When we think of concrete deck blocks, most of us think of the usual hollow or cored concrete block with two large square holes. However, there are several types of concrete deck blocks, differing in both size and shape. Some have hollow spaces while others are solid. Almost any of these can be used as a shed base to support the floor beams or even set as a solid floor or base.
You can use concrete deck blocks for a shed base as long as you remove the grass and add some gravel under the blocks.
Much depends on the type of soil and terrain you have. If you have sandy or loose soil, blocks may tend to sink over time unless they are set in firm footings. Likewise, on slopes it may be handy to level your shed by using blocks of a shorter height so that you can pile two on the high-end corners and three on the low end corners.
However, a slope leads to runoff in heavy rain which leads to erosion. This will eventually cause problems with the block stability and your shed may start leaning after a time. Your climate will also play a part, as frost heave can really play havoc with a shed standing on blocks along set along the bottom frame.
By the way, although some people refer to these blocks as cinder blocks, the ones you are looking at probably aren’t. Concrete blocks are made of cement and an aggregate of crushed stone or sand. Cinder blocks contain actual coal cinders and ash. They came into commercial use to take advantage of the huge number of cinders left over from steel mill processes.
They are lighter and usually cheaper than true concrete blocks but are not as strong. In fact, they are outlawed in building projects in some states. Nowadays they are more likely to contain volcanic pumice rather than coal cinders. It’s best to stick to true concrete blocks.
Can I Use Concrete Deck Blocks Instead of Footings?
You can use blocks instead of footings for small to medium-sized sheds up to 160 square feet and if your soil is level and solid. You still would benefit greatly from first digging into your ground to level the shed space and pouring in a few inches of gravel, then tamping it down firmly. Blocks set directly on soil are much more likely to sink or tilt over time, leading to an unstable shed.
If your climate makes your ground subject to frost heave, you won’t be able to just rest the blocks directly on soil or on a gravel foundation. You need to check with your local building code enforcement agency. They may require that the blocks be set a certain distance below ground, that they have solid concrete footings underneath them, or not allow them to be used in this way at all. Check out my article Can I Use Paving Slabs for a Shed Base? to see if paving slabs are a better option.
How Much Weight Can Deck Blocks Hold?
A single 12×12-inch deck block can hold up to 2,000 lbs. That may sound like a lot, but it’s tough to calculate how many you will need from that figure. It’s hard to know how much your shed will weigh, although if you buy a pre-built shed, you may be able to find out from the manufacturer.
You’re not done, though, because you still have to guess how much weight will eventually go into the shed. That lawn tractor is heavy, and even a few cans of paint can add a surprising amount of weight.
A good rule of thumb is to use one block support every three to four feet for a shed that will see normal use. The average 12′ x 12′ shed, for instance, will use at least 18 blocks. For a smaller shed that won’t be stuffed with heavy motorized gadgets, you can go to a support every four to six feet. This is one place in which too many is much better than too few.
Can You Stack Concrete Deck Blocks?
You certainly can stack concrete blocks. If you want to stack them, you can use cement to connect them or use a thick bead of construction adhesive applied with a caulking gun. To keep the foundation solid, it would probably be better to use solid blocks rather than hollow ones. If you do use hollow core blocks you can add stability by inserting rebar through the cores and filling them with cement. If your ground has a lot of slope check out my article How Do You Build a Concrete Pier Foundation for a Shed?
How to Use Concrete Deck Blocks for a Shed Base
The common deck blocks are typically 12 x 12 inches square with slots molded into the top to allow for the insertion of the foundation beams. Each block has two slots, allowing the insertion of a beam from any direction. They are laid over a level gravel base with one at each shed corner and along each foundation beam every three or four feet, unless you have a small, lightweight shed that won’t be used to store heavy things. In that case, fewer blocks can be used and spaced out more.
Start with the corners, then lay out your blocks. You’ll want to make sure all the blocks are level with each other in order to make sure your foundation beams will be level. Of course, use a level on the beams as well to double check.
Solid blocks that come in sizes such as 8 x 16 x 4 inches can be used as foundation blocks. They are usually listed as cap blocks. In this case, 4 x 4s are often used as the foundation joists. These joists actually just rest on the blocks but are attached to the floor frame. If you need to stack blocks
You can also use hollow core blocks that are often called fence blocks. They are the same size as the solid blocks but have cored spaces. However, they aren’t as strong as the solid blocks and may cost more.
The common hollow core block that sizes out at 8 x 8 x 16″ is often used as a shed foundation. They should be used and spaced out like the deck blocks. If you need to stack them, make sure the core openings face vertically. You can use cement or construction glue; however, the cement will make a more permanent bond.
