Do Cinder Blocks Need to be Filled?

In short, no. Cinder blocks do not need to be filled to be effective, but it depends on what you’re building. In some cases, it’s best not to fill them, and in other cases, they should be filled. There are a handful of factors to take into account when making this decision.

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Moisture is your enemy when working with cinder blocks. If you’re living in a wet climate or working during the winter, any moisture in your structure could cause problems.

If you’re working in the winter, the cold will cause the water present in your structure to expand, causing cracks.

If you’re working with a wet filler such as concrete, the mortar holding the blocks together will dry first, sealing in the water from the concrete. Concentrated moisture held within the structure can impact the structure’s stability and will take a very long time to dry.

What are Cinder Blocks?


Using wet fillers such as concrete is common practice, but you must allow enough time to let it thoroughly dry.

 If you’re working in a warm, dry climate, this shouldn’t be a problem, but if you’re working in a wet climate, the concrete filler could potentially take years to dry completely.

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Your Structure’s Purpose

What will your structure be used for? You don’t have to worry about filling the cinder blocks if you’re building something like a raised plant bed or a fire pit.

Because projects like this generally don’t require the structure to bear weight or pressure, you don’t have to worry about the extra reinforcement provided by filling the cinder blocks.

If you’re building something that must be strong and sturdy, like a retaining wall or a similar structure, filling the cinder blocks is recommended to ensure that it can withstand the pressure placed upon it.

Do Cinder Blocks Need to be Filled?

What are Cinder Blocks?

Cinder blocks are a type of building material all their own and should not be confused with concrete blocks.

Unlike concrete blocks made from a mixture of steel, stone, and sand, cinder blocks are made from concrete and cinder. ‘Cinder’ is coal ash used as an aggregate in the concrete mixture to keep the cinder block light.

Because cinder blocks are lightweight, inflexible, and not meant to bear a lot of pressure, they are generally best suited for small projects. Garden walls, plant containers, and fire pits are all brilliant candidates for cinder blocks.

Is It Safe to Fill Cinder Blocks?

Yes, it is entirely safe to fill cinder blocks, but you want to be careful with what type of filling you plan on using. Additionally, you should take your climate, timeline, and what you need from your structure into account.

Materials Used to Fill Cinder Blocks

If you are planning on filling your cinder blocks, you have a wide range of options that come with different benefits.

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Is It Safe to Fill Cinder Blocks?

Cement and Rebar

Cement and rebar are a classic combination when building retaining walls and other small structures that need a little more strength. This will produce a robust structure, but just make sure you have plenty of time to let it dry.

Sand and Gravel

Using sand and gravel is a cheaper option, and it is recommended to use a mixture of both. You don’t have to worry about moisture as the sand and gravel will be dry.

The gravel adds weight and strength to the wall while sand fills in the corners that gravel alone would leave empty.


This isn’t recommended as it comes back to the issue of moisture. Soil can hold quite a bit of water, and if you’re in an area that gets cold, that moisture will freeze and expand, causing cracks.

Spray Foam Insulation

Spray foam insulation is a great option, though it can be pricey. Spray foam insulation won’t add any strength to your structure, but it will keep your structure nice and toasty.

 As with concrete, though, spray foam insulation will introduce moisture to your structure. It’s best to use this type of filler in the dry season and be sure to give it ample time to dry to avoid problems down the line.

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Do cinder blocks deteriorate?

Eventually, yes. Water is a cinder block’s worst enemy, and it can eventually wear them down to the point of crumbling if they are not regularly treated.

How do you keep water from coming through cinder blocks?

Various barriers such as weeping tiles and masonry waterproofing paint can help keep water out of your cinder block structure. Some types of waterproofing need to be reapplied every few years to continue to be effective.

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Can cinder blocks support a house?

Cinder blocks are weaker than concrete blocks due to their lightweight nature and are generally not recommended for load-bearing structures like a house foundation.

There are occasional exceptions, though. Depending on where the house is being built, cinder blocks could be an option when laying a foundation.

Can cinder blocks support a house