A strip foundation is a continuous band of concrete under a wall that spreads the wall’s load over the ground. For an ordinary house on good ground, SANS 10400-H gives deemed-to-satisfy sizes, so you don’t need an engineer’s design. This guide explains those rules, shows how to take off the materials, and lists the situations where the deemed-to-satisfy route doesn’t apply.
One note first: the version of SANS 10400-H that is freely available is the 1990 edition. The current edition is from 2012. The figures below are from the 1990 text and match the ones commonly used today, but check the current edition or your plans where it matters.
The basic rules
- Concrete: at least 10 MPa at 28 days, or a mix of 1 cement : 4 sand : 5 stone by volume.
- Thickness: at least 200 mm, except on solid rock.
- Width: at least 600 mm under load-bearing or free-standing masonry walls, and at least 400 mm under internal walls that carry no load.
- Piers: foundations under piers project at least 200 mm all round.
The foundation must sit on firm natural ground. How deep that is depends on your site, which is why the trench depth is something you measure, not something a calculator can tell you.
When the simple rules don’t apply
Clay that swells and shrinks, collapsing sands, fill, soft ground, steep slopes and high water tables all need a foundation designed by an engineer. If the trench turns up any of these, stop and get advice before you pour. The cost of a design is small next to the cost of cracked walls.
Taking off a house foundation
Start from the plan and measure two totals: the length of external and load-bearing walls, and the length of internal non-load-bearing walls. Then:
- Footing area = external length × 0.6 m + internal length × 0.4 m.
- Concrete = footing area × thickness, plus waste. Trenches are rarely perfectly straight or level, so allow about 10%.
- Excavation = footing area × trench depth. This is the volume in the ground; dug soil takes up more space once it is loose.
- Foundation walls run from the top of the footing up to the damp-proof course. SANS 10400-K puts the DPC at least 150 mm above the finished ground, so the height is trench depth − footing thickness + DPC height.
- Bricks for those walls: about 104 per m² for double brick and 52 per m² for single brick, plus a little for breakage.
- Mortar: Class II, 150 kg of common cement and 0.6 m³ of building sand per 1,000 bricks.
Worked example
A small house has 40 m of external walls and 12 m of internal walls. The footing is 200 mm thick, the trenches are 600 mm deep, and the DPC sits 150 mm above the ground.
- Footing area = 40 × 0.6 + 12 × 0.4 = 28.8 m².
- Concrete = 28.8 × 0.2 = 5.76 m³, or 6.34 m³ with 10% waste. At the Concrete Society’s low-strength foundation mix of 290 kg per m³, that is 37 bags of cement, with about 4.1 m³ each of sand and stone, so order 4½ cubes of each.
- Excavation = 28.8 × 0.6 = 17.3 m³.
- Foundation walls = 0.6 − 0.2 + 0.15 = 0.55 m high. Bricks = (40 × 104 + 12 × 52) × 0.55 = 2,631, or 2,763 with 5% for breakage.
- Mortar = 2.763 × 150 kg = 414 kg, which is 9 bags of cement, with about 1.7 m³ of building sand.
The strip foundation calculator does all of this in one go, and the strip footing calculator handles a single run of footing with its own checks.
On site
- Peg out the trenches and check the diagonals before you dig.
- Dig to firm, undisturbed ground and keep the trench bottoms level. Remove loose soil from the bottom.
- Set level pegs for the top of the concrete so the footing is the right thickness all the way along.
- Pour each section in one go where you can, and keep the concrete damp while it cures.
- Build the foundation walls up to DPC level, then lay the damp-proof course. See damp-proofing: DPC, DPM and rising damp.
This guide is general information for estimating. It is not engineering, legal or tax advice. Check SANS 10400 (the National Building Regulations) and your local municipality’s building control officer for your project.