How to make a reliable formwork for concrete stairs

Building concrete stairs is a reliable and durable solution for both homes and outdoor spaces (for example, stairs to a house). However, to ensure that the final result is even, strong, and safe, it is very important to properly organize the formwork. The formwork acts as a “mold” that holds the concrete until it fully hardens. In this article, we will discuss how to make formwork for stairs by yourself, what materials will be needed, and what to pay special attention to in order to avoid mistakes.

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What is formwork for stairs and why is it important

Formwork for stairs is a temporary structure (most often made of wooden boards or plywood) that is shaped according to the desired configuration of the future steps. It must:

• Withstand the weight of the concrete (which can be a very significant load).
• Be airtight (so that the mixture does not leak and distort the shape).
• Match the dimensions (so that the stairs are of the required height and width).

Improperly made formwork can lead to deformation or cracking of the concrete, unevenness, and even emergency situations. Therefore, to create a strong and reliable structure, it is necessary to choose the right materials, make accurate calculations, and assemble everything carefully.

Materials and tools you will need

Materials

1. Wooden boards or plywood

   • It is recommended to choose boards with a thickness of at least 20–25 mm.
   • For the formwork of external stairs (stairs to a house), boards with moisture-resistant coating or plywood with increased moisture resistance will be suitable.

2. Self-tapping screws, bolts, and brackets

   • Used to fasten the boards together. Brackets will add rigidity to the structure.

3. Plastic film

   • Will prevent the concrete mixture from sticking to the wood and will also protect the wooden elements from moisture.

4. Sealant

   • Seals joints to prevent the mixture from leaking and to avoid voids.

5. Reinforcement (if necessary)

   • If you are making load-bearing concrete stairs or large flight structures, it is important to provide reinforcement for greater strength.

Tools

• Tape measure and square (for accurate measuring and marking).
• Pencil or chalk line (for marking).
• Screwdriver and hammer (for assembly).
• Saw or handsaw (for cutting wooden boards).
• Drill (for fastening braces to the floor or walls if necessary).
• Sander (optional, to remove splinters from the boards).

Step-by-step guide to making formwork for concrete stairs

1. Calculate the dimensions of the stairs

1. Determine the number of steps: for this, measure the total height (from the lower level to the desired upper mark) and divide it by the desired height of one step (usually 15–18 cm).
2. Determine the depth of each step: the recommended width of the tread (the horizontal part of the step) is usually between 25 and 30 cm.
3. Calculate the width of the flight: depending on whether the stairs will be internal or external, it is usually 80–100 cm (or more if space allows).

Tip: Draw a plan on paper with exact dimensions in advance, especially if you are making entrance stairs to a house or large stairs inside the house.

2. Prepare the base

• Clear debris and level the surface where the stairs will be located.
• If necessary, create a small sand-gravel cushion (for external stairs).

3. Cut the wooden boards

• According to your calculations, cut the boards into the required lengths.
• Remember that you need side parts (so-called “stringers” or side guides) and cross boards to form each step.
• Check the evenness of the cuts, as this affects the accuracy of the assembly.

4. Assemble the formwork

1. Create a frame: install the side boards at the edges of the future stairs.
2. Attach the cross boards (vertical panels for each step) to the sides. This is usually done using brackets and self-tapping screws.
3. Protect the inner surfaces: lay plastic film inside to prevent the concrete from sticking.
4. Use sealant at the joints of the boards to prevent the mixture from leaking.

Life hack: If possible, assemble most of the formwork on a flat surface (for example, on the floor), and then install it in place. This will allow you to better align the right angles and avoid misalignments.

5. Fix the structure in place

• Place the formwork where the stairs will be and level it using a level.
• Install braces (for example, wooden blocks) that will press against the floor, walls, or sturdy elements nearby. This is necessary so that the formwork does not “spread” under the pressure of the concrete.

6. Reinforcement (if necessary)

• If a concrete slab of significant thickness is planned, or if it is external stairs that bear special loads, be sure to use reinforcement.
• Position the reinforcement bars (diameter 8–12 mm) parallel to the length of the step. You can tie them together with wire at the top and bottom for greater strength.

