Concrete blocks come in many forms with different uses. However, when somebody mentions concrete blocks, what most people think of are the rectangular blocks with two holes. These are known as corner blocks.
There are also larger ones with three holes that are similar. Also available are many kinds of flatter blocks; however, these are not the kind most people use for a shed base. This article will discuss those and a couple of other forms.
In addition to go over the pros and cons of concrete blocks I will look at the different types of concrete blocks and their usage for a base for a storage shed.
There are a couple of block forms that come under the umbrella of concrete blocks. These are deck blocks, which contain slots to hold the lumber framing the base of the shed. Pier blocks have a built-in bracket that attaches to the frame boards. Some suppliers call these deck blocks also.
Both of these can be used to support both the outside framework and the joists. Some localities prohibit the use of these blocks, depending on the type of shed you build, while others have restrictions governing their use.
The Pros and Cons of Concrete Blocks
Concrete blocks provide a simple way to put a foundation under your shed. They are easy to use and place, and you don’t need a huge excavation to install a foundation. A shallowly dug, level base for some gravel to facilitate drainage under the blocks is all many people use.
There are problems with using blocks for a foundation. Some people simply grade out their ground and level it, then place the blocks right on the soil. This alone causes its own problems. Soil tends to settle over time, even if the place you picked for your shed is usually dry. Softer soils underneath may settle, causing sinking on the surface.
Freezing and thawing also tend to shift the soil and can actually raise and lower the shed a bit. When it’s lowered, there’s no guarantee it will be lowered in a level position but may actually be tilted. This will cause strain on the connections all over the shed and may cause the door to stick when opening and closing.
Additionally, people may think that they only need blocks under the corners. This leaves the centers of all sides and the center of the shed itself unsupported. This puts undue strain on the floor joists. A floor without sufficient support will tend to bounce and eventually sag. Even putting the shed directly on a gravel pad, while not supplying airflow underneath the shed, does provide support under the whole floor.
Blocks do not keep weeds from growing underneath your shed and do not keep out animals, such as rats and mice, and even skunks, from under your shed and perhaps getting inside. Without gravel underneath and around the shed, rainfall from the roof will splash against your shed siding. Heavy rain will also splash water and mud against your shed.
If you have wood siding, this will be an extra source of moisture you don’t need on your siding. Metal siding may rust prematurely if it gets a bath every time it rains. The mud that can splash up certainly doesn’t improve the shed’s appearance, either.
Additional problems that people may not realize include the fact that some shed companies discourage putting their products on blocks, especially in colder climates because of the freezing and thawing ground problems. Putting a shed on blocks may even void the manufacturer’s warranty on a prefab shed or one built to order. If you are needing a different type of foundation for your shed read my article What’s The Best Shed Foundation Option For You?
What is the Difference Between Cinder Blocks and Concrete Blocks?
Many people use these terms interchangeably, but concrete and cinder blocks are actually two different things. Concrete blocks are made from pure concrete and cement. The only aggregates used are finely crushed stone or sand. They often have holes in them to allow for further reinforcement.
Cinder blocks contain actual cinders and ash from coal. These came into being in the past when most houses and businesses burned coal for heat, producing a lot of cinders. Steel mills especially would create tons of cinders every day from their processes. Somebody came up with the idea to utilize this waste material in the production of blocks, which came to be known as cinder blocks.
Cinder blocks used to be somewhat cheaper than concrete blocks, but they have not been produced in mass quantities for decades. They are lighter than concrete blocks and not as strong. Because they were more prone to bucking and bowing, they gradually fell out of favor with reputable construction firms. Some building codes state that they cannot be used in construction and building projects.
Still, some builders will still use cinder blocks when permitted because they are lighter weight and easier for the workers to handle all day. These days, the blocks are far more likely to contain volcanic pumice than actual coal cinders.
How Many Concrete Blocks Do I Need for my Foundation?
For a shed foundation, using solid blocks instead of the common hollow model is a much better idea, as they will provide more support. Filling the hollow spaces with concrete is a good way to make the hollow blocks more sturdy and able to withstand the weight of the shed.
While it’s a bit more work, the hollow blocks are lighter than the solid ones and easier to handle, so you might decide to add the concrete after the blocks have all been placed, especially with a large shed.
Depending on what you put into your shed and what size it is, the hollow blocks may actually start to crumble over time. Concrete blocks should be used at each corner, then spaced about six to eight feet apart along the outside framework. Space the blocks under the joists the same way.
If you need a building permit for your shed, be sure to discuss your foundation with the building authority. They may want to inspect your shed placement for soil type and drainage and may not approve a concrete block foundation, either in that particular place or on your property entirely. For more information on building permits read my article Do You Require a Building Permit For a Storage Shed?
