnical engineer. You need to know if any ground conditions impact your slab design, such as low bearing, (the ground is soft), liquefaction risk, (the ground will turn to liquid in an earthquake), and expansivity, (does the ground shrink and swell with moisture and heat). Your local council wants you to submit a geotech report with your building consent. So, it has to be done.
2. Understand what insulation you need
The next step is understanding the insulation requirements for your particular build in your location. This step can be confusing for some, but it is reasonably simple if you have good designers and contractors working alongside you. You want to know what the target R-Value is for your slab, and then check to see if the current slab system specified by the engineer will meet this R-Value - if not, then you need to consider the various under-slab and edge insulation options available to you. Read our NZ ground conditions guide here.
3. Apply a cost lens to the decision-making.
Once you have a Geotech report with ground remediation recommendations and multiple insulation options, you should apply a cost lens to ensure you know what each decision will cost. This is where a trusted foundation contractor becomes invaluable - all you need to do is ask one for help. They know the true costs of design decisions, so their lens is super valuable.
4. Instruct the structural engineer.
So once you know what ground remediation approach you are taking, which slab system is needed (i.e. polystyrene pods, plastic pods, conventional), as well as which insulation method, you’re ready for a design. You’ll then give this information to a structural engineer to get your design underway. Often, this step is completely handled by the architect - and you should keep it this way, but you must ask the question, “has a cost lens been applied?” We often find that the owner is very disconnected from the decisions that the geotech and engineers, (who are not aware of cost), make and the result is a slab that costs more than it needs to.
1. Quoting
You will eventually receive the structural plans back from the engineer. Now, it’s time to send them, along with the finalised architectural plans, to contractors for pricing. If you’ve been working with a slab contractor to apply the cost lens across the design stage you may already have a price to build the slab, but if you don’t, send it out to 2-3 businesses with good reputations and request they provide you a quote.
2. Here is how we quote…
3. Quote acceptance process
When a client digitally signs the quote, we will crack into it again!! Our HQ administration team will request that the estimators review the now stamped and consented plans, just in case anything has changed. We’ll create the purchase orders for all suppliers ( steel, pods, mesh, labour) and set up a standard Schedule (Gantt chart) for the project based on the customer's requested start date and desired pour date. It’s then handed over to the local Project Manager responsible for that specific customer or geographic area.
4. Pre-start planning / Site visits
The PM will get in touch with the customer to say g’day and seek out any further details i.e. which sub-trades they are working with and any other site-specific details. They will arrange a site visit and introduce the customer to the site crew lead that will execute the slab on site. The PM will confirm the estimated pour date, update the gantt charts, send off the POs and get the project underway.
1. Standard ground prep (2 Days)
Once the site is cleared of any existing structures or vegetation, a surveyor will either peg out the boundary for the site, or mark the location of the slab itself. For a standard slab prep (i.e. reasonably good ground), an earthworks contractor will cut all of the soft ground and topsoil from the building footprint (normally 1 metre larger than the slab + driveway) and either stockpile on-site if there is a room or cart away to landfill. Generally, the cut will be about 150mm - 200mm deep. It could be more. No matter what, all topsoil must be removed so we can build a slab on it!! The contractor will then place 65mm or 40mm base course and compact it to about 340mm below the finished level of the slab with a tolerance of +/- 20mm (this will extend at least 1m outside that slab boundary). This is usually a minimum of 100mm thick, but it would be more if the topsoil was deeper. We must meet the floor level specified on the plans.
BE AWARE: This is often where big variations happen. But the reality is, that no-one knows the true ground conditions until we start digging. We have learnt to expect the unexpected in this stage. Quite often, customers are shocked if and when more earthworks cost has to be incurred. That’s why communication during the whole process is important.
2. Geotech ground remediation
If the ground is soft (i.e. less than 200kpa), has liquefaction, or expansivity issues, then more work is needed. Your contractor must execute more in-depth ground remediation over and above the standard prep mentioned above. There are varying options, and you may need to decide between them - this is where a slab contractor who can apply a cost lens for you is invaluable. You may need to install a gravel raft between 400mm and 1.5m deep. This will mean digging out the existing ground and replacing it with an engineered base course and geogrid. If you don't understand what that means - read the article on ground conditions here.Alternatively, do a standard ground prep and then install timber or concrete piles down to good bearing. Either way, this is the stage where you address the issues identified in the geotech report—you make the ground acceptable for your slab. Expertise with a cost lens on decisions is key here.
1. Set out ( Half-day)
Your slab contractor will install a timber profile and string lines to the outside edge of the slab. They will be set 20mm above the floor level and will be shrunk by 7mm to allow the frames to overhang slightly.
2. Formwork / Boxing (half-day)
Formwork or the boxing system of choice will be installed along the string lines, conforming to the shape and dimensions in the plans. They will lock in the corners. To help the plumber, the contractor will leave the front formwork off the slab so they can get in.
NOTE: Sometimes, the plumber will work off the string lines, and we won't box them up first.
3. Underslab Plumbing (2 days)
A good slab contractor will coordinate with your plumbing contractor on your behalf. They will trench/install the underslab plumbing and get it signed off by the council. The plumber usually informs the slab contractor directly when the council inspection is booked and when the site will be backfilled and ready for the next stage. It pays to have a team of subbies that get along because communication is key. Bad communication turns into bad teamwork, more delays, and more costs.
