Foundations and Walls for Your Garage Conversion

Converting your garage means you'll probably swap out that big garage door for a proper wall, perhaps with a new window or a doorway. This new wall needs a strong, reliable base.

Foundations for New Walls

When you add a new wall where your garage door used to be, you might discover the existing garage floor isn't strong enough. Most garages have a shallow concrete slab that won't adequately support a new wall that carries weight. So, you'll likely need to build a completely new foundation for this infill wall. To see how deep your current foundation goes, you can dig down next to it until you hit the very bottom.

A foundation's main job is to safely pass the weight of your building down into the ground. These are typically made from concrete, but their exact design changes based on what your project requires. You might hear terms like 'deep-fill' – where concrete fills almost the entire trench – or 'shallow-fill,' which uses just enough concrete to transfer the load effectively. If you're dealing with tricky ground, you could even need specialised foundations such as piles (long columns driven into the ground) or rafts (a large concrete slab covering the entire area). We highly recommend that you speak to a structural engineer or your local building control office for expert guidance on this.

Before you design your foundation, there are several key things you need to think about:

• The Type of Soil: The ground beneath your property needs to support the combined weight of the foundation and your new wall. Different soil types have varying strengths; some can hold more weight than others. For instance, clay soils can swell or shrink significantly as their moisture levels change. To prevent problems caused by this ground movement, foundations often need to be deeper than 0.75 metres.

• Nearby Buildings: Always consider how close your neighbours' properties are and what kind of structures they have.

• Trees: Tree roots absorb water from the soil, which can cause clay soils to dry out and shrink. This shrinkage can be very detrimental to foundations. If there are trees nearby, you might need to make your foundation significantly deeper. Be aware that cutting down or heavily pruning existing trees in clay soil areas can also cause the ground to swell over time, potentially harming foundations that are already in place.

• Drains and Sewers: If any drains or sewers fall within the '45-degree load spread area' (the zone where your foundation's weight is distributed), there's a risk they could get damaged. Generally, your foundation excavation should go down to at least the deepest point of the drain, sewer, or its trench.

• The Size and Material of Your New Wall: A wall that is heavier or larger will demand a stronger foundation. This means its depth and width will need to be adjusted based on how much weight the soil can bear and how thick your wall is.

• Existing Ground Condition: Your foundations must always rest on stable, undisturbed ground. Areas that have been previously filled in, perhaps to cover old drains or simply to make the site level, are not suitable. These spots usually contain soft, mixed soil and various debris, which makes them entirely unsuitable for pouring a foundation directly onto.

• Landfill Sites: If your property is built on a former landfill, you might need highly specialised foundations, such as piling (deep support columns) or a 'raft' foundation (a large, reinforced concrete slab). This is because landfill sites have very deep layers of unstable ground. A structural engineer is definitely the person to consult for advice in these situations.

Health and Safety Alert: When you're working in trenches, always be extremely careful. Trenches can collapse without warning, leading to severe injuries, so make sure you take all necessary precautions.

Walls Beneath the Ground (Your Substructure)

Whether you have a deep-fill or shallow-fill foundation, you might need to build a section of wall below the ground level. We call this the 'substructure.' Its purpose is to provide robust support for the walls you see above ground, known as the 'superstructure.' This substructure must be strong enough to hold up the superstructure and also durable enough to resist damage from frost and any sulphates found in the soil.