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.

Note: this post contains affiliate links. As an Amazon Associate I earn from qualifying purchases.


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.

See also  What Concrete Slump Do You Need?

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.

See also  Crushed Asphalt vs. Crushed Concrete

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.

Kraken Bond Fastcoat Spray Foam Insulation - Insulation Foam Spray, Polyurethane Spray Foam, Heat Insulation&Acoustic Spray, Self Expanding Foam, Foam Insulation Can, Gun/Cleaner NOT Included | 6 Pack
  • 【𝗘𝗔𝗦𝗬 & 𝗦𝗧𝗥𝗢𝗡𝗚 𝗕𝗢𝗡𝗗𝗜𝗡𝗚】✔️ Kraken Bond Fastcoat Polyurethane Spray Foam is paintable, and has excellent adhesion on most building materials. There are two nozzles to apply. Nozzle A is used for a vertical application that provides wedged spray, and a nozzle named B is for ceiling applications.
  • 【𝗤𝗨𝗜𝗖𝗞𝗟𝗬 𝗖𝗨𝗥𝗜𝗡𝗚 𝗧𝗜𝗠𝗘】✔️ You will no longer wait thanks to Kraken Bond Fastcoat Polyurethane Spray Foam of strong chemical bonds. Note that gaps and uneven surfaces may cause delayed bonding. Wait 24 hours for a full cure. ***𝗡𝗢𝗧𝗘: Store in a cool, dry location as high temperature and humidity can cause early aging of the substance. Do not store in refrigerator. STORE AWAY FROM CHILDREN AND ANIMALS.
  • 【𝗨𝗡𝗜𝗤𝗨𝗘 𝗦𝗨𝗣𝗘𝗥 𝗙𝗢𝗥𝗠𝗨𝗟𝗔】✔️ Kraken Bond Fastcoat Spray Foam Insulation is the most innovative, easy-to-use, polyurethan-based for high quality heat and sound insulation in the world. Kraken Bond Fastcoat Self Expanding Foam designed for professional workers, but it is also ideal for everyday users as well. The R-value measures the thermal resistance of a material. It's 5.66 per inch and thermal insulation value 0.025 W/(m.k).
  • 【𝗛𝗢𝗪 𝗧𝗢 𝗨𝗦𝗘】✔️ Kraken Bond Fastcoat Insulation Foam Spray can should be shaken very well before use. Apply the foam 12-18” distance from the wall at vertical applications. Spray the foam 6-8” distance from the ceiling for horizontal applications. The thicker application means the higher insulation value. It is recommended to keep the can attached to the applicator gun if you wish to resume on a later date. NOT recommended to use with other brands.
  • 【𝗖𝗢𝗠𝗠𝗢𝗡 𝗨𝗦𝗔𝗚𝗘 𝗔𝗥𝗘𝗔𝗦-𝟭】✔️ Kraken Bond Fastcoat Heat Insulation and Acoustic Spray ​can also be used on Drywall, Plaster, Stucco, Wood, Concrete, Brick, Sheetrock, Polyurethane, Glass, Carbon Fiber, Concrete, Stone, Granite, Metal, MDF, PVC, OSB, Automobile body, car trailers, boats, yachts, vessels, camper, sheds, kayaks, RV, and all kinds of sea vehicles.


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.

See also  How to Drill into Concrete with a Regular Drill

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.

Happybuy Tiles Interlocking 55 PCS Black, Drainage Tiles 12x12x0.5 Inches, Deck Tiles Outdoor Floor Tiles, Outdoor Interlocking Tiles, Deck Flooring for Pool Shower Bathroom Deck Patio Garage
  • 【HIGH QUALITY MATERIAL】- These deck tiles are made from high-impact polypropylene making them rigid and durable. This deck flooring is colorfast and features UV protection making the tiles resistant to fading and staining.
  • 【55PCS DECK TILES INTERLOCKING】- This listing includes 55pcs black anti-slip and thickens interlocking deck tiles, three more compared with other competitors.
  • 【NON-SLIP & WATERPROOF】- These floor tiles are designed for non-slip and slip resistance. They can support heavy loads such as SUVs, large pickups, and other large items. The four-side loop-to-peg interlocking system makes installation hassle-free and stable.
  • 【EASY TO SET UP & CLEAN】- These patio blocks are simple to set up and just "snap" together to create any size. No tools or glue required. You spray down with a hose for easy clean and maintenance.
  • 【VARIOUS USES】- Deck tiles interlocking outdoor are perfect for covering slippery concrete or wood surfaces, such as around the pool or to cover unsightly concrete, for locker-room, sauna bathroom, deck patio garage, and other wet area matting.

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