A concrete block foundation is one popular form of foundation to make sure your shed base is sturdy and solid. You may have wondered about using this type of foundation when you are planning to build a shed, especially if you have some blocks left over from your home construction or another project.
If your shed is going on solid soil and isn’t much larger than an 8 x 12 or so, a concrete block shed foundation may be just what you want. These foundations are certainly easy to construct and don’t cost much to install. We’ll look at the pros and cons of such a foundation and how to construct one for maximum benefit.
Pros of a Concrete Block Shed Foundation
Easier to level the shed: Concrete blocks provide a way to level your shed when it has to be installed or built on a slope or uneven ground, much like a pier and post system.
Keeps shed above ground: Blocks can raise your shed above ground enough to keep it away from moisture, insects, and even mice and rats. It can keep the snow from piling up against your shed, causing moisture problems, or even blocking you from getting inside for a time.
Blocks raise your shed above any erosion problems when combined with a good drainage system. They also provide needed airflow underneath your shed to prevent mold and mildew problems from forming inside your shed. This airflow can also keep your shed cooler in hot weather.
Cheap option: Blocks are a less expensive type of foundation than some others and certainly involve less work than, say, a concrete slab foundation.
Easier to move shed: Having your shed on blocks actually makes the shed movable if it becomes necessary at some point.
Cons of a Concrete Block Shed Foundation
More blocks needed to support heavier loads: Unless your shed is going to be a small one, such as those that are only used to store rakes, shovels, yard chemicals and the like, a typical concrete block installation that only goes under the corners of the shed may not give you the support you would want.
Most people who construct such a foundation don’t run a row of the blocks under the center of the shed, leaving less than optimal support for the center of the shed and heavier items such as riding mowers or snow blowers or even heavy sacks of cement or fertilizer. You may end up with a sagging floor. All in all, concrete blocks are useful as a foundation only for small or medium-sized sheds.
Blocks will settle: Unless you provide a solid base for your blocks, which will take excavation and a thick layer of gravel, your blocks may settle over time. If your ground is soft or prone to erosion, or if your ground is affected by freezing and thawing, you may find that your shed is sinking at some point. Since it’s more likely to sink on one side versus straight down, you may find that your door doesn’t open or close correctly.
Allows weeds and grass to grow: Blocks don’t keep grass or weeds from growing right up against your shed. The proximity of this growth will tend to keep moisture around your shed, possibly leading to mold or mildew problems down the road.
Lifts and move from frost: With the potential sinking from soft soil, erosion or frost heave, many shed builders try to discourage customers from using a block base. In fact, with some shed manufacturers, using a block foundation may actually void the manufacturer’s warranty.
This is especially true in the Northeastern part of the U.S. Since you’ll need to provide the foundation for most pre-built sheds, you need to check with your vendor as to whether they’ll approve of a block foundation.
May not be allowed in some areas: Any town or city building code may not approve of a block foundation, nor an HOA. In colder climates subject to frost heave, any foundation may have to be secured below the frost line, usually necessitating installing concrete piers as deep as three to four feet.
A concrete block shed foundation wouldn’t be allowed in such an area. Be sure to check with the relevant authorities before starting a concrete block shed foundation. For more information on permits read my article Do You Require a Building Permit For a Storage Shed?
Can I Use Cinder Blocks for a Shed Foundation?
The terms cinder block and concrete block are often used interchangeably, but these are actually two different products. In fact, true cinder blocks haven’t been produced in great numbers for several decades. Cinder blocks came into use back in the days when people used coal furnaces or wood stoves for home heating.
In addition, the large steel manufacturing plants used a great deal of coal in their steel-making process, leaving a lot of leftover ash, or cinders, behind. Homeowners used to have buckets full of ashes left to dispose of.
The ash began to be used in the manufacturing of cinder blocks. The ash was used in place of aggregate, or the fine gravel and sand used in concrete, making lighter weight and less expensive blocks. Unfortunately, this meant that the blocks weren’t as sturdy or reliable as true concrete blocks.
If you do find actual cinder blocks, they really aren’t up to the task of holding up your shed. They are more prone to bowing and buckling and don’t have much tensile strength. They just aren’t good for a foundation that you want to last.
In addition, cinder blocks are usually formed with the familiar two large holes through the middle, although concrete blocks may be formed this way as well. These are known as hollow core blocks. For a shed foundation, you’ll want to use solid, rectangular blocks, or you can also fill the hollow cores with some rebar and concrete to make solid supports. Other option can be paving slabs, you get get more information in this article. Can I Use Paving Slabs for a Shed Base?
How Many Rows of Concrete Blocks are Needed?
This depends on several factors. Some people with smaller sheds will only put blocks or a pillar made of blocks under the corners. Larger sheds need more support, with blocks every six to eight feet all the way around and a row down the middle.
