Learn What Determines Concrete Footing Depth, Why Founding Level Matters, How Soil And Site Conditions Change The Answer, And What To Check Before Excavation And Concrete Placement. This Guide Covers Strip Footings, Pad Footings, Piers And Residential Footing Planning In Australia.
A footing can have a depth below finished ground, a concrete footing thickness and a required founding level in suitable material. These are related, but they are not the same thing. The correct dimensions come from the footing design and actual site conditions.
Footing depth depends on the structure, the site and the soil. Use the approved design and actual excavation conditions together.
Wall, roof, floor, column and concentrated loads influence footing size and depth.
Stable, reactive, soft, filled or variable ground can require different footing systems.
The footing must bear on the material and founding condition required by the design.
Strip, pad, pier, bored pier and stiffened raft details use different geometry.
Sloping sites, cut and fill, retaining walls and changes in grade alter excavation depth.
Water, leaking services, poor drainage and reactive soils can affect footing performance.
Excavations, trees, services, boundaries and existing footings can influence the design.
Use the applicable plans, engineering, NCC requirements and referenced standards.
There is no single concrete footing depth that is correct for every Australian project. The depth you see on a drawing is normally part of a complete footing system that also specifies width, thickness, reinforcement, concrete strength, founding condition and detailing. Copying one dimension from another house, shed, fence or retaining wall can produce a footing that is too shallow, unnecessarily deep or simply the wrong type for the site.
A useful way to think about footing depth is to separate three different questions. First, how far below the finished ground or slab level must you excavate? Second, how thick is the concrete footing itself? Third, what material must the bottom of the footing bear on? These dimensions can overlap in a simple footing, but they should not be treated as interchangeable.
Footing depth often describes how far down the excavation or bottom of footing is from a reference level. Footing thickness describes the vertical thickness of the concrete footing itself. A footing can therefore be founded well below ground while the concrete base has a much smaller designed thickness.
Depth Below Ground ≠ Concrete Footing Thickness ≠ Founding RequirementThe founding level is the elevation at which the footing bears on the supporting ground or other designed founding material. On site, excavation should continue only as required to reach the specified founding condition. If the excavation exposes soft, loose, wet, uncontrolled fill or other material that does not match the design assumption, the correct response is normally to stop and obtain direction rather than simply pouring deeper.
This is one reason a footing depth shown as a neat drawing dimension can change in the field. The design may nominate a minimum or expected level, but the actual excavation has to be checked against the material encountered and the project documentation.
For many Australian residential projects, site classification and footing system design are closely connected. Standards Australia describes AS 2870 — Residential slabs and footings as covering site classification and the design and construction of footing systems for single dwelling houses and related structures within its scope. The standard is not a substitute for engineering on projects that fall outside its provisions.
The National Construction Code also sets building requirements and references standards and accepted construction solutions. Use the current National Construction Code, the project documentation and the requirements of the relevant state, territory or local approval process.
Soils do not all carry loads or react to moisture in the same way. Some sites are relatively stable; others contain reactive clays that move as moisture changes; some have uncontrolled fill, soft ground, erosion, old excavations or highly variable layers. The footing system is selected so the building and ground can interact within acceptable limits.
For this reason, “dig to firm ground” is not enough as a technical instruction. Ground can feel firm under a shovel yet still be unsuitable or inconsistent. Where a soil report, site classification or geotechnical recommendation exists, use it together with the footing design.
Sloping ground can create different depths along the same wall. A stepped strip footing may be used where shown on the design, while other projects use piers, retaining elements or suspended floor systems. Do not simply maintain one excavation depth below the existing ground surface if that conflicts with the design level or produces unsupported steps.
On cut-and-fill sites, pay particular attention to whether the footing is founded in natural ground, controlled fill or a combination. Transitions between materials can create differential movement and may require specific detailing.
“Frost depth” is a common footing rule in colder overseas building guidance, but it is not a useful universal shortcut for Australian residential construction. Local climate, site classification, the applicable code pathway and the engineered footing system should control the design. In alpine or unusually cold locations, project-specific requirements may still address frost or freeze-thaw effects.
Different footing systems use different depth, width and reinforcement relationships.
Continuous strip footings support walls or lines of load. Depth can vary with site class, wall type, load, founding material and any required stepping.
Pad footings support concentrated loads such as posts or columns. Their plan size, thickness and embedment are determined together.
Pier depth may be controlled by the need to reach a particular stratum, resist movement or transfer load below weaker near-surface material.
Residential slab systems may use edge beams and internal ribs rather than separate shallow strip footings. Beam depth is part of the engineered slab system.
Before excavation, read the footing plan, sections and schedules. Confirm width, depth, reinforcement, concrete grade, steps, piers, starter bars, service penetrations and any notes about founding material. Check which dimensions are minimums and which are fixed.
Set the excavation from a known reference level rather than measuring independently from uneven soil. A laser level, builder's level or other suitable method helps keep footing bottoms and steps consistent with the design.
Footing trenches and pier holes can intersect electrical, gas, water, communications and drainage services. Use the appropriate service-location process, including Before You Dig Australia where relevant, and follow site-specific safety procedures before mechanical excavation.
Excavate accurately so the footing bottom is not churned into loose soil. Avoid unnecessary over-excavation. If machinery disturbs the final founding layer, trim and prepare the bottom using the method required by the project.
Compare the exposed material with the expected site conditions. Unexpected fill, soft spots, water, old trenches, organic material or highly variable layers should be reported. The correct solution might be deeper excavation, engineered fill, larger footings, piers, drainage work or a revised design — but that decision should come from the appropriate project authority.
