Concrete Block walls

Home Inspector with Home Inspector/Structural Engineer/Mold Assessor


The building blocks of a good house

          When visiting with any of my seven nephews, I love to play with them and their toys.  It is a great way to connect and have fun, plus it satisfies my inner child.  One of the most popular toys I had forty years ago is also one of theirs;  Lego blocks.  We all had them everywhere, those wonderful little blocks than will make anything the mind can imagine.  They are small and light weight, go together quickly and easily, and have great strength.  They are waterproof, durable, and economical.  Plus, they look good once assembled and can be used in almost any environment.  Now that I have "grown up", I discovered my attraction to the little blocks never really went away, but merely transferred to what are called Concrete Masonry Units (CMUs), or simply concrete blocks.

          The first concrete block was produced in 1882 and has grown in popularity ever since.  As the name implies, a CMU is made of concrete along with other possible additives to give it varying characteristics of density, strength, fire resistance, texture, cost, color, and thermal insulation.  To make a CMU, a relatively dry concrete mix is poured into a steel mold, vibrated, and compacted.  Then, the CMU is removed from its mold and cured to achieve optimal strength and dimensional stability.  Curing is typically conducted at elevated temperatures inside a special chamber to speed up the process.  Following the curing phase, CMUs are placed in storage for a drying phase.  Once fully cured and stabilized, a standard CMU measures 8 inches tall, 8 inches wide, and 16 inches long.

          CMUs come in three categories; lightweight with each block between 25 and 30 pounds, medium at 35 pounds each, and normal at 39 pounds each.  The heavier block has a denser concrete with greater strength.  A normal CMU provides an insulating R-value of 3.8, and a three-hour fire resistance.  The amount of cement (the ingredient that bonds the concrete materials together) in each block ranges from is 8.5% - 12%.  Therefore, CMUs are usually manufactured locally since the small amount of cement can be shipped in, and the bulk of other components, like gravel, sand, and water, are readily available in most areas.  Local production also saves shipping costs.  Overall, CMUs are affordable, readily available and sustainable.    

          In 2007, according to the Portland Cement Association, nearly 8 billion CMUs were produced in North America.  After double checking my math, that means if all the blocks were stacked on top of each other, they would reach one million miles high; the equivalent to two round trips to the moon.  And, they would weigh 140 million tons, which equates to 580,000 fully loaded rail cars.  Wow, that's a building boom!

           While CMUs used in construction of houses have no snap-together joints like Lego blocks, they are bonded together by skilled masons using precise formulations of mortar that, once cured, create a wall with great overall strength.  In Florida, most houses built with CMU's receive a stucco finish to make their exterior surface more attractive and durable.  Sometimes a home is referred to as a "CBS" home, which can refer to either "concrete block structure" and/or "concrete block with stucco".

On the inside of a house, block walls normally have wooden studs, or furring strips, applied to them, with wallboards screwed to the furring strips.  On the exterior, it is easy to tell if a home is built with CMUs because it will have windows recessed from the exterior finished wall surface by about three inches.  In wood frame construction, windows lay flush with the exterior wall finish.

If you are considering purchasing or building a home, a properly built concrete block structure will be more expensive than a wooden one.  However, the extra cost buys durability, versatility, longevity, and resistance to fire, windstorms, water damage, and wood destroying organisms.  Reinforced masonry houses also receive larger reductions on homeowner's insurance.  So, building a concrete block house is a solid long-term investment, even though the initial expense is greater.

 Greg Bertaux is a licensed professional engineer and home inspector.  His company, ISLE   Management Corp., provides property inspection services along the entire Treasure Coast. For more information call (772) 569-2141, or visit

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