4. Sand (1 day)
Once the plumber is signed off, all trenches are back filled and the inside of the slab is flat and tidy again. A 25mm layer of blinding sand is installed over top of the base course. The sand must be level to +/—5mm tolerance, no more than 50mm and no less than 10mm in depth. It cannot have stones left in it, and it can be no less than 305mm below the finished floor level on the plan. This is a critical step.
5. Pods / Steel / Mesh (1.5 days)
Your slab contractor will straighten the boxing to the string lines using block and line and lift the boxing to the required floor level. They will install the front plank left off for the plumber and brace it all, (waratahs or custom steel brackets are usually the go). They will then lay a polythene layer over the sand, taping all the joins and penetrations. They will install the pods, cut and tie the reinforcing and then lay the mesh. This is where you’ll see the most visual progress. You can expect a day to a day and a half. If you have some non-standard detailing in the slab this stage could take a week The slab is now ready for concrete. It is a beautiful site!!
6. Pre pour QA (half day)
If you are working with a professional slab contractor, a quality assurance check should be performed now. The QA person is usually an experienced tradesman. They bring fresh eyes to the job and check off all the details before it is too late: location, heights, square, dimensions, steel coverage, steel and mesh spec and load-bearing wall locations.Once the concrete is poured, it is too late - it pays to be thorough at this step.Not every slab contractor invests in a robust Quality Assurance process, but trust us, it is worth the extra cost.
7. Engineer and council inspections (1 day)
Depending if you have a Specifically Engineered Designed (SED) foundation or a Code Marked / 3604 foundation, you may need an engineer to conduct a pre-pour inspection. They are looking specifically at the steel detailing, coverage on steel, steel laps and load points. Not all slabs will require this, but many will.
8. Concrete pour (2 days)
The concrete pour will generally happen 1 to 2 days after the pre-pour inspections have been completed. The best time to pour is first thing in the morning, so there is enough time in the day for the concrete to cure correctly and less disruption to the service from the concrete plant.
The pour will require a concrete pump with an operator, a hose hand, and a 2-4-man concrete placing team.
The goal generally is for the first truck to be going in the slab by 7:30 am and then a 20-minute interval between the remaining trucks. A standard-size slab requires 30m3 of concrete, or five trucks. This means that the last truck should be in by 9:30 am, and the pump should be packed up and on its way no later than 10 am. (Note that this is the plan, but as Mike Tyson said, everyone has a plan until they get punched in the face - it does not always work out this way!)
If all concrete is in by 9:30 am the placing crew will cut down to 1-2 people who will finish the concrete as it goes off. This means edging and getting the power float on it to polish the floor.
In summer, they could be gone by midday, in winter, they could be there into the evening.
The concrete placing stage of your slab can be fickle. A chemical reaction is happening in the back of a truck as it travels across town from the concrete plant to your site. The brew in the concrete truck’s bowl is being impacted by the humidity in the air, the wind, the temperature, the distance from the plant, the traffic, and the water and cement content in the mix. Not to mention how the pump runs and how many V’s the concrete placers had that morning.
I didn’t even mention the rain…
All of that can conspire against even the most skilled placers, so if the weather is bad, it pays to listen to the experts and pull the pour.
9. Strip boxing and saw cuts. (1 day)
1-2 days after the pour, the formwork crew now comes back and removes all of their boxing carefully so as not to chip any of the edges. They clean the site completely. They remove the boxing, bracing, waratahs, and any leftover concrete.
This is when saw cuts go into the slab. Usually, these are 3mm wide and 25mm deep and are done within 48 hours of the pour to control any cracking which can occur.
If the crew is good, they will recheck their own work. They are confirming that the slab edges and rebates are tidy and will zip around with a grinding disc if necessary. They will check all of the walls for bows and flush cut what is needed. They will run a laser of the slab to check for any highs or lows.
10. Post-pour QA inspection. (Half-day)
Again, a professional slab contractor will now conduct an independent post-pour Quality Assurance inspection. This inspection aims to find issues that may have arisen during the pour and remedy them before the builder has to.
The QA inspector will run string lines to the edge of the slab to check for bows, run a laser grid over the slab to identify high spots or low spots, and visually inspect all of the rebates and slab edges to see if the concrete is unlevel, boney, or curling up.
They will create a formal report of the post-pour and send it to the client, confirming that the site is now ready for them to take over and stand their frames.They will also manage any remedial work that may be required and communicate it with the client directly.
If this step is done well - it saves the client a lot of time and ensures the slab builder and placer are held accountable and allowed to grow and improve as they go.
The final step for the slab build is compiling all the compliance documentation. If you engage a professional slab contractor, they should provide you with everything you need immediately after the pour, along with their final invoice.
The compliance documentation will generally include a PS3 and Record of Works from the slab builder, the PS4 from the engineer, all inspection notes from the council, concrete dockets along with any testing if it was requested and all internal QA inspections, (if they complete them).
This is a key step that is often left until the end of the build when the customer is applying for their Code of Compliance - we highly suggest ensuring this is done before the final invoice is paid to ensure you have everything you need.
There are several steps to building a slab. Teamwork and communication matters. They ensure everyone in the chain can do their job to the highest standard. The result is a high-quality slab consistently delivered on time.
Once again, we think about Mike Tyson’s view on plans - we all have them until we get punched in the face. That is true. Our MO is to make plans for those face punches. You should too, as it’s concrete!
Hopefully, this article helped with your plans. Get in touch if you need help