How to Properly Prepare the Ground for a Steel Structure

When planning to install a steel structure—be it a garage, storage unit, or workshop—preparing the ground correctly is essential to ensure long-term stability, durability, and ease of assembly. A well-prepared foundation will help to protect the structure from shifting, moisture damage, and uneven weight distribution, ultimately extending its lifespan and performance.

There are several reliable methods for preparing the ground depending on the size and intended use of your steel structure. Below, we explore three common approaches, each with its own pros and cons, along with expert tips to guide your decision.

Method 1: Using Concrete Blocks (Best for Smaller Structures)

This is the most straightforward and economical approach. It’s ideal for smaller installations like garden sheds or lightweight garages, where minimal groundwork is sufficient.

Step-by-Step Guide:

1. Select and Mark Out the Site:
   • Choose a flat and stable area free from tree roots, waterlogging, or underground pipes.
   • Use stakes and string to mark out the footprint of the structure.
2. Level the Ground:
   • Clear away grass, topsoil, and debris.
   • Use a spirit level and rake to ensure the surface is as flat as possible.
3. Position Concrete Blocks:
   • Place large concrete blocks (paving slabs or pier blocks) at each corner and along key points under load-bearing sections of the frame.
   • Blocks should rest securely on compacted soil or a layer of gravel to avoid sinking.
4. Check for Alignment and Level:
   • Use a long spirit level or laser level to ensure all blocks are at the same height and evenly spaced.

Advantages:

• Cost-effective and quick to install.
• Requires minimal digging or concrete work.
• Suitable for temporary or mobile structures.

Considerations:

• May not provide adequate stability for large or heavy installations.
• Not ideal for areas prone to frost heave or shifting ground.

Method 2: Installing Concrete Footings (For Heavier or Permanent Structures)

This method is better suited to larger buildings, especially when the structure will remain in place for many years or needs to withstand strong winds or uneven terrain.

Step-by-Step Guide:

1. Design the Footing Layout:
   • Determine the number and location of footings based on the building’s support points.
   • Commonly used for corners, midpoints, and load-bearing sections.
2. Excavate Footing Holes:
   • Dig holes approximately 30–60 cm wide and deep (adjust depth based on local soil conditions and frost lines).
   • Ensure uniform spacing and alignment.
3. Install Reinforcement:
   • Place rebar cages or vertical steel rods inside each hole to strengthen the concrete and prevent cracking.
4. Pour Concrete:
   • Use a strong concrete mix (e.g. C25/30 grade).
   • Fill each hole with concrete, ensuring the top surface is level with the ground or slightly above.
   • Allow the concrete to cure for at least 48–72 hours before proceeding with construction.

Advantages:

• Provides superior structural support.
• Ideal for sloped or soft ground.
• Long-lasting and resistant to weather damage.

Considerations:

• Requires more labour, materials, and time.
• May need digging equipment for deeper holes.

Method 3: Laying a Full Concrete Slab (Most Durable and Professional)

A full concrete slab creates a completely level and load-bearing surface. This is the most robust foundation method and is commonly used for workshops, industrial units, and garages where flooring and internal equipment installation are needed.

Step-by-Step Guide:

1. Excavate and Clear the Area:
   • Remove turf, soil, and debris to a depth of 15–25 cm.
   • Ensure the ground is level and compacted using a plate compactor.
2. Add a Sub-Base Layer:
   • Spread 10–15 cm of crushed stone or Type 1 MOT hardcore.
   • Compact thoroughly to create a stable base.
3. Install Formwork:
   • Use treated timber or metal boards to build a perimeter frame around the slab area.
   • Ensure all corners are square and edges are level.
4. Lay Damp-Proof Membrane (DPM):
   • Roll out a polythene sheet across the base to prevent rising damp and moisture ingress.
5. Insert Steel Reinforcement:
   • Position rebar mesh or bars across the slab area, supported by spacers or chairs.
6. Pour and Level the Concrete:
   • Pour concrete evenly, working in sections.
   • Use a screed board and float to smooth the surface and eliminate air pockets.
7. Allow Proper Curing:
   • Cover the surface with plastic sheeting and allow curing for at least 7 days.
   • Avoid loading or installing the structure during this time.

Advantages:

• Creates a professional and permanent base.
• Suitable for heavy machinery and anchored structures.
• Reduces weed growth and prevents ground movement beneath the structure.

Considerations:

• Highest cost and labour requirement.
• Must plan for proper drainage and expansion joints if needed.

Additional Site Preparation Tips

• Drainage: Make sure water runs away from the foundation by sloping the surrounding ground gently (1–2% gradient).
• Anchoring: Once the structure is installed, use expansion bolts, anchor plates, or chemical fixings to secure it to the base.
• Legal Compliance: Always check with your local council whether planning permission or building control approval is required before construction.
• Frost Protection: In colder regions, foundations should be set below the frost line to prevent shifting or cracking.
• Access: Leave ample space around the structure for maintenance, vehicle access, or future modifications.