Question 1

How deep do I need to dig for a slab foundation?

Accepted Answer

A typical slab-on-grade foundation requires excavating 100–200mm (4–8 inches) for the slab itself, plus any additional depth for a gravel sub-base (usually 100–150mm). Strip footings beneath the slab go deeper — typically 300–600mm (12–24 inches) below frost line. Check your local building code for required frost depth, as this varies significantly by climate.

Question 2

What is working room in excavation?

Accepted Answer

Working room is the extra space you dig beyond the foundation dimensions to allow access for workers, formwork installation, waterproofing, drainage, and backfill compaction. A typical working room allowance is 600mm (2 feet) per side. This means the total excavation is the foundation size plus 1.2m (4 feet) in each direction. Some projects require more working room for deep foundations or complex waterproofing systems.

Question 3

How do I calculate foundation excavation volume?

Accepted Answer

The formula is: (Foundation Length + 2 x Working Room) x (Foundation Width + 2 x Working Room) x Depth. For example, a 12m x 10m foundation with 0.6m working room and 2.4m depth for a basement: (12 + 1.2) x (10 + 1.2) x 2.4 = 13.2 x 11.2 x 2.4 = 354.8 m³. Add strip footing volume separately if applicable.

Question 4

What is the difference between crawlspace and basement excavation?

Accepted Answer

A crawlspace excavation is typically 450–900mm (18–36 inches) deep — just enough space to access plumbing and utilities under the house. A full basement excavation is 2.1–2.7m (7–9 feet) deep to create a usable living or storage space. The volume difference is dramatic: a basement dig for the same footprint can be 4–6 times more material than a crawlspace.

Question 5

How many truck loads for a foundation dig?

Accepted Answer

This depends on the foundation type and size. A slab foundation for a typical house (12m x 10m at 150mm depth) produces about 22 m³ bank volume or roughly 30 m³ loose — about 3 standard dump truck loads. A full basement for the same footprint produces about 355 m³ bank volume or 480 m³ loose — about 45 truck loads. Always use loose volume (after swell) for hauling estimates.

Question 6

Do I need footings for a slab foundation?

Accepted Answer

Yes, most slab foundations require strip footings (also called thickened edges or grade beams) around the perimeter and under load-bearing walls. These extend below the slab depth to the frost line and carry the structural loads to the soil. Footing requirements vary by building code, soil type, and structural loads. This calculator can estimate the additional footing excavation volume separately.

Foundation Excavation Calculator

Results

How It Works

Foundation Types

Working Room

Strip Footings

Frequently Asked Questions

Related Calculators

More Excavation & Earthworks Calculators

Related Guides