This configuration allows for that all-important airflow under the shed that keeps moisture from condensing on the joists and underside of your shed. How many you need will depend at least partly on whether you need to use them to level your shed.
You may need three blocks per pillar on one end and two on the other. The shape of the blocks, whether square or flatter solid rectangles, and how high you want your shed raised figure into this as well. Most sheds take about 20 to 60 blocks unless you’re using the slotted foundation blocks.
How Deep into The Ground Do the Blocks Need to Be?
The blocks themselves actually don’t have to go into the ground. You should excavate your shed space enough to lay a base of four inches or so of tamped gravel first. The gravel will aid in drainage and help keep the soil from eroding around the blocks.
You can put taller blocks into the ground an inch or two, but most people who use them without a gravel base will only dig out the ground under them enough to make sure the blocks are level with each other.
What Do I Put Under the Concrete Blocks?
The best thing is a layer of tamped gravel a few inches deep. It should extend out all the way around your shed perimeter a foot or two to guide water around your shed and keep it from pooling around your blocks or underneath the shed. The gravel helps to keep soil from eroding around your blocks and aids in water evaporation.
How Do You Lay a Concrete Block Foundation?
Layout the shed location: Mark out your site with stakes and string, and make sure all the corners are square. Place a block in each corner and at the center point of the long sides and blocks down the middle if needed. Push your shovel down right along the edges of each block, then remove the growth from this space. Replace the blocks. If you are using hollow-core blocks, the opening should be on top.
Prep Ground: Remove all the growth, roots and rocks about three or four inches down inside the perimeter. Cover the space with landscaping fabric, using commercial-grade fabric if possible, to keep it from being punctured by your gravel. Put down three or four inches of sharp gravel, tamping it down and leveling it every two inches instead of trying to tamp the whole layer at once.
Install and level blocks: Choose one of your corner blocks, making sure it is level in both directions. This corner will be your reference. All the other blocks must be level and level to the reference block. You can use a long 2 x 4 placed from one block to another to help you check that the blocks are level with each other.
Remove soil or add gravel under each block to get the desired result. Once your blocks are level, you can fill the hollow cores with cement and perhaps a piece of rebar. Placing some rebar into the center of each hole and hammering it a foot deep before pouring concrete into the holes will really make sure the blocks are solid and aren’t going anywhere.
For a slope, you may want to dig into the slope to make a level base. If you have to dig very deeply, you may need to build a retaining wall to keep the soil from sliding down against your shed. The retaining wall may be built to be wider than your shed by at least a foot on each side to divert water around your shed.
Proper Drainage: You may also want to build a french drain system to divert water from pouring down the slope and eroding the soil from underneath your blocks or washing away some of your gravel. The drain can start at a distance behind your shed on the slope, then travel down one side at an angle, steering the water away.
If you don’t want to dig into the slope, you can use blocks of different sizes to level your shed, making pillars of blocks cemented together on the low end. If you find that you need to raise one end more than a couple of feet, you should probably switch to a poured pier and post system for that end rather than stacking more blocks.
You still need to take care to make sure all the blocks are level with each other at the tops. For very small leveling differences, some people have found that pieces of asphalt shingles make great shims. For sheds not dug into the slope, a french drain system would still be a good idea. Heavy rain can and will always run down a slope, potentially eroding the soil around and under your blocks. Check out my article What’s The Best Way to Add Drainage Around a Shed? for more options to property drain water from around your shed.
Solid foundation blocks. There are solid concrete blocks made just for foundations such as this. The ones most commonly used for shed foundations measure 8 x 8 inches square and are 16 inches tall. They come with slots molded into the tops to hold your main frame planks. Because the slots are molded in on all four sides, the center blocks can hold both your outside frame plank and a perpendicular foundation plank running down the middle of your shed floor.
These blocks are great for sheds to be located on a smaller slope or ground that is just uneven, perhaps with a low corner. Leveling your shed with these can be done by placing extra wooden pieces into the slots and fastening your frame planks to them.
The planks can also be placed so that the planks rest on top of the blocks instead of in the slots for leveling purposes, but this should only be done with a block or two when necessary. You should take advantage of the slots whenever possible for a more sturdy construction.
These blocks are usually placed on top of your gravel base. Some builders prefer to put two or three inches of tamped gravel only below the blocks. Your soil type and the likelihood of rain runoff through or around your shed site will dictate which method to use.
If your soil type is solid enough and your slope not too much, you can save quite a bit of expense and work using a concrete block foundation with a small or medium shed, such as an 8 x 12.
Conclusion
Be aware, however, that with larger sheds, especially those that will hold heavy lawn equipment, this type of foundation may not be suitable. You’ll need to take that into consideration in planning your shed, but this is definitely a popular and feasible form of foundation.