Foundation Work

Part of Lime & Cement

Designing and constructing durable building foundations using lime concrete and mortar for permanent structures.

Why This Matters

A building is only as strong as its foundation. The most beautiful timber frame, the most carefully laid stone wall, the most precisely engineered roof — all of it cracks, tilts, and eventually collapses if the foundation beneath it moves, settles unevenly, or fails to distribute the building’s weight to the ground. Every permanent structure in a rebuilding scenario starts with the foundation.

Foundations serve three critical functions: they spread the building’s weight over a large enough area of soil that the ground can support it, they prevent moisture from wicking up into the walls (rising damp destroys masonry from within), and they anchor the structure against lateral forces (wind, earthquakes, earth pressure). A proper foundation built with lime concrete provides all three — and unlike timber sills that rot or dry-stacked stone that shifts, lime concrete foundations can last indefinitely.

The techniques here do not require Portland cement, reinforcing steel, or industrial equipment. Romans, medieval builders, and colonial-era masons built foundations using the same lime-based methods described in this article, and many of their buildings still stand after centuries of service.

Site Assessment

Before breaking ground, understand what you are building on.

Soil Bearing Capacity

Different soils can support different loads. Build on the wrong soil and the building sinks, tilts, or shifts.

Soil Type	Bearing Capacity	Foundation Approach
Bedrock	Excellent (unlimited)	Build directly on rock surface
Dense gravel	Very good	Standard strip foundation
Compact sand	Good	Standard strip foundation
Stiff clay	Moderate	Wider foundation to spread load
Soft clay	Poor	Very wide foundation or raft; remove and replace if possible
Loose fill/topsoil	Unacceptable	Dig through to firm soil below
Peat/organic soil	Unacceptable	Remove entirely or relocate building

The Probe Test

Drive a sharpened wooden stake (2-3 cm diameter) into the ground with a heavy mallet:

• Firm soil: Stake penetrates slowly with many blows, resists strongly. Good for building.
• Moderate soil: Stake penetrates steadily with moderate effort. Acceptable with wider foundations.
• Soft soil: Stake sinks easily with few blows. Must dig deeper to reach firm soil or widen foundation significantly.

Water Table

Dig a test pit to your planned foundation depth. If water seeps in:

• If slight seepage: manageable with drainage (see below)
• If water floods in: foundation may be below the water table. Either raise the building level, install drainage, or relocate

Never Build on Topsoil

Topsoil is rich in organic matter that decomposes over time, causing settlement. Always dig through topsoil to the undisturbed subsoil beneath. This is typically 30-60 cm deep but can be deeper.

Foundation Types

Strip Foundation (Most Common)

A continuous strip of concrete under each load-bearing wall. Suitable for most buildings on moderate to good soil.

Dimensions:

• Width: At least twice the wall thickness (e.g., a 30 cm wall needs a minimum 60 cm wide foundation)
• Depth below ground: At least 45 cm below finished ground level (deeper in cold climates to get below frost line)
• Thickness: 20-30 cm of concrete

Construction:

1. Excavate trenches along all wall lines. Make trenches straight and level — use string lines and a spirit level (or a long board with a water level).

2. Level the bottom. The trench bottom must be level along its entire length. An uneven base causes differential settlement — one part of the wall sinks more than another, cracking the entire structure.

3. Compact the bottom by tamping with a heavy wooden ram or stamping.

4. Add a gravel bed (10-15 cm) of clean, compacted gravel. This provides drainage beneath the foundation and evens out minor irregularities.

5. Pour lime concrete to the full width and thickness. Use hydraulic lime concrete (with pozzolan) for best results. Pour in one continuous session if possible.

6. Level the top surface — this is where your wall will sit. Use a straight board to screed (flatten) the wet concrete. Check level with your longest available straight edge.

7. Cure for at least 14 days (preferably 28) before beginning wall construction. Keep damp by covering with wet material.

Pad Foundation

Individual concrete blocks under point loads (columns, posts). Used for light structures or post-and-beam buildings.

Dimensions:

• Typically 60 cm x 60 cm x 30 cm deep for light structures
• Increase size for heavier loads
• Each pad must be individually level and at the same height as all other pads

Raft Foundation

A single concrete slab covering the entire building footprint. Used on poor soils where the load must be spread over the maximum area.