7. Pouring concrete

1. Prepare the concrete mixture (or use a ready-made concrete mix). Usually, concrete of class not lower than C20/25 is used for stairs.
2. Carefully pour the formwork starting from the lower steps, gradually moving upwards.
3. Use a deep vibrator or tap the walls of the formwork with a wooden batten to remove air voids.
4. Level the surface of each step using a trowel or straightedge.

8. Care for the concrete and dismantling the formwork

• Give the concrete time to set: usually, initial strength is gained within 2–3 days, but full hardening can take up to 28 days.
• Regularly moisten the concrete (especially in hot weather), covering it with polyethylene to avoid premature drying and cracking.
• Dismantle the formwork carefully to avoid damaging the edges of the steps. Start with the cross boards and braces.
• After removing the formwork, if necessary, perform additional sanding, polishing, cladding, or finishing (for example, with tiles or wooden overlays).

Common mistakes when making formwork for stairs

1. Incorrect calculations of the height and depth of the steps
   • Leads to uncomfortable or even dangerous stairs.
2. Weak fastening of the side boards
   • The formwork may “spread,” and the concrete will lose the required shape.
3. Lack of sealing of joints
   • You risk having leakage of the mixture and deformed edges.
4. Too early dismantling of the formwork
   • The concrete must have sufficient strength; otherwise, the edges of the steps may crumble.
5. Ignoring reinforcement
   • If the stairs are large, cracks may appear without reinforcement, especially under increased loads.

Additional tips and recommendations

• Use a level and square at every stage of the formwork assembly.
• If you doubt your skills or do not have the appropriate tools, it is better to consult professionals.
• Before pouring the concrete mixture, carefully check all joints, corners, and braces.
• Take care of the concrete in a timely manner: optimal humidity ensures the necessary strength and long service life.
• After the concrete has fully set (28 days), you can additionally clad the stairs with tiles, wood, or other materials.

FAQ (Frequently Asked Questions)

1. Can I make formwork from plywood instead of boards?

Yes, using plywood is possible and often practiced. The main thing is to choose moisture-resistant plywood of sufficient thickness (from 15 mm and more) so that it can withstand the weight of the concrete and not deform.

2. How long does it take for concrete stairs to fully harden?

The initial strength of the concrete is gained in about 2–3 days. Full hardening (and achieving design strength) usually takes 28 days. During this time, it is advisable to protect the concrete from drying out and mechanical loads.

3. What type of concrete is best to use for stairs?

It is recommended to use concrete of grade not lower than C20/25. It is strong enough and suitable for most typical loads in private construction.

4. How to ensure a good appearance of the stairs after dismantling the formwork?

Use plastic film or special formwork lubricants. This will help prevent the concrete from sticking. After removing the formwork, you can level the unevenness by sanding, and later do a finishing decoration (with tiles, stone, etc.).

5. Is it necessary to do reinforcement for stairs?

For small internal stairs, in some cases, it is possible to limit oneself to concrete with fiber or a minimal amount of reinforcement. However, usually, for any monolithic concrete stairs, reinforcement is desirable as it significantly increases strength and durability.

6. Can I use leftover wood of different thicknesses for formwork?

It is preferable for the thickness of the boards to be the same, as differences in thickness can lead to misalignments and unevenness. If you still use different wood, carefully monitor the evenness of the structure.

7. Is a vibrator needed for pouring concrete into the formwork?

A vibrator allows for better compaction of the concrete and removal of air. If you do not have one, you can tap the walls of the formwork with a wooden batten or use other methods. However, for large and complex structures, it is recommended to use a vibrator.

Conclusion

Formwork for concrete stairs is one of the most important stages of construction, which affects the safety, convenience, and durability of the stairs themselves. Careful calculation of dimensions, proper selection of materials, and precise adherence to the step-by-step instructions will help you create quality stairs by yourself. If difficulties or doubts arise, you can always consult professionals, as quality work is a guarantee of reliability and comfort for many years to come.