How to Build a Shed Base With Concrete Blocks [Complete Guide]

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Ever driven through the suburbs and admired other people’s sheds? Wondered how they got them to sit level or how they got started?

In the past number of years I’ve built quite a few sheds for myself and others. I don’t consider myself an expert; I began with the same question as you. How to build a shed base with concrete blocks? Whether you build your own or buy a ready-made shed, you need to decide what it’s going to sit on.

Do I need a foundation? What are my choices? Can I use concrete blocks? What tools and materials do I need? Do I need a permit? These are all important questions to think about.

I’ll attempt to answer these questions as we walk through this article together.

Do I Need a Foundation for My Shed

The first thing you should do once you make the decision you want a shed, is where are you going to put your shed? This sounds pretty simple, but it’s one of the most important decisions you make about your shed. The site you choose determines if you need a foundation under it and if a concrete block foundation can be used.

Do You Really Need a Foundation for a Shed?

How to Build a Shed Base With Concrete BlocksWhen planning your shed, remember the climate you live in and what you are going to use the shed for. If the shed sits directly on the ground it will begin to rot within a couple of years. Even pressure treated lumber rots.

Moisture from the soil, insects, even rodents, can have a devastating effect on wood. Keep in mind that rain and snowmelt can pool around your shed making access wet and mucky. Snow buildup can make entry difficult too.

If you want your shed to sit level, then a foundation is a common method to create a level base. Most construction sites aren’t level. Building a foundation for your shed creates a level base and usually lifts it above water damage. It can also make access in winter easier too.

Can a Concrete Block Foundation Be Used?

Concrete blocks are a great way to create a level base for your shed. They can be doubled up to give a wide base, or stacked to build piers for higher clearance or uneven slopes.

Concrete blocks are ideal for flat or on-grade foundation support. They raise your structure off the ground and won’t rot.

What are Concrete Blocks?

Concrete blocks are a cast cement and an aggregate of sand and gravel. They are often referred to as concrete masonry units (CMU). The blocks usually have one or two hollow cores, but are available with solid cores – which are much heavier!

The blocks come in different dimensions, making them a flexible foundation solution. They can also be cut with Mason’s chisel or carbide saw blade.

What is the Difference Between a Concrete Block and a Cinder Block?

Difference Between a Concrete Block and a Cinder Block
Concrete blocks are not
cinder blocks. They are made from a denser material than cinder blocks which have industrial ash as an aggregate. A key difference between is the susceptibility of cinder blocks to winter freeze-thaw.

If exposed to moisture and freeze-thaw cycles, cinder blocks can begin to disintegrate. If using cinder blocks protect them with the shed base, or regularly treated with a water sealant.

Which is Better, Concrete Block or Poured Foundation?

Poured concrete reinforced with steel has more lateral strength than concrete blocks. However, if you are only using it to support and level a base for your shed, then it’s over-kill.

Block and poured concrete need the same ground preparation for leveling and drainage. An 8”x8″x16″ block is easier to place than mixing and pouring or having ready mix delivered to put into a form you’ve built.

How Many Concrete Blocks for a Shed Foundation?

You should have concrete blocks at every corner. Depending on the shed dimensions, halfway between the corners or about every 6’ to 8’ around the perimeter. Remember to support the middle area of the shed in a similar pattern.

The spacing of the blocks is also affected by the framing material used to support the structure. I had a buddy who figured he was only going to store bikes and yard tools in his shed. He didn’t support the middle on the perimeter or the floor. BIG mistake! Plan for today, build for tomorrow.

Benefits of Using a Concrete Block Base

There are many benefits to using concrete blocks for the foundations instead of other materials. Concrete blocks are readily available and pre-made. Once the ground is prepared, they make a great on-grade raised foundation for your shed. The blocks are easy to move around and have a wide flat surface which eases the leveling of your shed platform.

Stack the blocks on uneven ground will provide a level surface. Double up the blocks, or double stack them, to provide more structural support. Concrete blocks come in different sizes for more foundation solutions too. If using hollow core blocks, fill the core with concrete and rebar to create a very solid pier or support.

A 66-lb. bag of concrete mix makes a ½ cubic foot of finished product. A concrete block (16”x8”x8” nominal measure) has a volume of 0.58 cubic feet and weighs between 30 and 35 lbs. – much lighter. I’ve used both products.

The determining factors are access to the site and the size of the structure, and budget. Concrete blocks are less expensive than the concrete mix or ready mix. Most of my sheds are on concrete blocks.