Dimensions:

• 15-25 cm thick
• Extends 15-30 cm beyond wall lines on all sides
• Requires more concrete but distributes load evenly

Step-by-Step: Building a Strip Foundation

Marking Out

1. Drive corner stakes at each corner of the planned building. Check that corners are square using the 3-4-5 method: measure 3 units along one wall line, 4 units along the adjacent wall line, and the diagonal between these points should measure exactly 5 units. If not, adjust until it does.

2. Set up profile boards — horizontal boards fixed to posts set back 1 meter from each corner. Stretch string lines between profile boards to mark the wall center lines and trench edges. The strings can be removed during digging and re-stretched afterward to verify alignment.

3. Mark the ground under the strings using sand, lime, or a stick.

Excavation

1. Dig to the required depth. Keep the trench walls as vertical as possible.
2. Remove all loose material from the bottom.
3. In unstable soil, slope the trench walls outward or shore with boards to prevent collapse.
4. Check the depth at multiple points — it must be consistent.

Drainage

If the site is even slightly damp, install drainage around the foundation perimeter:

1. Dig a drainage channel alongside the foundation trench, sloping gently (1 cm per meter minimum) toward a discharge point downhill.
2. Fill the bottom of the drainage channel with gravel.
3. Lay clay pipes or a channel of stacked flat stones along the gravel bed to carry water away.
4. Cover with more gravel, then soil.

Good drainage prevents water from softening the soil beneath the foundation and causing settlement.

Concrete Mixing and Pouring

Foundation concrete formula:

Component	Parts by Volume
Lime (hydraulic or with pozzolan)	1
Sand (sharp, clean)	2
Gravel (20-40 mm)	4

Mix on a clean surface near the trench. Add water to achieve a stiff but workable consistency. Transport to the trench in buckets or a wheelbarrow.

1. Pour into the trench in layers of 10-15 cm.
2. Tamp each layer firmly with a heavy wooden ram.
3. Fill to the top of the trench or to the planned foundation height.
4. Level the top with a straight board (screed).
5. Check level immediately — once the concrete begins to set, you cannot adjust.

Damp-Proof Course

Moisture rising through the foundation into the walls (rising damp) is the biggest long-term threat to masonry buildings. Install a damp-proof course (DPC) between the foundation and the wall:

Options:

• Slate: Lay two courses of thin slate slabs across the full width of the wall, bedded in mortar. The slate is impervious to water.
• Dense mortar: A thick layer (2-3 cm) of very dense hydraulic lime mortar with crushed brick. Less effective than slate but widely available.
• Bitumen/tar: If available, a layer of hot tar or bitumen painted on the foundation top is an excellent DPC.
• Lead sheet: If available, extremely effective. Used historically in high-quality buildings.

Position the DPC at least 15 cm above finished ground level. This prevents splashing rainwater from bridging the DPC and wetting the wall above it.

Stepped Foundations on Slopes

On sloping ground, the foundation must step down to follow the terrain while keeping the top level:

1. Always step down in the direction of the slope — never slope the foundation to match the ground.
2. Each step should be at least 15 cm high (the height of one masonry course or one concrete pour).
3. Each horizontal section must be at least 30 cm long.
4. Overlap the concrete between steps — the upper section should extend at least 30 cm past the step.

Common Mistakes

Mistake	Consequence	Prevention
Building on topsoil	Settlement, cracking	Always dig to undisturbed subsoil
Uneven trench bottom	Differential settlement	Check level constantly during excavation
Too narrow foundation	Overloaded soil, sinking	Minimum 2x wall thickness width
No damp-proof course	Rising damp destroys walls within years	Always install DPC
Loading too early	Foundation cracks under wall weight	Wait minimum 28 days before building walls
Ignoring drainage	Water softens soil, foundation shifts	Install perimeter drainage on all but the driest sites

Take Your Time

A foundation is invisible once the building is finished, but it determines whether the building stands for a decade or a century. Extra time spent on getting the foundation right — level, square, properly cured, well-drained — is the highest-value time in the entire construction process.