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Cinder Blocks: 15 Things You Always Wanted To Know

Versatile, cheap, durable and lightweight – cinder blocks are your friend when it comes to a multitude of backyard projects.

Cinder blocks can be used for raised beds, potting tables, birdhouses, succulent gardens, firepits, grills, firewood boxes, storage shed shelves, benches, steps, pit smokers, plant shelves, garden borders, table legs, walls, water features, fences, outdoor kitchens, compost bins or swimming pools.

So in honor of our friend the cinder block, here are 15 things you’ve always wanted to know about them (but forgot to ask)…

Cinder Blocks

What Are Cinder Blocks Used For?

Versatile cinder blocks can be used for: raised garden beds, potting tables, birdhouses, succulent gardens, firepits, grill tables, firewood containers, storage shed shelves, benches, steps, outdoor fireplaces, and in pit smokers.

Did we have you at raised garden beds? No? Here’s more: plant shelves, garden borders, patio table legs, privacy walls, privacy walls with vertical gardens, water features, fences, outdoor kitchens, table centerpieces, compost bins, or swimming pools.

Don’t like the look of cinder blocks? Try cementing shells, pebbles or cut glass to the outside. Paint striking modern designs or use stencils. Contrast the light grey cinder block with other materials like rough wood, dark wood, slate, or terracotta. 

Use mounting putty, hardwall hangers, exterior self-adhesive hooks, landscape block adhesive, concrete screws, or expansion bolts to add stuff with colors or textures. Cement on other veneers like bricks, tiles, or stone. Add a wrought iron element.

Here’s a video with 40 cinder block ideas.

Which is Stronger Cinder Block or Concrete Block? (What’s The Difference?)

A hollow concrete block is stronger than a coal cinder block. Coal cinder blocks weigh less because they are made from an aggregate of coal cinders (aka breeze), and not an aggregate of sand and small pebbles

What is a traditional cinder block? Cinder is actually what it sounds like – a small piece of something that burned. In the case of cinder blocks, the thing that’s burned is coal. The coal cinders are leftover products from coal-burning power plants – the fly and bottom ash portions. 

Coal cinder blocks are light-weight, low density concrete masonry units (CMUs). They are made from Portland cement and an aggregate of burned coal cinders. The mixture is poured into molds and then heated or air dried. Traditionally, coal cinder blocks have either two or three hollow interior areas.

What’s the difference between concrete blocks and coal cinder blocks? It’s the aggregate. A hollow concrete block is made of two things – an aggregate and Portland cement. For concrete blocks, the aggregate is made of sand and small pebbles. For coal cinder blocks, the aggregate is coal cinder. 

Coal cinder is very lightweight, while sand and pebbles are not. 

But the truth is, the terms concrete block and cinder block have become synonymous. Many stores have hollow, cored or celled blocks made of concrete that are labeled “cinder block”. Check the fine print – your cinder block may not have been made with coal cinder. More likely, it’s concrete.

Coal cinder blocks are ideal for small scale garden and home projects. Coal cinder blocks should not be used for anything load-bearing. Check your building codes if you are planning to build an entire structure with them, because they might not be strong enough. 

But hollow concrete blocks are a different story. They are commonly used in buildings.

Where Can I Get Cinder Blocks?

Cinder blocks are incredibly useful, so every hardware store has them. Check Home Depot, Lowes, or your corner hardware or building supply store.

You can even buy used or free cinder blocks on Craigslist or your neighborhood social website. They last for decades, so people have them hanging around forever.

What is The Average Price of a Cinder Block? (Are They Cheaper Than Concrete?)

Standard, plain hollow concrete blocks start at about $1.00 each. Some fancier ones range up to $4.00 each.

Experts say that the price of a hollow concrete wall is around $1.50 per square foot. The price of poured concrete is just under $3.00 per cubic foot.

Hollow concrete blocks are cheaper, need less special equipment and are easier to lay for DIY home and garden projects.

How Long Do Cinder Blocks Last?

Cinder blocks will last for years and years – up to 100 years

Are Cinder Block Walls Strong?

Coal cinder blocks are plenty strong for DIY garden projects like raised garden beds and fire pits. If you are building a tall, load-bearing structure, use hollow concrete blocks.

If your block project is tall or very long, consider strengthening it with rebar and poured concrete. 

Is is Safe To Grow Vegetables in Cinder Blocks? (Are They Good Planters?)