Important Things to Consider Before You Start

Before you begin building your foundation, there are some important things to consider. You should check local ordinance, soil types, drainage, slope, and the local utilities.

Local Ordinances

Unless you live in an Undeveloped Municipality, there will be Building Codes and a Building Inspector. A check online or phone to the Municipal Offices should provide you the information. It’s better to check than have to pay a fine or to remove what you worked hard to build.

Soil Types

There are basically 6 soil types. Soils are usually found in combination with each other but occasionally can cover a large area. Your soil type can impact the type of footing you will require, and if you can build where you want.

  • Peat: Often found along the swampy or reclaimed land. Can be dust dry in summer and spongy during wet periods, it offers weak support for a foundation.
  • Sand/Gravel: Widely distributed by glacial and ancient watercourses. It drains well due to the large particles. But, small particles can wash away creating holes under foundations. When this soil is compacted, it holds together well and makes good foundation support.
  • Silt: Usually found in old watershed areas or floodplains. It retains moisture causing it to expand. This puts pressure on foundations and can weaken them. Not a good foundation support.
  • Clay: Found in many locals it is a poor supporting soil. It’s pliable when wet, retains moisture, thus increasing pressure against foundations. When dry, it shrinks. This can cause up and down movement on foundations.
  • Loam: Widely distributed as soil, it is a combination of clay, silt and sand. Due to the combination, it retains enough moisture to keep its shape but also drains well. It is good for supporting a foundation.
  • Rock: Large or small they come from bedrock. Some are granite, limestone, sandstone, and shale. It could also be bedrock. Their depth and stability make a firm foundation support. They can be challenging to level though.

Drainage

The soil type and lay of the land, or its contours, affects how rain and snowmelt leave your property. It should be considered when selecting your foundation site for your shed. It will also impact on the type of foundation you build. If it drains slowly but is the only spot you can build, then you may want your shed higher off the ground.

Slope

The slope of your ground also affects drainage and foundation type. Build on a slope and you may have more to excavate at one edge and need different levels of concrete blocks. If the hill is bedrock you can’t excavate, so you must rely on concrete blocks to create your level base. A slope can also impact drainage and need retaining walls and/or drainage ditches.

Any Other Problems Near the Construction Site

There are some other things to keep in mind when planning your construction project? Are there trees overhanging the site which may be in the way or could damage your shed once it’s built? Are they on your property or a neighbor’s?

Connected to all trees are their root systems which can play havoc with your excavation. The roots can also damage and shift foundations. Cutting out the roots may also hurt or kill someone’s favorite tree.

Rocks are another issue to keep in mind. They can make excavation complicated, or require foundation alterations. During the digging for one of my sheds, I found a rather large piece of granite where I wanted my shed. After breaking a sledge hammer on it, I decided it was bedrock. I altered the foundation plan and shifted it a bit so the rock fit between floor joists.

Depending on where you live, there may be underground utilities you are unaware of. A quick call to your local utility company and they’ll come and check. Check utility clearances. There may be setbacks or allowances that you need to respect.

Concrete Block Shed Foundation Cost

The materials used and the shed dimensions determine the cost of a foundation. Here are some comparisons for an 8’x12’ shed:

  • Concrete Block: 8”x8”x16” hollow core blocks cost $1.50 to $2.50 each. Solid core blocks can cost double. This foundation needs 6 blocks so between $9.00 and $30.00 if hollow or solid core.
  • Gravel Pad: A 6”x8’x12’ gravel pad requires 1.78 yards of ¾” crushed stone. Costs between $45.00 and $75.00; your shed still sits on the ground.
  • Concrete Pier: For stability/frost it should be 4’ deep. You need 6 – 4’ long 8” diameter tubes plus 4’ of rebar for each, and concrete. Some suggest gravel for the bottom of the tube too. A tube is $6 -$8.00, rebar about $0.25 a foot, and 3 bags ($4 – $5.00ea) of concrete/tube. Your lowest cost is $114.00.
  • Poured Concrete Foundation: A 4”x8’x12’ concrete pad requires 4.7 yd³ of concrete. My last load was $225.00/ yd³; I’ve heard $125.00/ yd³ “someplace”. Add rebar or steel mesh, a gravel sublayer, vapor barrier, and the cost climbs.

