Understanding Cob Floor Mixes
Cob floors offer a durable, low embodied energy surface built from subsoil, sand, fiber, and natural oils. Builders blend these ingredients in specific ratios to achieve compressive strength, abrasion resistance, and crack control. The calculator above multiplies floor area by layer thickness to determine total volume and distributes that volume among clay, sand, and straw according to a mix ratio. Densities of the constituents then convert volumes to mass, while the straw mass is divided by a bale weight to yield bale counts for planning supply deliveries.
The raw volume \(V\) of a cob layer is computed by \(V = A \times t\), where \(A\) is area and \(t\) is thickness. When thickness is entered in centimeters, the script converts it to meters before multiplication to retain unit consistency. After volume is known, the ratio string is parsed. For example, a user entering "1:2:0.15" intends one part clay, two parts sand, and 0.15 part straw by volume. Summing the parts yields a total of 3.15 parts. Each component’s volume portion is then \(V_i = V \times p_i / \sum p\). Because clay and sand have similar bulk densities near 1600 and 1500 kg/m³ respectively, their masses can be approximated by \(m_i = V_i \rho_i\). Straw is extremely light, roughly 100 kg/m³, so even a small volume constitutes only a few kilograms.
Many builders reference mix ratios by weight rather than volume. Volume based ratios are more intuitive in the field when mixing with shovels or buckets, but they do not account for variations in material densities. The calculator uses typical bulk densities, yet actual densities may differ based on moisture content or particle size. Users should adjust densities if they have measured values. Including adjustable density inputs would complicate the interface, so defaults are provided to maintain simplicity while still reflecting reasonable estimates for planning purposes.
Straw plays a key role in controlling shrinkage as the clay dries. The fiber interlocks the matrix, distributing tensile stress and preventing cracking. Too little straw results in fissures; too much compromises compressive strength. A volume fraction between 5% and 15% is common. When entering ratios, a number like 0.1 corresponds to 10% of the solid ingredients. After computing straw mass, dividing by bale mass yields the number of bales. Because bale masses vary widely, the input allows customization. Builders often use small square bales weighing 18–23 kg. The calculator defaults to 20 kg but accommodating other bale sizes avoids surprises.
To illustrate, consider a 25 m² room with a 5 cm base layer at a ratio of 1:2:0.15. The total volume is 1.25 m³. Clay accounts for 0.397 m³ (1/3.15), sand for 0.794 m³ (2/3.15), and straw for 0.059 m³ (0.15/3.15). Multiplying by densities gives 635 kg of clay, 1,191 kg of sand, and about 6 kg of straw. Dividing straw mass by a 20 kg bale yields 0.3 bales, meaning a single bale is sufficient with surplus for subsequent layers. Such calculations assist in scheduling material deliveries and estimating labor, as moving a ton of sand requires more manpower than mixing a few straw flakes.
The following table demonstrates how varying thickness influences mass requirements for the same 25 m² area using the 1:2:0.15 ratio. It highlights the significant increase in materials for thicker structural layers.
| Thickness (cm) | Total Volume (m³) | Clay Mass (kg) | Sand Mass (kg) |
|---|---|---|---|
| 3 | 0.75 | 381 | 714 |
| 5 | 1.25 | 635 | 1191 |
| 7 | 1.75 | 889 | 1668 |
Besides structural layers, builders often apply a finer finish coat using screened clay and sand with minimal fiber. Though thinner, these coats cover the entire area and thus require accurate planning. Finishing oils like linseed or tung oil penetrate the cured surface to harden and waterproof it. The calculator does not estimate oil volumes, but understanding base material quantities helps approximate cost and drying times before sealing.
Mixing cob can be labor intensive. Some crews mix on tarps with bare feet, while others use cement mixers or mortar mixers. Knowing the total volume helps plan mixing batches. For instance, a typical wheelbarrow holds about 0.07 m³; thus our example 1.25 m³ project would need roughly 18 wheelbarrow loads. Each load might take 10 minutes to blend and deliver, translating to several hours of work for a small team. Accurate estimation avoids running out of material mid-project or over-ordering heavy ingredients that must be moved again later.
In mathematical terms, the script’s calculation steps are:
,
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where \(p_i\) represents each component’s ratio part and \(\rho_i\) its density. The straightforward implementation makes the tool easy to adapt. Developers could extend it to output cost estimates by multiplying masses with local material prices or to model layered floors by running calculations sequentially. The open-source nature invites experimentation and refinement for diverse building contexts.
Earthen floors embody sustainability by using locally sourced ingredients and minimal processing. They moderate indoor humidity, provide thermal mass, and offer aesthetic warmth. With proper planning aided by this calculator, self-builders can accurately gauge material needs, minimize waste, and allocate labor efficiently. By quantifying components with simple formulas, traditional building practices blend with modern computational convenience, empowering more people to adopt low-impact housing solutions.