How to build concrete formwork: A step-by-step D.I.Y. guide

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.

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.

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.

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.

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.

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.

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.