How to Build a Shed Base With Concrete Blocks

concrete block shed foundation
Image courtesy: Facebook

What You Will Need

Your foundation choice will impact the tools you need and the supplies you require. My focus is building a shed base with concrete blocks. Here’s what you need:

Tools: Shovel and small garden trowel

Supplies: Concrete blocks (6) – hollow core or solid. If a sloped site has several thicknesses

  •  Gravel (optional) – pea gravel or gravel dust
  •  Asphalt shingle or two as shims

Prepare Your Building Site

Prepare Building Site

  • Clear your build site of obstructions – rocks, trees, shrubs, junk
  • Hammer a stake in at one fixed corner and outline the shed perimeter with the string and 3 other stakes.
  • I add a foot to each dimension (8’x12′ becomes 9’x13’) so I won’t have to move the stakes when placing corner blocks.
  • Square your perimeter with the measuring tape. Measure 3’ along the string from your fixed corner toward another corner and tie a knot. Measure 4’ along the string in the other direction from the fixed post and tie a knot.

You can use a marker to mark the 3’ and 4’ points. Move the two corner posts connected by the string to your fixed corner until the diagonal distance between the two knots needs to be 5’. This will ensure a square corner. Repeat with the other corner post and you should end up with four 90 degrees (square) corners. This will work for any size shed.

Laying a Concrete Block Foundation for a Shed

Once you prepare your build site and have squared your perimeter, you’re ready to begin.

  • Place a concrete block in each corner and one at the center point of each long (12’) side.
  • Push the shovel into the soil around the block. Move the block, use the shovel to remove the grass and then replace the block (hollow cores should be up if used).
  • You can remove all the sod within the perimeter. Roll out a weed guard membrane, cover with 4”s of gravel; then place your concrete blocks.

We have reached the most important part of laying your foundation. Level the blocks themselves, and to each other. A flat building site levels more easily than a sloped build site.

Here’s a helpful video:

Flat Grade Site

  • Pick a corner block. This will be your reference corner. Use the 2’ level check that the block is level long ways and short ways across the top. Use the garden trowel to remove soil or place gravel under the block to level it.
  • Rest the 2”x4”x8’ board on your leveled corner block and across to either of the blocks within reach. Place your level on the top of the board to check for level between blocks. If it is high, remove soil from under the non-reference block until level. If it is low, add gravel until it is level.
  • Once you have achieved level, level this block as you did your reference. Recheck with the 2”x4” that the span is still level – this can be very picky work. Having a helper saves time and energy.
  • Repeat this process from the reference block to the other block reachable with the plank. You now have 3 blocks level. Work your way around the perimeter from block to block until all are individually level, and level with each other.
  • You now have a level concrete block foundation on which to build your shed.

Sloped Site

There are two ways to build a foundation for a sloped site.

1. Dig into the slope to make a level base for your shed. You may need to build a retaining wall to keep the soil from falling in on your shed over time. You may also need a drainage ditch to divert water away from your shed.Excavate your site and prepare your retaining walls. Follow the procedure for a Flat Grade Site above to build your foundation.

2. The second way is to build “on” the slope. This may still require a drainage ditch but probably won’t need a retaining wall.

  • Like the Flat Grade, but pick the corner block highest on the slope. This will be your reference corner. Use the 2’ level to check that the block is level long ways and short ways across the top. Use the garden trowel to remove soil or place gravel under the block to level it.
  • Rest the 2”x4”x8’ board on your leveled corner block and across the slope to the other corner. Place your level on the top of the board to check for level between blocks. If it is high, remove soil from under the non-reference block until level. If it is low, add gravel until it is level.
    If the difference is great enough to add another block, level the existing block and add another on top. I often have several blocks of different thicknesses for this purpose. Ensure the stacked blocks are level vertically too! Use a square of shingle for minor horizontal adjustments.
    Recheck with the 2”x4” that the span is still level – this can be very picky work. Again, having a helper saves time and energy.
  • From your reference corner use the 2”x4” and level to set the mid-way block. Follow the process used with the other corner. Ensuring stacked blocks are level horizontally and vertically.
  • Repeat this process until all blocks are set. You now have a level concrete block foundation for your shed.
  • I recommend doubling up your blocks and alternating each course (set) for heights of more than 16”. This will create a more stable foundation. If it is more than 24” high, then I recommend filling hollow cores with concrete or switching to a concrete pier. Ensure stacked blocks are level horizontally and vertically.

Conclusion

I hope you found the article enjoyable and informative. The more preplanning and site preparation you do before you start, the easier the foundation will go.

Review the list of what you will need. Every interruption to go get something is a frustration. Follow the steps. You want your foundation to be level and stable before you build on it.

Your comments are appreciated. If you know someone who is thinking about building a shed, share with them if you liked it.

 

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