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An image of a leveled rectangular area with wooden formwork, secured by stakes and red string.

Overview

If you're planning to pour a concrete slab, setting up sturdy and accurate formwork is essential. Formwork acts as a mould that holds the wet concrete in place until it cures, ensuring a smooth and level finish. Whether you're laying a foundation, a garden path, or a garden shed base, follow these steps to create a reliable formwork structure.

Safety tip: Always wear the appropriate safety equipment (safety glasses, gloves, ear muffs and a mask, for example) and always follow the instructions for the product or equipment.

Steps

1Mark out the area

Start by measuring the area where your formwork will be built. Use a tape measure to determine the length and width, ensuring you have the correct dimensions. Once measured, mark the corners of the layout using spray marking paint. If you're creating a 4m x 2.2m rectangle, spray a mark at each corner where the formwork will sit. A clear, accurate layout helps prevent mistakes when setting up the timber boards.

A person wearing a red shirt using a tape measure to mark the area with blue spray paint. ]

2Establish setback stakes

Place stakes roughly one metre away from each corner mark. This offset is known as a setback, and it serves as a guide for your formwork without getting in the way of the actual workspace. Check the distances between stakes that are in line with each other to ensure they are equal—this helps maintain a square layout.

A Bunnings Team Member is placing a setback stakes one metre away from each corner mark using a hammer.

3Run a string line for guidance

Take a string line and run it between the stakes, outlining the shape of your formwork in construction. This will serve as your visual guide to ensure the boards are positioned correctly. Ensure the string is tight and level, as this helps keep everything straight and even. To check that everything is square, measure diagonally between opposite corners—both measurements should be the same.

A person wearing red shirt is running a string line between the stakes to outline the shape of your formwork.

4Check and adjust ground levels

Before placing your timber, check the ground levels using a spirit level. If one side is higher than the other, you may need to remove soil using a shovel if the ground is too high or add crushed rock to raise lower areas and create a stable base. If you’re working near a house or other structures, consider allowing for water runoff by ensuring the formwork slopes slightly away from buildings. If your formwork sits between finished surfaces, such as an existing slab or driveway, match the working height rather than aiming for a perfectly level frame.

A Bunnings Team Member is checking and adjusting ground levels using a spirit level.

5Cut and prepare the timber

Now, select your timber boards. Typically, 90mm x 35/45mm timber is suitable for most concrete formwork, but this may vary based on your slab size and thickness. Cut the timber to the required lengths according to your layout. Ensure the boards are straight and free from defects, as warped timber can cause uneven concrete results.

A person in a red shirt is using a Ryobi circular saw to cut timber to the required lengths according to your layout. 

6Assemble and secure the formwork

With your timber pieces ready, position them inside the workspace, following the string line as a guide. Join the corners using treated pine screws and an impact driver to form a solid frame. Double-check alignment to ensure each board sits flush with the string line. If needed, adjust the positioning before securing everything in place.

A person is using a Ryobi power drill to fasten two timber pieces together following the string line as a guide.

7Reinforce the formwork

To ensure your concrete formwork holds its shape under the weight of wet concrete, reinforce it with steel posts at the corners and at regular intervals along the perimeter. Drive steel posts into the ground just outside the formwork and secure the timber boards to the posts using hex head screws. Make final adjustments to ensure the boards are level, straight, and firmly supported.

A Bunnings Team Member is using a hammer to secure concrete formwork, reinforcing it with steel posts to maintain its shape under wet concrete.

8Now that you've built formwork

Health & Safety

Asbestos, lead-based paints and copper chromium arsenic (CCA) treated timber are health hazards you need to look out for when renovating older homes. These substances can easily be disturbed when renovating and exposure to them can cause a range of life-threatening diseases and conditions including cancer. For information on the dangers of asbestos, lead-based paint and CCA treated timber and tips for dealing with these materials contact your local council's Environmental Health Officer or visit our Health & Safety page.

When following our advice in our D.I.Y. videos, make sure you use all equipment, including PPE, safely by following the manufacturer’s instructions. Check that the equipment is suitable for the task and that PPE fits properly. If you are unsure, hire an expert to do the job or talk to a Bunnings Team Member.