If you’re using them in stacks to even out a slope, try using cap blocks instead or a combination of both. On a slope, you’ll probably not find that the drop from one end of the shed to the other is an even eight inches, or the height of a block. The shorter cap blocks enable stacking in smaller height increments.
You’ll need to dig out underneath your block placement to both provide a gravel base and level each block stack. Using shorter blocks in the stacks will cut down on the digging as the slope progresses. If this seems like a lot of work, it is. If you have a noticeable slope, it may be much better to use a post and concrete pier system.
This entails pouring a round cement pier with part of the pier sticking up above ground. A bracket is set into the top before the concrete sets to hold the pier. This system is often used as a shed foundation on sloping ground.
How to Use Concrete Deck Blocks for Posts
You’ll need to find hollow concrete blocks that are preferably square and have a square opening in the middle large enough for your posts. Some deck blocks are formed with both slots and a post opening. To secure the post, a good solution would be to use either a hammer drill or a drill with a hammer drill function to drill a hole into the block for a screw to secure the block to the post.
An alternative is to use some good construction adhesive around the post to prevent any chance of wobbling. Ask your supplier what is recommended in this case. Remember that the glue is not considered to be any support, but rather is temporary in this case.
How Much Do Concrete Deck Blocks Cost?
The 8 x 8 x 16″ hollow core concrete blocks everyone thinks of usually cost $1.50 to $2.50 each. If you plan to stack blocks to help with a slope, you might choose a similar block that is only four inches tall. These cost around $2.00.
A cored block is usually square, about 8 x 8 x 8 inches, with a square or rectangular hole in the middle. They run around $2.00 per block.
Deck blocks with slots for beams and a square opening in the middle for a post are about seven inches tall and twelve inches square or sometimes a little smaller. They cost about $10 per block.
Solid blocks can be used to support the foundation frame as well. They usually come in 8 x 16-inch sizes with a height of four inches, but there are other sizes. They cost $1.70 to 2.80. Similar blocks have three holes in them and are often called fence blocks. They may cost the same as the solid blocks or may be a little pricier.
The Problems with a Concrete Block Foundation
Installing a shed on a concrete block foundation may lead to problems down the road. It’s especially important to take the extra steps to dig and level the shed foundation space and to use well-tamped gravel in the space to support the blocks. If you just place your blocks directly on the soil, you’re just asking for tilting and sinking problems later on. In fact, some building codes won’t allow you to do this.
Setting blocks on less than a firm foundation can lead to stability problems. Even if your soil seems solid, it does shift and erode over time. Freezing and thawing of the soil can also cause problems. The first sign may be that the door doesn’t seem to close properly.
This means that a lean is forming, even if you can’t see one. You may have to lift a corner of the shed or a whole side to fix the problem. If your shed blocks are sitting on soil, this problem will probably crop up again.
You may have seen smaller sheds set on just one concrete block on each corner. Even if your shed is lightweight, though, and you don’t have room for heavy equipment inside, you’d be surprised at how much weight will end up in it. If the center isn’t supported well, you may be in for an unpleasantly sagging floor in time.
Blocks do allow airflow underneath your shed, but without a landscape fabric underneath the gravel to block out unwanted growth, you will have things growing underneath the shed, even if it’s just moss and lichen.
Grass and weeds will grow close around the shed, blocking the airflow. Mud can splash onto the shed during heavy rain, and algae can grow on the siding from dampness. These can deteriorate the siding.
Building a good gravel pad first does not only provide a better foundation for the blocks but making it a foot or two wider than the shed all the way around will keep water from running off along and underneath the shed and steer the runoff from the roof away from the shed. It will also prevent splashing from the roof runoff.
A dirt foundation underneath the shed will allow animals to take shelter there, from mice to skunks. While gravel underneath won’t guarantee the prevention of this problem, it will discourage the wildlife.
If you are buying a pre-built shed, you may run into other problems. Some installers won’t put a pre-built shed on such a foundation. In addition, it may void any warranty provided by the manufacturer. Last, but not least, your local building codes may not allow such a foundation, especially if the blocks are put on bare soil. For a more detailed look at concrete blocks read my article Pros and Cons of Using Concrete Blocks for Shed Base.
Using concrete blocks for a foundation can provide a good base for your shed as long as the shed isn’t too big, the soil type and slope are suitable, you don’t have problems with frost heave, and you prepare the ground underneath with landscape fabric and a few inches of well-tamped gravel.
Local building code restrictions also can play a large part in this decision. While it won’t be as simple as laying a few blocks down on the ground, it can save you money over concrete footings. It’s something to be considered.