If they do allow the block foundation, they may require additional foundation or drainage materials. It’s important to check on this possibility unless you live in a very rural area with no restrictions.
How Expensive are Concrete Blocks?
The standard concrete block with the two holes costs a little less than $2 each. Deck blocks can run from $7 to $10 each. The pier blocks with brackets run around $8 each. Some are manufactured so that you can attach various kinds of brackets to fit your needs.
Do I Need to Put Gravel Under Concrete Blocks?
It’s really a good idea to put a gravel pad under the blocks. Not only with it help keep the shed from shifting with the soil, but a gravel pad will help with drainage and keep weeds from sprouting up under the shed. Properly installed, a gravel pad will also keep water from splashing up and affecting your siding.
In addition, a gravel pad will help distribute the weight of the shed and keep the blocks from punching through the top of the ground and sinking. A vapor barrier may be installed under the gravel to protect the shed’s bottom. Landscaping fabric under the gravel will block weeds from growing through. You may find my article What’s The Best Way to Add Drainage Around a Shed? helpful with using gravel under your shed.
Will the Type of Soil Affect a Concrete Block Foundation?
The type of soil and the topography of the site will both certainly affect a block foundation. The slope or lack of it will affect drainage and may cause your shed to sit in a big puddle after heavy rain. The soil type likewise is very important and impactful.
Peat is often located in areas of swampy or reclaimed land. It can be totally dry during summer but get spongy when the rains come, offering less than optimal support for a shed. Silt comes from washed-up soil occurring on old floodplains. It can expand with moisture and weaken shed supports. Clay likewise is not good soil for support.
It holds moisture and is pliable and even can be slippery when it gets wet. When clay gets dry it shrinks and can even crack. This causes your foundation to move.
Sandy soil or soil with gravel has good drainage, but the particles can wash away, creating dips and holes in the soil. If this soil is compacted well, it does make good foundation support. Loam is a combination of silt, sand and clay. It’s usually what is marketed as topsoil. It keeps its shape when moist but also drains well and is good for foundational support. Rock, of course, makes for a very stable foundation. Getting it level, however, can be impossible. Check out my article Should I Build a Shed in a Sloped Yard? if you have a sloped area where you putting your shed.
Are Concrete Blocks Easier to Install and Level?
Concrete blocks are fairly easy to handle for many people, especially the hollow type. Whether you install them on a gravel pad or directly on the soil, all that’s needed is to make sure the base is level. Any discrepancies may be addressed by simply adding or removing soil or gravel to make the blocks level.
How to Install Concrete Blocks
You’ll need a shovel and garden trowel, measuring tape, string, stakes, a long level, hammer, wheelbarrow, and a straight board eight feet long. If you have rocky soil, you may also need a pick.
Clear out your side of growth, rocks, roots and any other debris. Put a stake in one corner of your side and use the string and the other stakes to outline the site. A good tip is to add a foot all the way around so you won’t have to move the stakes when placing the blocks. Make sure your space is square. Measure three feet on the string from your first corner and mark it. Then mark four feet in the other direction.
The distance between the two marks should be five feet. If it isn’t, move your other stakes accordingly until you get the proper distance. Repeat the process with the opposite corner and your space should be squared.
Place blocks at each corner and at the center point of the sides. Hollow-core blocks should have the holes up. Score the dirt around the blocks, then remove them. Use these score lines for your excavation. If you’re just using blocks, you won’t need to remove much soil; you’ll just need to make sure the base is level.
However, since it’s best to install a gravel base, dig down enough so that you can pour four inches of gravel. Smaller gravel will aid in tamping down and leveling. Before pouring the gravel, install a vapor barrier large enough to go into the depression, up the sides, and over the edge somewhat.
If you don’t wish to install a vapor barrier, at least you should put down landscape fabric to prevent growth under the shed. Make sure your gravel is tamped down firmly and is level.
Place your blocks down as before. Using a corner block, make sure this block is level. Use the long board to stretch to an adjacent block to make sure it is level and that the span between is level. Add or remove gravel to get this block level.
Work your way from block to block making sure that each block is level and that the span between blocks is level. Once you are done, this is the time to add concrete into hollow blocks to add stability. Add some gravel around the shed space to drain water away from it.
While some install blocks only around the outside frame of the shed, it’s a good idea to install blocks to support joists inside the frame as well to take the strain off the joists in the center. Sheds usually store items that add up to quite a bit of weight. Joist block supports will prevent eventual sags in the shed.
Conclusion
Taking care with constructing your shed base and foundation is easily the most important part of building or installing a shed. If something is lacking in the foundation it can eventually lead to leaning or sinking, or at least put a strain on the shed. Make sure to take the time to construct your foundation so it is level and sturdy.