There is more than one reason why you might want a concrete pier foundation for your shed. Of course, if your shed is going to be built on a slope, concrete piers are the answer to enable you to build a level base for your shed.
This is not the only reason to use this method, however. Concrete piers provide a lot of stability for a shed without the extra work and cost of a full concrete slab foundation.
You may want to raise your shed up for open storage underneath. You may have to install your shed in a place that gets damp or has some occasional water flow, and you want to get your shed raised above the problems.
You may want to just get it above ground a few inches to provide that all-important airflow underneath to prevent moisture problems. Whatever the reason, concrete piers provide the needed stability.
There are two main types of concrete piers. There are precast piers that sit on top of the ground or are partially buried. They usually are formed with holes to fit 4×4 wooden posts and hardware to enable attaching them. Some are formed so that the actual floor joists can be fitted in.
The other type is the kind we will be discussing here. They consist of concrete poured into holes with attaching hardware set into the top before the concrete cures to hold your 4×4 posts.
This is the kind used for shed foundations set on uneven or sloped ground, and the type that provides the most stability. While installing these concrete piers doesn’t take the work that a concrete slab does, it does take some elbow grease and a little skill. Be sure to scroll down to the end of the article for a step-by-step plan for building a concrete pier foundation.
How Deep Should Concrete Piers Be for a Shed?
Basically, your piers should go below the frost line to prevent frost heave, or the piers moving with the freezing and thawing of the soil. Plan to dig your holes at least a foot below this frost line. Your building authority should have information on the depth of the frost line in your area.
Many pier holes are dug around four feet deep. While this takes quite a lot of work, even with a post-hole digger or auger, you’ll make up for it later with the sturdiness of your foundation and the peace of mind this will bring.
Some people prefer to simply dig holes large enough to sink large coffee cans filled with concrete into the holes. This, however, is not really sturdy enough except for perhaps a very small, lightweight shed such as a plastic shed meant to only store rakes, shovels, and a few yard chemicals. This method will probably not satisfy any building or zoning regulations, which is another reason to just do it right and dig enough to install piers the proper way.
How Many Concrete Piers Do I Need for My Shed?
This depends on the size of your shed. Common spacing distance is about eight to ten feet apart. An 8 x 12 ft. shed typically has six piers, one on each corner and another in the middle of the long sides. You also need to also take into consideration what you plan to put in your shed.
If you are going to store heavy lawn tractors or woodworking equipment, you may want more support. You may want to plan your floor joists and space out the piers according to this placement. If your shed will contain heavier items, you may want to space your joists at 16 inches apart, then space out your piers according to this distance. Of course, if your shed is pre-built, you need to plan according to its joist spacing.
What Size Sonotube Do I Need?
SonoTubes and other such tubes made for pouring round concrete posts and piers come in many sizes. They are made from moisture-resistant cardboard or paper and may be waxed inside. They are usually available at any home improvement store or lumberyard. Again, your building code enforcement may come into play here, as they may have specifications as to what diameter your piers should be.
The usual rule of thumb is to use a tube that has a diameter three times the width of the post. Following this formula, for 4 x 4 posts, you’ll need tubes with a diameter of a foot. Other sources will use a foot-wide pier with an 8 x 8 post. If you’re building on a slope or your soil is not very stable, you’ll likely use the wider tube width for added stability.
You’ll want tubes that will go the depth of your holes and stick up above ground six inches. You can trim them to size if they are too long.
Can I Make the Form from Wood?
Most forms for concrete slabs are made from wood planks, which are later removed once the slab has fully dried. However, with piers, you are not pouring a slab. You can make square piers with forms made from scrap lumber, but wooden forms are usually utilized for much shallower pours.
You may also need to have these wooden forms inspected by the building authority (yes, some authorities have regulations on these), so check with them before deciding to make the forms this way.
It’s much easier to dig round holes when digging the deeper ones required by a concrete pier foundation for sheds and use SonoTubes or other such products made for just this type of work. Round piers offer more support than do square ones, so this should also be a consideration. You can also make the piers from pressure treated wood to save money, check out my article How to Build a Post and Beam Shed Foundation for more information.
Do I Need to Put Rebar Inside the Concrete Pier?
This is a matter of some discussion. Some people urge inserting rebar into the concrete while others don’t think it necessary. Basically, it comes down to soil and weather conditions. If your soil is unstable, you may want to use rebar in the piers to prevent the concrete cracking.
Likewise, if your area is prone to hard or long freezes and there may be a problem with soil heave from the freezing and thawing, rebar will help keep the concrete in one piece. Usually, a couple of pieces of #4 rebar per pier is sufficient unless you pour some really wide ones.
Do I Need a Base for the Concrete Pier?
You don’t need a base per se, but you should put four to six inches of gravel into the base of each hole and tamp it down before pouring the concrete. This will help with drainage and help keep underground water from pooling around the bottom of the pier as well as providing a firm foundation for the pier.
What Will It Cost for Concrete Piers?
The approximate cost is $3 to $7 per square foot, but this is the cost if you pay someone to build them for you. If you are putting these piers in yourself, it basically comes down to the cost of the concrete mix, or cement, sand and aggregate if you are mixing your own from scratch.
The only other costs will be the pier brackets and the form tubes. The brackets range from under $10 to $25 per piece. There are many choices as far as design and material. The tubes start at around $12 for an 8 x 48-inch tube and are available at most home improvement stores. Longer ones are also available at builders’ supply stores.
How to Build a Concrete Pier Foundation
Step 1] Permits and setbacks. Unless you live in a really rural area outside of any town or village limits, your locality will probably have some rules for building or locating a shed. Even placing a pre-built shed will have some regulations you need to follow. Depending on the size of the shed, you may need a building permit.
