Strip footings are the simple, long foundations placed under load-bearing walls. They spread wall loads into the ground and are common in low-rise masonry or framed buildings.
This article explains practical steps to assess soil, estimate loads, size widths and depths, and detail reinforcement. The aim is clear, usable information to help make sound decisions on site.
Assessing the site and soil
Good strip footing design starts with knowing the ground under the building. Soil type, bearing capacity and water conditions change how wide and deep a footing must be.
Take time to collect direct information about the site. Even simple tests and observations reduce guesswork and lead to safer, more economical foundations.
Soil bearing capacity
Soil bearing capacity is the load the ground can carry without excessive settlement. Typical values vary widely: compact sand or gravel can carry much more than soft clay.
When possible, use a site report. If that is not available, perform a simple plate load test or rely on conservative published values adjusted by local experience.
Groundwater and drainage
High groundwater raises the risk of bearing capacity loss and frost heave. It can also cause uplift in some soils.
Check drainage patterns and seasonal levels. Where groundwater is high, increase depth or improve drainage to protect the footing.
Calculating loads and footing width
Design begins by adding vertical loads from walls, floors and roof. Then divide total load by safe bearing capacity to find required footing area.
Keep calculations simple but conservative. Allow an additional margin for unexpected loads or weaker soil patches.
Estimating vertical load
Vertical load is the sum of dead loads and imposed loads acting on the wall line above the footing. Dead load includes wall weight and floor finishes; imposed load covers live loads and any concentrated loads.
- Determine wall load per metre length (kN/m).
- Add tributary loads from floors and roof that the wall supports.
- Include any point loads or heavy openings that concentrate load.
Finding footing width
Required footing width (B) is roughly equal to the total vertical load per metre divided by soil safe bearing capacity.
Compute B = V / q_allow, where V is load per metre (kN/m) and q_allow is allowable bearing (kN/m2).
- If V = 120 kN/m and q_allow = 150 kN/m2, B = 0.8 m.
- Round up to a practical construction width and check edge distances to avoid eccentricity.
Depth, reinforcement and detailing
Depth and reinforcement choices control bending and shear capacity and reduce settlement risks. Minimum thickness is set to resist cracking and to ensure adequate embedment of steel.
Design depth often depends on local frost depth and the need to reach firm soil. Reinforcement layout then follows to resist bending and shear from loads.
Minimum thickness and cover
Common practice sets minimum thickness of strip footing between 200 mm and 400 mm depending on load and local rules. Thicker footings reduce bending stress but cost more.
Concrete cover to the top and bottom bars protects reinforcement from corrosion. Typical cover is 50 mm for ground-contact elements unless local codes require more.
Reinforcement layout
Place continuous longitudinal bars along the footing length to resist bending. Add distribution bars perpendicular if needed to control cracking.
- Typical detail: 2 or 3 bars at bottom (tension face) and single bars near top if uplift or eccentricity is expected.
- Use standard bar sizes (e.g., 10–16 mm) depending on bending moment and spacing.
Construction sequence and common mistakes
Correct construction sequence saves time and prevents defects. Simple mistakes on site often lead to uneven settlement or cracked walls.
Focus on excavation, base preparation, proper formwork and continuous concrete placement to maintain structural integrity.
Excavation and base preparation
Excavate to the planned level and remove loose material. Compact the base or replace it with well-graded fill if the native ground is weak.
Lay a thin blinding concrete if the ground is dusty or wet; this provides a clean surface for placing reinforcement and prevents contamination.
Pouring, curing and backfill
Place concrete in one continuous pour along a run to avoid cold joints where possible. Vibrate or tamp to remove voids and ensure full consolidation around bars.
Cure concrete properly to reach design strength. After curing, backfill carefully in layers and compact to avoid creating voids that cause differential settlement.
Common checks and adjustments on site
During work, validate assumptions made in the office. Simple checks reduce risk of costly rework.
Measure actual soil conditions, confirm footing alignment and monitor concrete strength as it cures.
Settlement monitoring
Place simple markers on walls or use settlement pins to check movement during and after construction. Early detection lets you fix problems before finishes are applied.
If settlement exceeds expectations, investigate the cause: poor compaction, variable soil or overloaded footings are common culprits.
Adjusting for eccentric loads
If wall loads are not centered on the footing, check for eccentricity. An eccentricity reduces effective area and can cause punching or tilting.
To correct this, increase footing width on the loaded side or provide a thicker slab to restore uniform stress under the footing.
Conclusion
Strip footings are an efficient way to support load-bearing walls when soil conditions are suitable. Careful assessment, simple calculations and good site practice produce reliable, economical foundations.
Use conservative assumptions when data is limited, and verify key on-site conditions early. With clear checks on load, soil and construction, strip footings perform well for many small to medium structures.
Frequently Asked Questions
What is the minimum width for a strip footing?
Minimum width depends on load and soil bearing capacity. Commonly widths start around 300–400 mm for light walls, but calculation is required. Always check local rules and soil data.
How deep should a strip footing be?
Depth must clear the frost line and reach firm ground. Typical depths are 500–1000 mm but vary by region and soil. Where groundwater or weak soil exist, depth or ground improvement must increase.
How much reinforcement does a strip footing need?
Reinforcement depends on bending moments and shear. Simple footings often use two or three longitudinal bars near the tension face and distribution bars across. Choose bar size and spacing to meet calculated moment and crack control.
Can strip footings be used on clay soils?
Yes, but clay can settle and swell. Use conservative bearing values, provide deeper footings, or improve the ground. Consider drainage and whether a raft or piled solution is more suitable in very soft clays.
When is a wider but shallower footing better than a deeper one?
Wider shallow footings work when soil bearing is moderate and frost depth is shallow. They reduce excavation and are easier to build. Deeper footings are needed when upper soils are weak or groundwater is high.