Standing water can soften some soils, collapse trench sides and interfere with inspection or concrete placement. Maintain site drainage and prevent runoff from entering open footing trenches. If groundwater or seepage is persistent, use a site-specific dewatering and stability plan.
Over-wide excavation increases concrete volume and may change how the footing behaves or how reinforcement is positioned. Narrow, unstable or collapsed sides can also compromise cover and geometry. Repair excavations according to the project instructions before reinforcement is installed.
Reinforcement should not sit directly on soil unless the design explicitly provides for that condition. Use the required chairs, spacers and supports so concrete cover and bar position are maintained during the pour.
Many projects require an inspection or approval before footings are poured. The inspector, engineer, certifier or builder may check depth, founding material, reinforcement, dimensions, services and cleanliness. Do not cover work before required inspections are complete.
Remove loose soil, rubbish and water; recheck trench dimensions and reinforcement; confirm access for the concrete truck or pump; and make sure no rain, traffic or excavation activity has damaged the footing bottom since inspection.
| Site / Design Condition | Why It Matters | What To Do |
|---|---|---|
| Loose or uncontrolled fill | May not provide reliable long-term support | Follow geotechnical or engineering direction |
| Soft or wet near-surface soil | Can deform or lose bearing support | Stop and verify the required founding solution |
| Reactive clay | Moisture-related movement can affect foundations | Use the footing system designed for the site class |
| Sloping site | Finished levels and founding strata vary across the building | Use stepped or engineered details shown on the plans |
| High concentrated load | Column or post loads can require larger/deeper foundations | Use structural footing dimensions |
| Nearby excavation or retaining wall | Can change support conditions and soil stability | Obtain project-specific structural/geotechnical advice |
| Unexpected old trench or service | Previously disturbed ground may cross the footing | Assess and repair using an approved method |
| Tree or moisture influence | Can alter soil moisture and movement patterns | Use the site and footing design recommendations |
Do not casually shovel loose excavated soil back into the trench. Loose backfill can compress after the footing is built. The repair method depends on the design and site conditions and may involve approved compacted material, lean concrete, additional structural concrete or another engineered solution.
Remove standing water and assess whether the founding material has softened, eroded or become disturbed. Some soils recover after drying and recompaction; others may need to be removed or treated. If the founding condition has changed, obtain direction before concrete placement.
Deeper is not automatically better. Extra depth can increase excavation risk, concrete cost, soil disturbance and interaction with services or neighbouring structures. A footing should be as deep as the design and founding conditions require — not arbitrarily deeper.
Use the observed ground to verify the design assumptions, not to invent a new footing size on site.
May provide a straightforward founding condition when it matches the design and site classification.
Footing systems are designed to manage expected movement; moisture management around the building remains important.
Confirm whether fill is controlled, tested and suitable. Unknown fill should not be assumed to provide reliable bearing.
Investigate drainage, groundwater and loss of support before concrete is placed.
| Mistake | Why It Is A Problem | Better Approach |
|---|---|---|
| Using one universal depth | Ignores soil, load and footing system | Use the actual design and site classification |
| Confusing depth with thickness | Can lead to the wrong excavation or concrete section | Read footing sections and dimensions carefully |
| Pouring onto soft fill | Can cause settlement or movement | Verify suitable founding material first |
| Backfilling over-excavation loosely | Creates compressible material under the footing | Use an approved repair method |
| Ignoring water in trenches | Can soften soil and disturb concrete placement | Control water and recheck the founding surface |
| Skipping required inspection | Non-compliant work may be covered before verification | Complete inspections before concrete |
| Changing depth around services without design review | Can interrupt the structural load path | Coordinate service crossings with the project team |
| Assuming deeper is always safer | Adds risk/cost and may create new site problems | Build to the required depth and founding condition |
For the work around a footing, see How to Prepare Ground for Concrete, How to Build Concrete Forms, the Concrete Foundation Calculator, Concrete Footing Calculator, Concrete Volume Calculator and Reinforcing Steel Calculator.
Quick Answers About Footing Depth, Founding Level, Soil, Excavation And Residential Foundation Planning.
There is no single universal depth. Use the footing design, site classification, founding requirements and applicable building documentation for the actual project.
No. Depth commonly refers to the position of the footing below a reference level, while thickness is the vertical thickness of the concrete footing itself.
It is the level where the footing bears on the supporting material required by the design. The actual material encountered in excavation should match the project assumptions.
Do not use one generic dimension as a universal rule. A dimension that is suitable for one footing or site can be unsuitable for another.
Many residential and structural projects rely on site classification or geotechnical information. Whether testing is required depends on the project, approval pathway and design.
Stop and obtain direction from the appropriate project professional. Do not assume that simply pouring deeper or adding loose gravel is acceptable.
Standing water and softened soil should be addressed before placement. Follow the project requirements for dewatering, inspection and founding-surface condition.
Unexpected site conditions sometimes require changes, but structural dimensions should not be altered casually. Confirm any change through the required design or approval process.
Not necessarily. They carry loads differently and can have different width, thickness, reinforcement and founding requirements.
Frost depth is not a useful universal Australian rule. Use the applicable project, climate, code and engineering requirements for the location.
AS 2870 — Residential slabs and footings — is a key Australian standard within its scope. Use the current edition/status together with the NCC and project documentation.
No. This is an informational guide page only and contains no calculator, input fields or generated structural result.
Use The Current Building Code, Referenced Standards, Project Engineering And Site Information For Structural Footing Decisions.
Residential slabs and footings standard covering site classification and footing-system requirements within its scope.
View AS 2870 ListingAustralia's building code framework and referenced construction requirements.
Open NCCAustralian concrete industry technical resources and construction guidance.
Visit CCAAService-location information to use before footing, pier and trench excavation.
Visit BYDA