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How to make a concrete slab foundation for a garden room

So you have decided to build your dream garden room. You’ve decided on the size and picked the perfect position in the garden to enjoy the perfect sunsets relaxing in the garden room of your dreams. Before you can sit back and relax you have some work to do. It’s time to create the base for your garden room.

When it comes to creating a solid base to support a garden room you don’t get much more solid than concrete. Creating a concrete slab foundation for a timber floor is an excellent way to ensure that your garden room has a strong and stable base that will last for many years.

Here is a guide on how to create a concrete slab foundation for a garden room timber floor:

Step 1: Site Preparation

The first step is to prepare the site where you will be building the concrete slab foundation. This involves clearing the area and levelling the ground. Use a spirit level to check that the ground is level, and make any necessary adjustments by adding or removing soil.

Step 2: Mark Out the Area

Once the ground is level, use wooden pegs and string to mark out the area where the concrete slab foundation will be poured. Make sure that the area is slightly larger than the garden room, allowing for a small gap around the edges.

Step 3: Excavate the Area

The next step is to excavate the area where the concrete slab foundation will be poured. Dig down to a depth of around 150mm, ensuring that the bottom of the excavation is level. Remove any rocks or debris from the area.

Step 4: Install a Damp Proof Membrane

Once the area has been excavated, install a damp proof membrane over the entire area. This will prevent moisture from seeping up into the concrete and causing it to crack or crumble. Ensure that the membrane overlaps the edges of the excavation by at least 150mm and fix it in place using a suitable adhesive.

Step 5: Install Formwork

The next step is to install formwork around the edges of the excavation. This will contain the concrete and ensure that it is poured to the correct depth and level. Use wooden planks or boards to create the formwork, ensuring that they are securely fixed in place.

Step 6: Add Hardcore

Once the formwork is in place, add a layer of hardcore, such as crushed stone or gravel, to the bottom of the excavation. This will help to provide a stable base for the concrete and prevent it from settling or cracking.

Step 7: Mix and Pour Concrete

The next step is to mix the concrete and pour it into the excavation. Follow the instructions on the bag of concrete mix to ensure that you get the right consistency. Pour the concrete into the excavation, ensuring that it is level and fills the entire area.

For a single-story lightweight garden room in Scotland, a standard concrete mix with a compressive strength of 20 MPa should be sufficient for the foundation slab. You can achieve this by using a mix with a ratio of 1:2:3 (one part cement, two parts sand, and three parts coarse aggregate).

It is also recommended to add a plasticizer to the mix, which will improve the workability of the concrete and reduce the amount of water needed. This will help to prevent shrinkage cracks and improve the overall durability of the slab.

In addition, it is important to ensure that the concrete mix is suitable for the local climate and soil conditions. In Scotland, where the climate can be cold and damp, it is recommended to add an air-entraining agent to the mix to improve its resistance to freeze-thaw cycles.

It is always best to consult with a building professional or structural engineer to determine the appropriate concrete mix for your specific project. They can help you calculate the required strength and thickness of the foundation slab, and ensure that it meets local building codes and regulations.

Step 8: Smooth and Level the Concrete

Once the concrete has been poured, use a float or trowel to smooth and level the surface. Work from one end to the other, ensuring that the surface is even and free from any lumps or bumps.

Step 9: Allow to Cure

Finally, allow the concrete to cure for at least 24 hours before removing the formwork. Once the concrete has cured, you can lay the timber floor directly on top of the slab foundation.

Materials Required:

  • Concrete mix
  • Damp proof membrane
  • Formwork
  • Hardcore

Tools Required:

  • Spirit level
  • Hammer
  • Saw
  • Drill
  • Screwdriver
  • Float or trowel

In conclusion, creating a concrete slab foundation for a timber floor requires careful planning and attention to detail. By following the steps outlined above, you can create a strong and stable base for your garden room that will provide a comfortable living space for many years to come.