How to pour a concrete slab for a patio

🧱 Concrete Patio Slab Guide for Australian DIYers

What is involved in pouring a concrete patio slab in Australia?

Pouring a concrete slab for a patio is a practical outdoor upgrade that can create a durable base for entertaining, paving, or outdoor furniture. For DIY homeowners, success comes down to proper site preparation, correct formwork, reinforcement, and finishing — not just the pour itself.

This guide covers the full process for a typical ground-level patio slab, using Australian methods and terminology. For structural slabs, reactive soils, steep sites, or slabs attached to a dwelling, engineering advice and compliance with AS 3600 and local council requirements may be necessary.

What skill level is required to pour a concrete patio slab?

Intermediate — Estimated time: 2–4 days including excavation, setup, pouring, and curing

What tools are essential for pouring a concrete patio slab?

What materials are needed for a concrete patio slab?

Safety Requirements

Step-by-Step Guide

Step 1: Plan the slab size, height, and fall

1️⃣ Set Out the Patio Area

Mark out the patio with pegs and string lines. Check the finished size carefully, including clearance from fences, paths, and house walls.

A typical patio slab for DIY work is often 100 mm thick. For light residential use, this is common, but soil conditions and load requirements matter. If the patio will carry heavy outdoor kitchens, spas, or masonry features, get engineering advice.

Set the finished slab height so water drains away from the house. A practical fall is around 10–15 mm per metre. This is critical for outdoor slabs in Australian conditions, especially during storm events.

💡 Keep the slab at least below weep holes and damp-course level when adjacent to brick veneer walls.

Step 2: Excavate and prepare the base

2️⃣ Excavate to Correct Depth

Excavate the area to allow for:

Remove all grass, roots, soft spots, organic matter, and loose soil. The base must be firm and uniform.

Compact the subgrade first, then spread the road base in layers and compact again with a plate compactor. The better the compaction, the lower the chance of future settlement and cracking.

If the site is wet, clay-heavy, or poorly drained, improve drainage before pouring. On problem sites, local practice and engineering advice are important.

💡 Plate compactors are usually better hired than purchased for a one-off slab. Hire is more economical unless you do regular landscaping or paving work.

Step 3: Build and brace the formwork

3️⃣ Install Formwork to Finished Levels

Use straight formwork timber such as 90 x 45 mm if it matches your slab depth, or deeper boards where needed. Set the forms to the exact finished height of the slab.

Brace the forms with pegs at regular intervals so they cannot bow or move under concrete pressure. Check levels, diagonal measurements, and falls before continuing.

Apply release oil lightly to the inside faces of the forms so they strip cleanly later.

If the slab meets a wall, post, or existing concrete, install compressible foam joint material to allow movement and reduce stress.

💡 Spend extra time here. Poor formwork causes more DIY slab problems than the actual pour.

Step 4: Install vapour barrier and reinforcement

4️⃣ Place Membrane and Mesh Correctly

For many external patio slabs, a vapour barrier may not always be essential, but if required by site conditions or design, place 200 micron polyethylene over the compacted base. Lap joints sufficiently and keep it flat.

Lay reinforcement mesh such as SL72, or as specified, and support it on bar chairs so it sits within the slab — not on the ground. The mesh should finish roughly in the upper half of the slab depth, with proper cover maintained.

Overlap mesh sheets according to manufacturer recommendations and tie them securely with tie wire.

Do not allow the mesh to be dragged to the bottom during the pour. Reinforcement only works if it stays at the correct height.

Reference structural reinforcement requirements to AS 3600 where applicable.

Step 5: Order, mix, and prepare for the pour

5️⃣ Get the Concrete Delivery Right

For small slabs, you may use a mixer and a general-purpose mix ratio of 1:2:3. For higher-strength or structural applications, use 1:1.5:2.5 or, preferably, order ready-mix to the required specification.

For most patio slabs beyond a very small area, ready-mix is easier, faster, and gives more consistent results. Confirm:

Have all tools, labour, water, and finishing gear ready before the truck arrives. Concrete work moves quickly once placed.

💡 If the slab is more than a small DIY size, ready-mix and a hired concrete vibrator or extra labour can be cheaper than battling multiple hand-mixed batches and inconsistent finish quality.

Step 6: Pour and place the concrete

6️⃣ Place Concrete Evenly and Quickly

Pour the concrete into the forms starting at one end and work progressively across the slab. Spread it with shovels and rakes rather than overworking it.

Keep the placement even and avoid dumping large heaps in one spot, which can displace mesh and formwork. Lift mesh gently into position as required, but do not leave it unsupported.

Fill to slightly above form level so screeding can strike off excess. If the slab is large, work in sections and maintain a consistent pace.

Do not add excessive water to make concrete easier to place. This weakens the slab and increases shrinkage cracking.

Step 7: Screed, float, edge, and finish

7️⃣ Finish the Surface Properly

Strike off the slab using a straight screed board resting on the forms. Move it in a sawing motion while pulling across the slab.

After screeding, use a bull float or darby to flatten ridges and close the surface. Do not overwork the slab while bleed water is present.

Once the surface firms up and bleed water has evaporated, finish the edges with an edging tool. Then choose your surface finish:

For most patios, a broom finish is a smart choice because it improves grip in wet weather.

💡 Outdoor concrete should usually have a non-slip finish. Smooth steel-trowelled patio surfaces can become slippery after rain.

Step 8: Cut control joints

8️⃣ Control Cracking with Planned Joints

Concrete will crack somewhere, so it is better to control where it happens. Install control joints at suitable spacing, commonly at panels close to square in shape.

A general guide is spacing joints at about 24–30 times slab thickness in mm, subject to slab layout and conditions. For a 100 mm slab, that often means joints around 2.4–3 m apart.

Joints can be tooled during finishing or saw-cut after the slab has hardened enough, usually within 6–24 hours depending on weather.

Joint depth should generally be about one-quarter of slab thickness.

Step 9: Cure the slab

9️⃣ Cure for Strength and Durability

Curing is essential. Once the surface is hard enough, apply curing compound or cover the slab with plastic sheeting to retain moisture.

Keep the slab curing for at least 7 days where possible. Proper curing improves strength, reduces dusting, and lowers the risk of surface cracking.

Avoid heavy traffic too early. Light foot traffic may be possible after 24–48 hours, but furniture and heavier loads should wait longer, depending on weather and concrete strength gain.

Hot, windy Australian conditions can dry the surface too fast, so plan pours early in the day and protect fresh concrete from rapid moisture loss.

Step 10: Strip forms and tidy the edges

🔟 Remove Formwork and Finish Surrounds

After the slab has gained enough strength, strip the formwork carefully without damaging the edges. Backfill around the slab if needed and grade surrounding ground so water drains away.

Check edges, joints, and surface finish. If you plan to seal the slab, wait until the concrete has cured fully and follow product directions.

A finished patio slab can now be left as plain concrete, sealed, tiled with compliant preparation, or used as the base for outdoor coverings.

Common Mistakes to Avoid

Pro Tips

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