We checked loads of gardening websites to see if people had bad experiences. We didn’t find any. Lots of people use cinder blocks to grow vegetables, plants and flowers.

For coal cinder blocks, there is a small chance that the coal cinder may have small amounts of toxins. But most people just sealed their blocks before planting.

For hollow concrete blocks, there is also a small chance of lime contamination. Sealants would take care of that. The cells are a perfect size for single plants and the cement won’t ever rot, like wood.

One of the many benefits of using cement blocks is that they stack so nicely. You can grow your vegetables in raised beds. Raising your beds helps with water drainage and – your back.

Here’s a video on how to build a cinder block raised bed garden.

Do You Fill Cinder Blocks With Concrete?

Filling a coal cinder block or a hollow concrete block with cement strengthens it a lot.

For small projects, like retaining walls or raised garden beds, people add strength with a length of vertical rebar at the corners. Then, cement in the rebar.

Can I Build a Fire Pit With Cinder Blocks? (Are They Heat Resistant?)

Cinder blocks are great for building fire pits. Concrete is a non-flammable material. In fact, a cinder block fire wall will absorb heat and protect you from sparks.

If you are shopping for hollow concrete blocks, see if you can find some that are fire rated. The blocks shouldn’t be sealed. Concrete blocks are porous – and that’s a good thing for a fire pit. The fire pit blocks will get wet – in the rain or with the hose – and water will get inside the pores. 

When the fire pit is burning, the water (or steam) needs to escape through the pores. When water can’t escape, small explosions can happen, like with river rocks.

You can start your fire pit without mortar – just stack the blocks on a level surface. If you like the design and the location, make it permanent with mortar.

Can Cinder Blocks Support a Car? (How Much Weight Will They Hold?)

Nope, not your car. Yeah, sure, we’ve all seen it done lots of times. But what you may not see is when the cinder block fails and the car crashes to the ground – not something you want to risk, particularly if you are working underneath it. 

Just. Don’t. Do. It. Experts say use car ramps.

Are Cinder Blocks Structural? (Can They Be Used For Construction?)

Coal cinder blocks are not strong enough for load bearing walls. Hollow concrete block walls are. Experts say to reinforce your hollow concrete wall with rebar and cement inside the cells.

Cinder blocks in a pile

Do I Need Rebar For a Cinder Block Wall?

Rebar is very handy for hollow concrete blocks. Some people use lengths in the corner to help stack the blocks.

Rebar (steel reinforcing bar) is routinely added to concrete to correct a concrete failing – tensile strength. Concrete is strong when a load is applied directly from the bottom or the top. It’s not so strong if you pull on it. Pulling happens when the concrete isn’t supported, and it tries to bend or sag. When this happens, the concrete fails and cracks.

Rebar has the strength that concrete lacks. In fact, it’s strength perfectly complements the failure of cement – and vice versa. Rebar will bend under pressure, but you can’t stretch it in a million years. Rebar and concrete make the perfect high-strength marriage.

How much and where you put rebar in a wall depends on your project size. Here’s a good reference.

How Do You Insulate Concrete Blocks?

Because hollow concrete blocks are – hollow – they are easy to insulate. You can spray foam insulation into the cells, inject foam insulation, fill them with polystyrene beads, use foam boards, or use loose-fill masonry insulation.

Be sure to put a cover on the top of the wall – don’t leave the cells open. Just providing a top cover will help the insulation of the cells.

How High can You Stack Cinder Blocks?

Cinder blocks are ideal for stacking. Experts say that cinder blocks shouldn’t be free stacked higher than 6 feet, 8 inches. If you want to construct something hide and wide, consider adding some rebar both horizontally and vertically.

Can You Paint Cinder Blocks?

Absolutely. Consider a coat of sealant or primer first because cinder blocks are super porous. Then, use exterior paints, either spray paints or liquid paints.

Final Thoughts

All hail the cinder block – the versatile building block for all backyard projects.

Strong, light, cheap and easy to use, what’s not to like when it comes to the humble cinder block?

And don’t tell anyone, by my office building is built on top of some. It’s not supposed to work but it does – and nothing’s moved yet in over ten years.

But hang on, what’s that rumbling sound beneath my feet?! 🙂

Mark H.

Homeowner and property investor Mark H. aspires to bring you the very best outdoor living content, based on his years of experience managing outside spaces. Read more >