The Silent Explosion in Your Home

The Inspector’s Perspective

The clay block construction method for houses, and in particular, units, has been very popular over the past couple of decades. It was promoted by masonry manufacturers as a strong, low cost, and low maintenance method of building. The product has proven to be all of these things as time goes by, however, be very cautious of the words “low maintenance”. This does not mean “no maintenance” as some people are very sadly finding out. There is one simple maintenance procedure required with this product, that if ignored, could lead to the silent explosion of your house’s structure.

Clay block construction is a very strong method of building houses. In this construction method the block wall is hollow initially, with steel reinforcing bars running through it at regular spacing. The hollow cores of the blocks that contain the steel reinforcement are then filled with concrete. This creates a structural wall that does not require a timber frame. This is very different to the more familiar brick veneer construction, as bricks do not contain the heavy steel reinforcement, but rely on a timber frame for the wall’s structural integrity. Brick veneer walls are not subject to the problem discussed here.

The big issue that many owners of these buildings do not know about, is the need to keep the block walls well sealed from moisture penetration, by either painting or regular application of a clear masonry sealer. This was specified by the block manufacturer, but many owners are not aware of the requirement, or the serious repercussions of not adhering to it.

With the absence of a suitable sealant, moisture from rain is absorbed through the blocks, mortar and core fill concrete and eventually reaches the steel reinforcement within the wall. As you would expect, this causes the steel to rust, and when steel rusts, the rust material grows to many times the size of the original steel. This growth is powerful enough to crack the concrete core and the clay block wall, which can accelerate the process even further by allowing the steel even more exposure to the elements. If left unchecked, this process can lead to severe structural loss to the building requiring extensive repairs or reconstruction of whole walls.


Exploding clay block wall caused by rust to internal reinforcement.

You can test the porosity of your clay block wall yourself by splashing a glass of water onto the wall.

If the water does not run off in little beads like on a polished car but sucks in to the wall, then it needs treating with a masonry sealer. This product is available from hardware suppliers – follow the directions on the label and retreat at the recommended intervals.

If you have any questions or concerns about this or any other building matter, you can call Brian at BPI Building and Pest Inspections on 07 5499 2477 visit or email us on

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