How do you fix a leaking cinder block basement wall? Here is a simple guide.

Cinder blocks look very solid, but looks can be very deceiving. Even though they have a rugged appearance, these blocks are made with very porous materials. This means it is easy for water to pass through them.

Moisture in Cinder Block Walls

You can find pinhole water leaks in cinder blocks after there has been a heavy downpour of rain. The leakages come from little dot-like holes known as pores which are known to form inside cinder blocks.

If your cinder block basement walls are leaking, then keep reading. In this article, we’ll be discussing how you can seal these leaks by using the simplest approaches.

What You Can Use To Seal Cinder Block Leaks In Basement Walls

Pinhole water leaks in cinder blocks can be fixed simply by filling them with a masonry waterproofer.

These materials are made with particles of silica or epoxy which bonds with the cinder block surface, hence creating a waterproof boundary against the pores that lead to the pinhole leaks.

This sealing material is much thicker than latex sealers and interior paints. They are also built to soak deep into the pores of the cinder block walls. It is there it forms the water-tight barricade.

Why Do Cinder Block Walls Leak?

When compared to a regular concrete block or poured concrete, you will find that cinder blocks are a lot more porous. These blocks are permeable to liquid and water vapor (+10 perms).

Heavyweight concrete blocks are classified as semi-permeable (5 perms), and even lower when the void cores are filled with a mix of about 2 perms of concrete.

When compared to blocks, a high-quality freshly poured concrete foundation wall of about 10 inches in thickness is impermeable to water.

Cinder Block Basement Walls Leaking

If your wet basement foundation walls are built with cinder blocks, then they are very likely to leak, at least significantly more than a poured concrete wall.

A newly built cinder block basement foundation wall could spend 5 to 10 years without issue but be rest assured that after all the waterproofing coating or damp-proof membranes have been depleted, the cinder blocks will eventually be exposed and will have to resist the moisture on their own.

Sadly, the blocks don’t hold up well against moisture.

As for the exterior, high-grade cinder block walls, they aren’t prone to hydrostatic water pressure issues.

Nonetheless, in times of wind-powered rain, and other heavy rains, large volumes of water are forced into the cinder blocks and eventually through the walls.

Why Applying A Concrete Sealer Is Vital

As the old saying goes – Prevention is much better than cure.

It is cheaper and easier too, which is why you must take the vital steps to improve your cinder block wall’s ability to resist water by applying a deep penetrating concrete sealer before any leakages develop.

If preventive care is not taken, the pores in the cinder block walls will expand over time, the pinholes will start to develop, and efflorescence will appear on the surface.

If you allow this to happen, the expanding pores will become more difficult to seal in the future.

However, when the cinder locks are sealed on time with a concrete sealer, the lifespan of the basement walls will be significantly extended, thanks to its re-enforced water-resistant capabilities.

When water seepage becomes extensive, soil and silt can be carried through the concrete and start to plug the pores of the cinder blocks. This will make it harder for the concrete sealer to waterproof the substrate.

A clear indicator of soil deposits or build-up inside the cinder block walls is a reddish or brown colored pattern on the interior wall’s surface.

The plug pores of the blocks can prevent the sealers from getting in deep into the pores and sealing them.

When the blocks can no longer be sealed from inside or outside with a penetrating concrete sealer, waterproofing paint, coatings, or rubberized membranes, then you should need to employ more extensive approaches to solve the problem.

This would of course be more expensive, and that is why taking preventive steps remains your best option as far as maintaining the integrity of your cinder block basement walls is concerned.

Why Are Cinder Blocks Different?

Just like concrete blocks, split face blocks, and other lightweight blocks, cinder blocks are also classified as concrete masonry units (CMUs).

They are meant for the building of load-carrying foundation walls, partition walls, exterior walls, retaining walls, non-load carrying partition walls, as well as basement walls.

With the use of strong reinforcement bars and hollow-core fillings, you can have strong and sturdy structural walls. That being said, all CMUs appear the same, but there are major differences between them.

A standard high-density concrete block is made from cast concrete (cement, gravel, and sand).

When compared to regular structural concrete, they are developed with a larger percentage of sand and a smaller percentage of water and gravel.

This gives it a stronger mix that maintains its shape after it has been removed from the mold.

A regular high-density concrete block of 8 x 8 x 16 inches with 2 cores weighs around 36 to 42 lbs per block. As for lower density blocks, industrial wastes may be used as an aggregate, as opposed to using fine gravel or sand.

These industrial wastes include bottom ash or fly ash.

The term “cinder block” is a very old term that was coined from the days when power plants in steel mills produced large volumes of cinder from burning coal.

They were patented as far back as 1917.

As for modern cinder blocks, they are made with volcanic pumice rather than cinder, except there is a coal-fired power station or cement kiln close by to supply the pulverized cinders or fly ash.

How To Seal Cinder Block Basement Walls

Now that you have a clear idea of what cinder blocks are and the components that make them up, you can easily learn how to seal them with the steps we will be provided below.

Step 1: Remove any efflorescence

If there is efflorescence developing on your cinder block walls, then you have to get rid of them first before trying to seal the leaking pores.

If they are left to stay, the sealing material will not penetrate as deep as you would want it to go. You can use a hard bristle brush to scrub it off.

It is easy to identify efflorescence, they are white powdery and crystalline substances that form on damp walls.

Step 2: Prepare the surface

After the efflorescence has been cleared, you can now take off all the broken cinder block pieces and mortar with a wire brush.

The time you will spend doing this depends on the size of the area you are planning to seal. You can have an extra set of hands help you out if need be.

You should also chisel the cracks and holes you find in the wall with the pinhole leak. Create a triangle or inverted v-shaped with the chisel and hammer.

Step 3: Mix the sealing cement

Now it’s time to apply some water to the cracks using a big sponge. Mix the hydraulic sealing cement thoroughly before you apply.

Be sure to follow the instructions on the product’s pack as you mix.

Step 4: Press the cement into the holes

Now that you have a thorough mix, fill up the chiseled holes with it. Be sure not to stir the cement after filling, as it will expand on its own when it makes contact with the water.

Step 5: Protect the floors from stains

You can set a drop cloth on the floor and use tape to hold it in position.

Step 6: Stir the masonry

You can do this with a paint stick until it has developed a uniform color. Spread the mixture across the cinder block wall using a paintbrush.

Make sure you go over all the nooks in the wall so that all of the exposed pores will be covered. If you miss a few leaking pores, then the leaking will continue, even though in smaller amounts.

You will have to wait for some time before the first coat of the masonry waterproofer is cured. The time it will take should be written on the pack of the product.

Step 7: Apply the second coating

Now it’s time to apply the second coating of the masonry waterproofer. You can also use a paintbrush for this.

Step 8: Observe the wall

When all this is done, observe the walls to see if it’s curing the right way.

If you notice any untouched spots, you can apply an extra coating of masonry paint as a form of reinforcement.

The way it cures is a direct effect of how well the sealers and coatings were applied.


There you have it guys, that’s how to seal leaking cinder block basement walls. Follow the instructions in this article to get it right.

Thanks for reading.

Leave a Reply

Your email address will not be published. Required fields are marked *