If you are building the shed, you probably will need one unless your shed will be really small. There may be rules specifying the size limit, height limit, materials used, foundation type, and so on. If you live in an area governed by an HOA, you’ll need to get their approval as well.
When you are buying a pre-built shed, you still have to build your own foundation. There may be rules governing the type of foundation required, depending on your location. For instance, in places subject to frequent windstorms, your local building authority may require sturdier foundations than those required in other places and may even require tie-downs for your shed, especially if you plan to have a lightweight shed such as a plastic one.
There will likely be restrictions on just where you can place your shed as well. There will be limits as to how far your shed needs to be from the property line, other buildings, overhead or underground utilities, and so forth. You may need an inspector to check out your proposed location.
There may be other restrictions, even going so far as to specify the minimum diameter your piers should be or inspecting any wood forms you may want to use for pouring the concrete. Get everything checked out before you start.
If you don’t and end up violating a code or restrictions, you may end up being fined daily until your violations are fixed to code satisfaction, up to and including tearing down the shed and starting over. Your HOA can add its own penalties.
Building or installing a shed without a needed permit carries some surprisingly steep penalties. In some places, the fines can be as high as $50,000, which will probably be far more than the shed itself costs. Some penalties even include jail.
The best bet is to find out what you need to do and secure any needed permits or inspections. This means having detailed plans of your shed and foundation to submit if they are required. It only takes one nosy neighbor calling up code enforcement to get the ball rolling. If you don’t fix the problem after you’ve been caught, you can eventually end up in jail or the town can put a lien on your property until the fines are paid.
Step 2] Prepare a plan on paper and show the layout for the piers. Not only will this make things easier as you start your project, but you’ll need plans for other purposes. You will need plans to show your building authority if you need to submit an application for a permit. If you are just building the foundation for a pre-built shed, you may need to show the foundation plans to get zoning approval.
Step 3] Get the ground ready. Stake out your ground and prepare it by leveling it out. It would be a good idea to dig out a few inches so you can lay some gravel for drainage, even though your shed will be raised up a few inches. The gravel helps keep the water flowing out and also helps with evaporation.
Step 4] Determine the required depth of piers. This is partly determined by the frost line. Most building codes specify sinking the piers at least a foot below the frost line in your area. Even without this consideration, plan on digging holes at least four feet deep. Keep in mind that your holes will also have four to six inches of gravel in the bottoms, so the hole depth should allow for this.
Step 5] Mark out the holes. Figure how many piers you’ll need and stake out their placement. Put the stakes outside of the planned space for the holes and mark the center of the holes on the ground. You will need to leave the stakes in as you’ll need them later to make sure the pier tops will be level. The stakes should also be spaced in even lines to help you line up your holes.
Step 6] Dig the holes. You can use a post hole digger or an auger the width of the holes you want. For deep holes, you may want to rent a hydraulic auger. This will help especially if you have gravelly soil or thick clay which will make manual digging nearly impossible.
Try to make the holes about two inches wider than the diameter of your form tubes. This will give you a little adjustment room if needed. Flare the bottom a bit if possible to give a more stable base.
Pour in about four to six inches of gravel and tamp it down. It doesn’t have to be level, but tamping it down gets rid of air space and creates a sturdier base for the concrete.
Step 7] Install and brace the Sonotube forms. You can easily cut them to the proper length. If possible, insert them in the holes first, then trim them. You want the tops of all of them to be level with each other. This is where those stakes come in handy. You can tie strings to the stakes at the proper level, which is about six inches above the ground surface. Level the strings and you will more easily level the tops of the tubes.
Of course, the tubes also have to be level vertically. This is where the little wiggle room you left comes in handy for adjusting. Pour in soil around the tubes, tamping it down as you go. This is where a helper comes in handy, as one person can hold the tube steady while the other fills and tamps down the soil.
Step 8] Mix and pour the concrete. Renting a concrete mixer will help with this, especially if you have several piers to form. The concrete mixer has another advantage, in that it keeps rotating the wet mix, keeping it from setting up while you pour your piers.
Using bagged concrete mix will save you a lot of time and effort. Your supplier can advise you on which kind to get when you tell him what your plans are. Some concrete mixes have reinforcing fibers mixed in for more strength.
While you can mix cement, sand and aggregate together yourself to make your own concrete mix, it’s much easier to just buy concrete mix. Besides, if you make concrete mix from scratch you need to mix your concrete ingredients dry before mixing it into the water, which makes for more work. Once you try figuring out how much you need of each ingredient, you’ll probably want to go for the concrete mix.
Figure how much water you’ll need for the concrete batch you are mixing, then pour in half that amount to start. Slowly add your concrete mix, then add water gradually as needed. It should have the consistency of thick oatmeal. You may not even need all the water you figured or you may need more.
Once you have it mixed to the right consistency, wait about three to five minutes before pouring it into your wheelbarrow or straight into your forms. Once you pour the concrete, this is when you set in the rebar if needed. Wait for the concrete to dry a bit, then set in your post brackets.
A concrete pier foundation for a shed is the second sturdiest base you can have, second only to a concrete slab. Unlike slabs, however, they are great for placing sheds on sloping ground. When formed at the proper depth, they can withstand almost anything your terrain or weather can throw at them.
They allow your shed to be raised enough to keep moisture and rainwater away from any wooden joists or other parts of your shed. They also allow airflow underneath your shed, which is important, especially in damp places or humid environments.
The air allows evaporation and keeps moisture from settling on the underside of your shed. While they do take some work to install properly, they are well worth the trouble. Once they are installed, you won’t have to worry about them. They are a good option to consider.