When installing a transportable building, what sits underneath it matters just as much as the building itself. A properly designed footing layout provides the structural stability needed to support the building safely over time. Without the correct footings in place, issues such as movement, uneven floors, or structural stress can occur.

While every site is different, there are several key factors that engineers and installers assess before a building is delivered. Below are four of the main considerations we look at when determining the footing layout for a transportable building in WA.

Step 1: Assessing the Wind Region and Terrain Category

One of the first factors engineers assess when determining footing layouts is the wind region of the installation site. In Western Australia, sites are classified as either cyclonic or non-cyclonic based on the wind speeds a building may be exposed to. Higher wind loads mean stronger anchoring systems and carefully designed footing layouts are required. For a deeper explanation, see our guide on cyclonic vs non-cyclonic transportable buildings.

Two key elements are assessed:

• Wind Region
• Determines the maximum wind speeds a building must withstand
• Cyclonic regions require stronger tie-down systems and engineered footing layouts
• Non-cyclonic regions still require anchoring, but design requirements are typically less extreme

• Terrain Category

• Open terrain, such as farmland, coastal land, or remote sites, allows wind to travel faster and exert greater pressure on the building
• Built-up areas with trees, structures, or natural barriers can reduce wind exposure

By assessing both wind region and terrain category, engineers can calculate the correct footing spacing, tie-down strength, and anchoring system needed to safely secure the building.

Step 2: Soil Classification and Ground Conditions

Another critical factor in determining footing layouts is the soil classification and ground conditions at the site. Different soil types behave differently under load, which affects how footings must be designed.

Engineers typically assess soil based on Australian classifications such as:

• Class A – Stable sand or rock with minimal movement
• Class S – Slightly reactive soils with minor ground movement
• Class M – Moderately reactive clay soils
• Class H – Highly reactive soils that expand and contract significantly

These classifications help determine the bearing capacity of the ground, which influences how deep and wide the footings need to be. Poor ground conditions may require deeper or reinforced footings to ensure the building remains stable.

In some cases, a geotechnical report may be required to properly assess the soil conditions before installation.

Correct site preparation plays a major role in preventing structural movement. Our guide on how to prepare your site for donga installation explains what needs to be done before delivery.

Step 3: Building Size and Structural Load Points

The size and design of the transportable building also play a major role in determining the footing layout. Different buildings create different structural loads, which must be properly supported by the foundations underneath.

A small, single transportable office typically places less weight on the ground and may only require a limited number of footings positioned at key structural points. Larger buildings, however, require a more detailed layout.

Engineers design footing plans based on the load-bearing points of the building’s structural frame, ensuring weight is distributed evenly across the foundations.

For example:

• Single-site office or small donga – fewer footings positioned under primary structural beams
• Multi-module office complex – significantly more footings to support the combined weight and structural joins between units

By aligning footings with the building’s structural load points, structural engineers ensure the building remains stable and level over time.

Step 4: Tie-Down and Anchor Bolt Positioning

Once the footing layout is determined, the next critical step is ensuring the tie-down and anchor bolt positions are correct. Tie-down systems are designed to secure the transportable building to the concrete footings, preventing movement caused by wind loads or environmental forces.

Each transportable building has engineered tie-down points built into the structural frame. The anchor bolts installed in the concrete footings must align precisely with these points so the building can be safely fixed into place during installation.

This becomes especially important in cyclonic regions, where buildings must be capable of resisting significantly higher wind forces.

Accurate footing layouts ensure installation teams can position and secure the building efficiently on arrival.

Common Mistakes That Cause Installation Delays

Even with engineered drawings in place, installation delays can occur when footing layouts are not followed correctly. A few common mistakes can quickly slow down the delivery and installation process:

• Incorrect footing spacing that does not match the engineered layout
• Footings poured in the wrong locations, making it impossible to align with structural load points
• Inadequate curing time, where concrete has not fully set before installation
• Missing or incorrectly positioned anchor bolts needed for the tie-down system
• Poor site preparation, such as uneven ground or restricted access for delivery equipment
• Ignoring engineer drawings and using assumed measurements instead

Careful planning and following the approved footing plan helps avoid unnecessary delays, extra costs, and rework when the building arrives on site.

Getting your Footing Layout Right

Footing layouts aren’t guesswork. They’re determined through careful engineering that considers wind regions, soil conditions, building size, and the precise positioning of tie-down systems.

At Instant Transportable Offices, we help clients across WA plan and install transportable buildings with the correct footing layouts from the start. Our team supplies demountable buildings for hire and demountable buildings for sale, along with guidance on site preparation, delivery, and installation so everything runs smoothly.

If you’re planning a new transportable office, donga, or modular building, talk to our team today. Call 08 9406 6600 or explore our range of transportable buildings to find the right solution for your project.