How to Lay a Concrete Slab
– From Idea to Done

If you’re looking to create a sturdy base for an outdoor project, you can’t beat a concrete slab. Durable, versatile and cost-effective, they’re the perfect foundation on which to build and grow your alfresco dream.

Whether you want to pour a foundation for a shed or greenhouse, create a patio with a modern aesthetic or build a heat-strong base for your fire pit, BBQ or pizza oven, a concrete slab is always a great choice.

The good news is laying one is a job you can DIY yourself. If this is your plan, but you’re after some more info to get started, you’ve come to the right place.

As well as showing you how to lay a concrete slab, Jack will help you understand what approvals you might need, work out how much it will cost and offer advice on how to pour with perfection.

This is always the first question you should ask, and the answer is: maybe.

Sorry, not as helpful as Jack usually is, but whether or not you need approval depends on where you live in Australia or New Zealand and the type of project you’re laying a concrete slab for.

Because of this, make sure you get in touch with your local council first.

These days, council approval is almost always required for a shed slab and larger areas such as carports. But you likely don’t need permission for smaller projects, such as creating a garden fire pit or BBQ area.

Saying that, it’s always better to check than get caught out and risk council making you remove it after all your hard work.

As a general rule of thumb, a concrete slab for an outdoor DIY project should be 100mm (10cm or 4 inches). This is strong enough to take the weight of things like garden sheds, BBQs and fire pits.

If you’re laying a concrete slab under a carport or as a driveway, you need to up this to 150mm, so it’s tough enough for cars to drive and park on.

Importantly, if you’re laying gravel first to improve drainage, which we recommend, the concrete should be 100 mm (or 150 mm), and the gravel should be an extra 50 mm underneath that.

How long is a piece of string?

The cost of a concrete slab depends on the size of your project. On top of buying the concrete mix itself, you need to fill your trolley with several other products, materials and concreting accessories.

Let’s look at the concrete first.

Concrete costs around $8 per 20kg bag at leading hardware stores. The total cost of a slab depends on the size of the area you wish to concrete.

Time for some maths.

Concrete is measured in cubic metres, or m3 for short. As a rough guide, 1.1 cubic metres at a depth of 10mm equals 108 bags of concrete.

To work out exactly how much you need for your garden project, multiply the length x width x thickness of your intended slab.

If you’re pouring a concrete slab for an area 2 metres x 4 metres x 100 mm, this equals 0.800 cubic metres, which is 93 x 20kg bags.

Based on these calculations, the concrete for your slab would cost roughly $744.

Numbers not your thing? Don’t worry; Jack’s got your back. This handy Cement Australia calculator will do the sums for you. Note that the calculation allows for 5 per cent wastage and is rounded up to the nearest whole bag.

Laying a concrete slab requires proper preparation. For this, you’ll need three key garden materials: a weed mat, gravel and two-way edging.

You’ll also need specialist concreting products, including Reo Mesh, Reo Bars and Bar Chairs, to reinforce your slab and avoid cracks. Not only do cracks look ugly, but they compromise its stability. And don’t forget the Rod Tying Tool and Rod Ties.

Once again, the exact cost of these materials will depend on the size of your project. As a rough guide, an 1800 x 1000 x 200 x 7mm reo mesh costs around $50.

Other tools you need include a shovel, wheelbarrow, hand trowel, edging trowel, spirit level, drill, hammer, mattock, tape measure and string. Hopefully, you’ve already got most in your garage. If not, ask a friend.

While laying a concrete slab will set you back a few hundred dollars or more, think of all that money you’ll save in man (or woman!) power by doing it yourself.

Before you pour your concrete slab, first prepare the area.

This includes marking it, digging the hole to the right dimensions, fitting the edging, filling it with gravel then laying and tying the reo mesh and bars.

Next, it’s time to mix and pour. Premixed bags are perfect for DIY projects and simple to use. Follow the packaging instructions, use your wheelbarrow and shovel as your DIY concrete mixer, and then pour into the hole.

Start in one corner and work your way backwards. Consider using a concrete vibrator to avoid any air bubbles.

Finally, smooth the concrete down with your shovel, then screed and level. If you want a smooth finish, use a float tool.

We’re often asked, ‘Can I pour concrete slabs in sections?’

Well, our answer is yes, you sure can. Pouring concrete in sections makes it easier if you’re doing the project solo. It’s strenuous work. But you need to do it pretty quickly as the concrete will start setting, making blending more difficult.

So, don’t have a cup of tea in between sections!

Now you know everything you need to know about how to lay a concrete slab, it’s time to get started on that garden project.

Once approvals have been checked, the next stop is a trip to your nearest Bunnings or local supplier to pick up your Jack products.

If you’re happy with how your slab turns out, why not share it on socials and tag @meetdiyjack?