To interlock or not to interlock

by Dan Powell


     I take issue with three of the alleged advantages of the interlocking block system which are the following:

  1. Earthquake resistance and   

  2. Ease of construction because they are self aligning.

  3. Ability to produce thinner walls

   There are some big liabilities that go with the interlocking system that I can see. First is the dependence on cement to make them strong enough to not break the overlapping flange in the event of lateral wall movement, but the cement and shape also makes them subject to shattering under the same circumstances. With the loss of flexibility and the shattering induced there is also introduced the potential for catastrophic failure where a section of block blow out causing a wall collapse. Obviously this hasn’t yet been proven but with the increase of earth block construction worldwide we will soon see the actual performance characteristics empirically. The vertical compaction also introduces problems with the need for mortar increasing cost as well as failure potential due to differential materials fatigue. I assume that the block are vertically pressed which would be the only way to make an interlocking block, (see my explanation at )

    Another liability to interlocking block is the difficulty in making corners, intersections, buttresses, and arches. Not to mention domes and vaults which are very important to earthquake resistant architecture as well as aesthetics.

   The last questionable advantage to interlocking block is the ability to construct thinner, steel reinforced earth block walls. Although this would appear to be an advantage you would be in the same league as either a fired brick or concrete masonry unit. The embodied energy, energy inefficiency and CO2 emissions in any cement or steel intensive product is too high to justify a thinner wall for any reason.

            My dad, W.C Powell, designed an interlocking block press for Reickerman Thailand back in 1992 and the principal was sound enough but the energy economics were not, with the increase in cost for cement and steel as well as the high cost of a much more complex machine the program was apparently scrapped. We have had the ability to produce ILB machines and have chosen not to for the above reasons and have begun to think of the ILB as a gimmick to make a product that is already adequate “feel” better. A rectangular block system made of 100% soil can more easily weather a lateral movement because of the built in slip joints without shattering or crumbling. This is similar to the crumple zone of a car that transfers the energy of an impact away from the occupants. In a wall 20’ long there are 400 to 600 slip joints to distribute the shift without concentrating the energy on the weakest link. This is not to say that the wall won’t fail but it won’t fail as quickly as the thinner cement bonded walls. The answer is thicker, mortar- less walls. Another aspect of the ILB vs. POB (plain old block) is the moisture transference issue. Soil in its pure form breathes moisture as well as CO2, cement stabilized block does not neither is it as comfortable. The ability of earth block systems to save energy is directly related to Thermal Mass, with a rectangular block I can build a wall from 12” to 48” in width, make any form I wish including vaults and arches and reliably build a home that will be less expensive to operate, and will last for centuries and the speed of construction is actually faster. How can you improve on that? There are a lot more reasons but I have covered the basics. For more information on embodied energy check out our information page at .