Concrete Block Foundations – What You Need To Know

Posted on Friday, February 6th, 2015 by Nathan Weinstock

During a home inspection, you want to have a really good look at the foundatiofoundation-step-crack-300x228n. After all, the foundation is what does the job of carrying the house. Most houses in the Ottawa area sit on a perimeter wall foundation made of concrete, either poured concrete or concrete block.

Concrete is one of the better construction materials having good strength in compression. In other words, concrete does a great job of holding up a house. Concrete is also fire resistant and rot resistant. So basically, concrete is good.

The thing about concrete block is that, as it ages, the mortar joints in the wall can deteriorate. The foundation can lose some of its ability to hold the blocks together and thus resist lateral forces (the soil pushing sideways). This deterioration is caused by water seeping through the mortar joints and washing the cement part out of the mortar. That cement is the binding part of the mortar that holds the concrete blocks together. Weaker mortar leads to cracking of the joints (see the picture). At some point, the foundation could require remedial work to reinforce the wall.

So, a significant factor affecting the foundation’s longevity is the amount of water that passes through the wall, contributing to the deterioration. The less deteriorating water, the longer the wall life. This means that a dry block wall can perform well. On the other hand, a cracked, leaky wall may not provide long term dependable service.

The existence of cracks does not necessarily indicate a serious problem. Cracks should be monitored for changes in size (getting larger) and water penetration (leaking). If either of these occur, then further investigation is needed and remedial work may be required. That way, you can figure out what the problem is and then decide what the best fix would be.

It is important not to cover the wall and obscure the foundation from view.  Don’t finish the wall unless you are sure that it is in good condition and has no water going through it. Maintain viewing access on the interior to monitor and assess the foundation condition and performance. Eliminate any water migration through the foundation.  This usually means installing foundation water proofing and changing the exterior grading. Make sure that the ground around the foundation on the outside slopes away from the house. This would provide proper drainage around the foundation.

If you have any foundation cracks, they should be repaired to allow for better monitoring and to help prevent leaking. Once repaired, they should be monitored for any reoccurrence or change in size or length. Any cracks that are found to be leaking should be repaired more reliably. Any cracks that are found to be growing should indeed be investigated. This is critical to ensure that these cracks are not structurally significant. If this is the case, remedial work (which can be significant) will be required.