Can You Fill Cinder Blocks With Soil or Concrete? Is It Safe?

Whether you are building a raised bed or a wall, cinder blocks are a cost-effective and sturdy way to do so. Old cinder blocks used to be literal blocks; cubes or cuboids with no holes as such. However, modern cinder blocks (and even bricks) have holes in them.

There are several reasons manufacturers do this; from making them lighter and easier to use all the way to easy alignment. Looking at those holes, it is common to wonder if you can fill cinder blocks, and whether it is safe to do so with soil or even with concrete. There are two schools of thought about this; both right on their own accord.

It’s not recommended to do so if you live in an area where you get frost. As the moisture within freezes, it will expand and could crack or potentially split the cinder block. On the other hand, concrete is actually added to increase the strength of a retaining wall. So, you can add soil or concrete within, but be careful about the climate you’re in.

What Are Cinder Blocks?

Can you fill cinderblocks with soil, concrete, gravel, sand, or foam?

Cinder Blocks are basic concrete masonry units used in almost every construction project. These blocks come in various sizes and are referred to by their thickness. Standard sizes in the US include:

  • 4-inch blocks
  • 6-inch blocks
  • 8 inch blocks
  • 12 inch blocks

These standard blocks are usually 16 inches long and 8 inches wide.

Standard sizes in the UK include:

  • 440 x 215mm
  • 390 x 190mm

As mentioned above, cinder blocks have holes in them to help with alignment and in some cases, the reason is actually to add soil or concrete within, provided the conditions are right.

When Is It Safe to Fill Cinder Blocks?

A cinder block being picked up by a construction worker
A cinder block being picked up by a construction worker

It is common practice to fill the holes of cinder blocks with rebar and then add concrete within to strengthen them. However, when doing so, it is important to take note of the climate you’re working in. For example, if you’re working during the winter season or even monsoon (not recommended, by the way), filling the holes is only going to create more problems for you.

If, however, you’re working in the middle of summer with little to no humidity, feel free to fill cinder blocks with soil or concrete.

The reason filling is not recommended in most cases is that as the joining solution (mortar or concrete) you use to join cinder block solidifies, whatever you add within will diffuse the moisture inwards. Not only with that, even if you’re adding soil, there will be moisture within. Concrete, of course, will be wet.

This moisture can take a year or two to dry out completely. This moisture will freeze over as the frost comes, leading to cracks in the cinder blocks. This is particularly dangerous when building retaining walls.

If you are building a raised planting bed, it doesn’t really matter if your cinder block develops cracks. What’s the worst that can happen, right? But if you’re working on a bigger project where the cinder blocks need to hold up immense weights, these cracks can be devastating.

Can You Fill Cinder Blocks With Sand?

There is no doubt that sand is much easier (and inexpensive) to pour than anything else – even the more traditionally preferred sand and gravel mixture. When pouring sand, your goal should be to fill all voids and ultimately seal the bottom. There should be no place left for the sand to go because it won’t be wet and therefore will have a tendency to flow.

Try to compact the sand as much as possible to ensure a sturdy ‘infrastructure’ for your cinder blocks to rely on. If done right and compacted enough (8-12 inches), you won’t really need to pour concrete. This can be done in wet areas as well. Just make sure you don’t overwater your mortar or joining solution.

Here’s something that we have actually tried and has worked wonders for us:

Build the wall or structure you are looking to build. Finish it and then drill two holes; at the top and the bottom of the wall (atop the holes you’re trying to fill). Place a screen or filter at the bottom hole such that only water can flow through it, not sand.

Pump a sand/water mixture and wait. The water will carry the sand into every nook and cranny and compact it. Use a pump to make sure it’s compact. Let the water drain out and you’ll only have sand left behind!

Can You Fill Cinder Blocks with Dirt?

Dirt isn’t really recommended for cinder block fill. It can retain a lot of moisture and therefore impact your structure’s integrity. We would suggest that you go with sand instead of dirt if possible.

Can You Fill Cinder Blocks with Gravel?

Filling cinder blocks with gravel alone – especially pea gravel – is known as a lazy-fill. Yes, it works, and it increases the wall’s mass and strengthens it. This is great for places like Florida or Spain (sub-humid). However, the wall won’t really insulate.

So, basically, you’ll get a sturdy wall, but you’ll most likely hate that particular wall during the winter season.

Instead of using just gravel, we would recommend you use a mixture of sand and gravel, which actually takes care of that issue. The sand seeps into the nooks and crannies left by the gravel while the gravel adds mass into the wall and strengthens it. You get the best of both worlds with very little extra cost, considering that sand doesn’t really cost that much.

Just remember to add water into the sand when adding so that the mix compacts well. Give the mix about 2-3 days so that water drains out and only then seal the exit.

When mixing, for every 3 bags of gravel, add 4 bags of sand. However, the mix is entirely up to you, depending on the degree of strength-to-insulation you are looking for.

Can You Fill Cinder Blocks with Foam for Insulation?

Spraying foam insulation into a ground area
Spraying foam insulation into a ground area

The answer to this is a resounding YES. People fill cinder blocks with foam regularly for insulation. It isn’t the best idea out there, but it is perhaps the next best thing after a sand and gravel mix. However, we would recommend doing so only when the summer season is at its peak.

It is quite common for the foam to be a bit too compressed at one spot and thus not really dry out completely. When this happens, you will start seeing wet patches across your wall. This mostly happens at the bottom of cinder block walls and ends up damaging the paint on that particular area.

We recommend you not seal the top for a few hours – or at least a day – after filling it with foam to let it dry out completely.

Filling cinder blocks with foam is ideal for insulation, not to mention how easy it is. However, the venture can get really, really expensive, depending on how large the wall is.

Can You Fill Cinder Blocks with Concrete?

If you are working in the middle of summer, are a patient DIY-er (or professional), or are building a retaining wall, you simply can’t go wrong with filling cinder blocks with concrete. The goal of adding concrete within cinder blocks is to strengthen the wall.

The process itself isn’t particularly difficult, nor is it as expensive as adding foam. By adding concrete into the cinder blocks, you are basically mimicking poured concrete walls. Just remember to give the wall enough time to cure before you apply plaster, distemper, or emulsion.

One way to ensure that the wall dries up properly is to build and fill one layer of cinder blocks at a time, but let us ask you, do you really have time or patience for that?

Summary

Considering everything, filling your cinder blocks isn’t exactly necessary, but it is completely safe, assuming you don’t add dirt in it. Filling cinder blocks can help you make the structure sturdier or improve insulation, while a mix of gravel and sand can actually help you achieve both.

About Charlie D Paige

Charlie is a massive DIY fan, with dozens of DIY projects under his belt - ranging from tiling to electrics, and concrete pads to walls. Charlie loves tinkering, seeing how things works, the outdoors and playing with power tools... so is it any wonder that he's completed so many DIY jobs over the years?

Charlie loves spreading his hard-won DIY experience with the world via this blog.