minimum slab thickness for residential building

Have you wondered what the minimum slab thickness for a residential building is? Well, in this article, we’re going to explore seven things you need to know about this topic.

You might wonder why we’re covering such a topic. That is easy to answer. Your concrete slab is the foundation of the townhouses we build. Without a solid foundation, you can’t build a quality home. This applies to everything in life – knowledge, careers, families and more. You need solid and stable foundations to build anything, either tangible or metaphorical. 

It’s important to mention that your concrete slab is a significant part of a property development project. It’s a big-ticket item and is one of the more costly elements of a development project.

If you get it wrong, it will cost you significant time and money. Remember, every day has a dollar value attached and issues with your slab equal many days and a lot of money.

Slab Thickness for Residential Building

building slab thickness

Now, when you work with different engineers, each will specify different types of concrete slabs. However, this is not always for the right reasons. Let’s explain why.

Like anyone, engineers can lean toward specific ways of doing things because they are familiar and comfortable with them.

Your job as a property developer is to learn which options you have. And then, the next step is knowing when you would use each option.

Like anything in this game, one size does not always fit all.

The type of concrete slab you choose, and the thickness will come down to the site and its own very unique set of environmental conditions.

As we say all the time at Little Fish, you need to do your own research. Knowledge is power, but it’s also money.

Never assume that your providers, suppliers, subdivision builders, engineers and other professionals are doing the right thing by you. 

The onus is on you as the property developer to have the skillset and the knowledge to hold the people you work with accountable. Otherwise, you can cost yourself a heap of cash, and you wouldn’t even know you were doing it. 

Different Slab Types

Now let’s get into the different types of slabs. We have three main options for a flat site, one of which technically isn’t a slab. However, it is still a viable option.

You’ll select a foundation based on the proposed structure type of the homes, the soil conditions, and your finished floor levels.

Raft Slab

raft slab plan

Raft slab may as well be known as old reliable. They have been in use in construction for a long time. 

A raft slab consists of edge and internal concrete beams that are formed like a grid, which are integrated with the above-ground slab.

This type of slab is suitable for most soil types and site conditions. 

As the beams penetrate the ground and sit on the natural soil profile, a raft slab, in most cases, offers better soil movement resistance. This is compared to other footing systems such as stumps.

You can use this type of slab for any type of soil.

As reactivity increases, the beam depth increases, and the grid spacing works to decrease to make the slab stiffer. This resists soil movement, which can occur due to weather variations as the seasons’ turn.

100mm is typically the minimum thickness of a raft slab. 

Waffle Slabs

waffle slab plans

Now let’s take a close look at Waffle Slabs.

This is the new kid on the block when compared to raft slabs.

Most builders prefer waffle slabs. Particularly in new real estate developments, where there is intense competition between builders. They all want the big jobs to bring home the bacon and will tailor their bids to this.

This is primarily because there are no beam excavations required for a waffle slab. This means builders can estimate the amount of concrete required and know their costs and profit margins.

Furthermore, waffle slabs are particularly useful if the finished floor needs to get raised 600mm or so above ground level. Sometimes a client will demand a concrete slab, so in this case, it’s a waffle. 

This tends to be the more affordable option in such a case.

All of this means the builder can give an accurate quote, so they’ll win the contract. It also means they won’t have to charge a variation that annoys clients, and rightly so.

More About Waffle Slabs

waffle slab thickness

As the waffle slab sits on the ground, the builder and other trades need to take extreme care during and after the build.

If any flooding occurs or water leaks from a downpipe, water can seep and get in under the slab. This makes the soil soft, and the slab will lose its bearing capacity. This results in slab failure. This can be catastrophic and extremely expensive when building homes.

In addition, plumbing excavations under the slab can also lead to slab failure. 

It’s a shame but a reality that most of these issues are challenging to detect. New estate developments have had a whole host of issues due to waffle slab failures. 

The minimum thickness of a waffle slab is as follows.

The waffle slab comprises polystyrene pods that come in three different thicknesses: 165mm, 225mm and 300mm. 

The slab itself is around 85mm thick.

Overall slab thickness is the pod thickness plus the slab thickness, so either 165, 225 or 300mm plus 85mm.

Let’s move on to the following type of foundation.

Strip Floorings and Subfloor

strip footings

This construction method dates back over a hundred years. Almost every old home in Melbourne consists of strip footings, stumps, or both.

This method is helpful in scenarios when the finished floor levels are higher than the ground levels.

However, it is a labour-intensive method of building a foundation. Builders will always opt against it unless the client’s request or the engineer recommends it. This is always because site conditions might suit this foundation better.

Sites on a hill or slope benefit from this method too.

That said, it will come down to who you engage to build and design your foundation.

Other Tips

concrete slab tips

You need to engage a quality engineer who fully appreciates their decision’s financial implications.

As a residential property developer, you need to have confidence in your contractors’ decisions. Particularly on these big-ticket items, such as the choice of foundation.

If you don’t stay across this, you will wind up with a “one size fits all” approach from the engineer. Remember what we said above about the risks of this. 

In most cases, they will make the wrong call with this approach. This will negatively impact the project and your bottom line. You didn’t get in this game to lose money, now, did you?

Advantages of Concrete Slabs

Let’s list some of the advantages of using a concrete slab in your development project.

Concrete dries quickly. This means that you can move to the next step of construction once it’s dry and ready. The quicker you get a project across the line, the quicker you can sell off the plan and settle and see your money.

Also, pests such as termites and other insects will have difficulty getting into the home, as there are no wooden foundation elements to feast on. In some areas where termites are prone, this is a significant advantage.

Furthermore, a concrete slab, while a big-ticket item, can still be cheaper than strip floorings and subfloor stumps. Remember, this is because the latter foundation is labour-intensive. 

Disadvantages of Concrete Slabs

concrete slab thickness standards

Let’s detail a few disadvantages of slabs. This isn’t to scare you off but merely to inform you. Remember, knowledge is power.

Pests can still make it into a home through the walls. Unfortunately, pests are a part of homes, and you can’t always avoid them entirely.

Any ducts for heating and cooling will run through the ground floor ceiling instead of the floor, which means the ceiling requires heavy insulation to retain the heat/cool air. This is an added cost to the project.

Furthermore, heating and cooling units will need to be installed on walls or on the roof, as they can’t be installed below the home.

And as we mentioned above, the slab can crack or lose integrity due to various issues. Any structural integrity issues are catastrophic and crazy expensive to fix. Tree roots, soil displacement, earthquakes and leaks can cause slab cracks.

concrete slab advantages

A Concrete Conclusion

In addition, we also shared some advantages and disadvantages of concrete slab foundations. Ultimately you need to understand all of this to become a successful property developer

We’ve shared the minimum slab thickness for residential buildings in this helpful article and seven things you need to know. By now, you’re informed about the different foundations and how they work. 

If you want to know more, reach out to the team at Little Fish. We’re always available to answer any questions you might have. 

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