Are concrete blocks waterproof?

Are concrete blocks waterproof?


Are concrete blocks waterproof? How do you waterproof your concrete blocks? Concrete is a porous material by itself. It has a tendency to absorb water even when it is fully set and cured. Hence concrete blocks are still porous and will absorb water. This is the main reason you see the damping of walls made out of concrete blocks. When rain hits the concrete a portion of it is absorbed by the concrete. This water then evaporates slowly over time.

The concrete block is not affected by the damping as much. Your walls made out of concrete blocks will not start dissolving over time mind you. Once the concrete sets it doesn’t dissolve. But the damping of the concrete block can affect the drywall in your house as well as any paint that is applied to the concrete blocks.

When concrete blocks get damped they can contain lots of moisture. When the concrete blocks are in contact with drywall they can slowly start dissolving away the gypsum-based drywall. The drywall will start thinning in the area of dampness as the gypsum is dissolved slowly. You may also see white spots on the floor right under the dissolving gypsum. This is dissolved gypsum that has now been set on the floor after the moisture evaporated. This can cost a lot of money to fix if it happens repeatedly.

Water-soluble paints are commonly used in single-family residences as they are cheaper than oil-based paints. These will easily dissolve over rainwater soaked concrete blocks and hence you would see a phenomenon called flaking.

Flaking is the chipping away of water-based paint as the surface in contact with the dampened concrete block dissolves and leaves a gap between the concrete block and the outer paint surface. This gap is filled by air and the paint starts to puff up. On minimal pressure the puffed up paint surface gives way and the chip of paint is removed very easily leaving behind a bare wall. This is also a big problem.

Also, damped concrete blocks start giving a characteristic smell to the property. The smell alone is the biggest problem as it can render a multi-million dollar building into an unsellable asset.

Are concrete blocks damp-proof?

As a matter of fact, they are not. Concrete blocks get damped quite easily when they are left out in rain. While concrete in concrete blocks will not dissolve they still get wet and damp. So leaving concrete block walls bare without any coating or plastering can cause problems.

Do concrete blocks absorb water?

Yes, they do absorb water. If you don’t believe me take a jug of measured water. Drop a concrete block in the jug and cover it. Leave it for 24 hours. When you revisit the jug the water in it will be grossly less. That water is absorbed by the concrete. While the dimensions of the concrete block will not change, this experiment will prove that concrete blocks are porous and do absorb water.

Do concrete blocks need to be sealed? How do you waterproof a concrete block wall? What do you use to seal concrete blocks?

Yes, concrete blocks need to be sealed. This can be achieved in two ways:

  • Use a Sealent to coat the walls. Many sealers such as Ion Bond etc. are available in the market. It is best to go to a local hardware store and get the cheapest one available in stock.
  • Plastering the walls. After plastering the walls you can further waterproof the wall by applying a framer over them if it is an outside wall. If it is an inside wall then you can get great waterproofing and smooth walls by applying wall putty on the wall.

Can concrete blocks be used for the foundation?

No, concrete blocks can not be used for the foundation of a house. The foundation of the house requires a material that is structurally intact and can absorb lots of pressure. It is also a plus if it is non-porous and does not absorb any dampness. Due to these reasons, I would always recommend building a foundation by throwing hard rocks in the columns/slabs around iron bars (steel rods). The remaining gaps will then be filled with concrete to strengthen the structure even further.

This approach has been proven time and again and you should shy away from using non-conventional materials for the foundation as they are not tried and tested over hundreds of years. If the foundation fails the house will crumble over and this can be a great disaster.

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