There’s once a couple that visited our offices wanting to get designs for their dream house. Halfway through the visit, the husband unknowingly opened a can of worms and ‘threw’ an undesired substance to the fan’ when he suggested that in order to cut costs, they use ‘zile blocks za mchanga’…… ‘Wewe unataka kunijengea nyumba ya matope! %:”@*^%$!!!!!!’ What ensued was a bitter exchange that tested our mediation skills.

‘Blocks za mchanga’ is in an ideal sense a compressed block made out of INORGANIC dry soil and stabilized with Portland cement/simiti. Stabilization is a technique of improving the properties of soil in such a way that it will possess the adequate strength, durability and dimensional stability.

In lay man’s terms, it is making the soil/mud block retain its shape and size both in dry and moist conditions and does not need to be fired/burned in a kiln. Stabilization techniques also greatly vary and some do not limit it to cement and regular soil.

Just over 10 years ago, soil block technology had taken the country by storm as the then starters of the technology claimed that the technology would ‘save’ a home builder up to 40% of their total construction cost. The most popular block is the Interlocking stabilized soil blocks (ISSB), also referred to as Compressed Stabilized Earth Block (CSEB)/ Stabilized Mud Blocks (SMB). The ‘wow factor’ of ISSB is that they essentially don’t use mortar (cement and sand mix) but instead lock into each other and hence has potential savings as we will see.

So, lets demystify this walling technology.

ISSBs are basically used for walling. So, let’s question, what is walling for? Why do we need it? With a wall you can control many things, like inside climate (direct unwanted sunlight, temperature, humidity, speed of the air, hold materials in suspension like slabs beams, some walls are just there to look good, etc., you can restrict the movement of unwanted animals like keep out fisis/hyenas from attacking your daughters, also used as a protection barrier from unwanted persons like thugs and politicians etc.

High density materials like concrete, bricks and tiles have a high thermal mass. Thermal mass is the ability of a material to absorb and store heat energy. A lot of heat energy is required to change the temperature of high-density materials like ISSB. This then means that ISSB are good at insulating a house, hence work well to keep heat out when its hot and keep it in when it’s cold.

ISSB are made in molds. By the name mold means that there is a degree of flexibility. They can be made either straight of rounded and that is certainly a big advantage.

Our biggest worry with ISSB is quality control during their ‘manufacture’. It is of essence that the soils be INORGANIC. ‘Inorganicness’ means soil that has never seen anything like a seed/ plant or any form of living organism. This is a tough call, as such soils are deep inside the earth and only get exposed during mass excavations like deep foundations or road construction.

Imagine waking up one good morning and you hear your daughter ‘Mami, kuna maindi inamea ndani ya socket’. If you have a soil block wall and a plant is sprouting from a wardrobe then you are in big trouble. Soils must also be well sieved to ensure the mix is uniform. Sieving eliminates many of these organisms but cannot be done 100%. Chemical treatments exist but then affect cost.

Compression is also hard to monitor. Compression is done either manually or by a machine. In small scale (like building your own house), it is cheaper to make blocks manually. Mechanically compressed/zenye zimefinywa na mashini are however more consistent in compression strength as the compression machine can’t lie. Introduce a loose Kamah or Mutua for some manual compression and quality assurance is not guaranteed. A laborer compressing blocks needs a great deal of energy and lets face it, some days, motivation can get low. Just picture some laborers compressing some blocks and they don’t have lunch money ‘Ai, mimi sina nguvu ya kufinya hizi vitu mtu nguyaz, sina doh ya lunch. nitafinya mpaka nusu’ (my people, I don’t have strength to compress these things, I don’t have lunch money. I will compress halfway). Then you have a compromised block.

Cement ratios are also tricky to monitor. Construction sites are known well for pilferage (or filperage in Makueni). Hence doubts of block quality may arise.

Have you ever wondered why all shags latrines have gaping holes? Yet these rooms are meant to be enclosures with the highest amount of privacy. Since ISSB does not have a mortar bond, it goes to say that technically, there are gaps in between the blocks. We have seen at least two interlocking soil block walls that one could see through. Gaps in such a wall are, of course, a quality control problem in manufacturing and laying. So, when using this technology, quality control is everything.

A big challenge with ISSB is Electrical wiring and piping for water. We have observed that the blocks are quite brittle and don’t respond well with kuchonga/chasing to lay water pipes and electrical conduits. Chasing that is done with a grinder works well, on well-made and laid blocks though. In many cases, we see contractors placing pipes and conduits on walling surfaces; a serious eye sore. You can imagine you get visitors at your house with pipes popping out everywhere then the visitor’s toi asks his mom ‘Mami kwanini akina kiokoooh wako na nyoka mingi kwa nyumba yao?

In our observation, there is approximately a 30% saving in MANUALLY compressed soil block walling as compared to conventional ‘Ndarugo’ machine cut stone. (For 6 inch walling). We have also observed from our experiences in practice, that walling as a cost component in a building is approximately 10% of the total cost of a building. Depending with the complexity of the walling type this varies though. For example, a zero joint super smooth high grade Isinya yellow stone of varying stone heights with intermittent bush hammered ribs will definitely jerk up the cost of walling. It is then notable that walling is a small cost component in building. That’s why we always wonder how some companies substitute masonry/stones/blocks/bricks with concrete panels and claim it’s for ‘cost saving’. Truth be told, concrete panels are more expensive than conventional walling.

As we have mentioned before (, build your house to respond to your needs and geo-physical and environmental factors, in your own budget. Don’t fall for marketing gimmicks. Whether it is soil blocks, stones/masonry, paper or panels, ensure you have professionals to advise you. It is cheaper in the long run.