Are you planning on building a shed but can’t decide what would be a good foundation for it? Or maybe you have some ideas but still need a little more information to make a good choice.
Being here, you’re more than likely considering precast concrete piers for the shed foundation, among other options, and depending on the situation, it might be a working option.
A precast concrete pier foundation is the most stable foundation besides a concrete slab. It is much more stable than skids, and you can be sure your shed will be stable for years to come.
There are also solutions to protect concrete pier foundations against frost heaving if that is a problem that might happen. Piers go some way into the soil, so if your soil is vulnerable to frost heaving, we can change it for a more suitable one.
Pier and beam foundation summary
Concrete piers don’t work alone; they need something that transfers the weight of the floor, walls, and roof into the ground. Wooden beams are most often used for that as they are cheap, depending on the size, and can handle a lot of weight depending on the x and y measures.
The precast concrete piers are embedded in the wet concrete footings and support 4×6 pressure-treated timbers once the concrete has cured. Piers should lift it off the ground high enough so the moisture won’t get into the structures.
Air can flow under the shed, and pressure-treated timbers are also moist-resistant.
The pressure-treated beams on top of the piers will support the shed.
Sounds good, so how do we install the concrete piers?
- Here we will assume a little. You have a plan for your concrete shed. How big and many piers does it need? What size beams do you need for the floor, wall, roof, and usage weight? Don’t forget to add in the stuff you’re going to store if it weighs a lot.
- Once you know what you’re building, you lay out the outlines of your shed and excavate it to a depth of 3 to 4 inches.
- Find out how deep is your frost line. From the ground studies you had before building the house or paying someone to find it out. Either one works. The key features are how fine the soil is and how much capillary water it holds. You might need to change some for dry gravel if it’s fine-grain soil like clay.
- Then dig holes 10 inches below the frost line for footings.
- If it’s fine soil, dig 20 inches instead and fill 10 of it with gravel you compact with a tamping rammer or something like it. This will help protect against frost heaving.
- Pour the concrete for the footings of the precast piers and then set the precast piers in the wet concrete.
- Check with the laser or level that all the precast concrete piers are on the same level; this will make it easier to continue with the 4×6 beam.
- Once the concrete cures, lay down landscape fabric and a gravel drainage bed, then install the beams. If the ground drains well and stays frost-free all year round, you will not need to make the gravel drainage bed. If you don’t have good drainage, you will need to do this, and I will explain how later in this article.
Everything above is overly simplified here. For a shed outside a small house, 4-6 pieces of concrete piers might be enough, but when the shed starts to be a size of a small house, you need to calculate everything correctly, so it’s safe.
That being said, if you properly compact the soil around the shed area and under the concrete piers, piers can hold a lot of weight, as concrete is solid when it comes to straight pressure.
How many pier blocks for the shed foundation do you need, then?
Technically one well-done concrete pier on solid soil should hold the weight of a regular tool shed on the side of the house. Still, we need beams for the structure, so four concrete piers should be enough for a small cube. For a more extended cube, 2 rows of 3 could be good so that it would be six.
For pier and beam foundation spacing, you need to figure out the structure’s shape and size of your beams. Do they need more than 2 support points? Which way do they go?
What is the size of the concrete piers, and how much weight can they hold without giving in and sinking in the soil?
There are also local regulations that might give you the answer or straight out tell you how much you need. Considering regular building materials distance of 6-10 feet between concrete piers seems reasonable.
More won’t hurt; less might, as I like to think about it.
How to build concrete pier and beam foundation in more depth
So the last part was the summary, and while I was writing it, I felt in-depth text might be needed for those who like to read and maybe know things more in-depth.
1. Start by laying out the concrete pier foundation design on paper or in nature
Every project should begin with planning, if possible, on paper. I won’t look down on the people who manage to do this without planning, but for regular folks, it’s for maintaining project size and cost.
You draw the shed on the paper, you know how many piers you should have between the distance, you see how many beams you need, and you can calculate the walls and roof materials from that.
Next, you check the hardware store for prices, and if it’s too much, scale down or think of other solutions. This is how I like things, and I won’t end up with that many unfinished projects.
So now we have everything on paper and make it for real. We got the materials needed and moved the project outside.
We first must lay out our project, as we have already done. Then we remove the sod and 3 to 4 inches of soil from the site. This will help us to install the drainage later.
Use your batter boards and string line and put masking tape or mark every 4-6 feet to help lay out where your piers will go. It’s essential to get it right, as no one wants to move the piers and footings once they are done. So take your time measuring everything correctly.
Run string lines across the taped points. Square the lines by applying the 3:4:5 triangle rule. If you don’t know, every triangle with 3:4:5 sides is a right triangle. This way, you can make your shed any rectangle shape.
With a plumb bob and powdered chalk, mark the locations of piers on the ground where the lines intersect.
2. Dig holes for concrete footings
Now the footings are like shoes for the concrete piers. They apply the weight of the piers to a broader area so the piers won’t sink into the ground quickly.
Next, we need to dig some holes for the concrete footings. These need to be 12 to 14 inches square and 10 inches below the frost line.
As mentioned in summary, if the ground is fine-grain soil, you should dig 10 inches deeper, 20 inches in total, and fill the 10 inches with gravel. Compact it with a tamping rammer or something else so the pillars won’t sink when the weight starts to add up.
Why do we do this? Fine soil, like clay, has good capillary properties for water to rise higher. We don’t want that, so we change it for rougher material.
If the soil is ok, we shovel 4 inches or so of gravel into the holes for drainage and tamp it down with a 2×4 or something similar. Just have the bottom level, so it’s easy to set form on top of it if we use one.
Let’s assume you do want to make your concrete forms. We build square forms of 1×6 material for the top of the footings and seat the forms over the holes and level the forms on all sides and with each other. If necessary, we should use stakes to hold the form on the bottom.
Another pro move here would be to measure the distance between the top of our form to the supposed 4×6 beam height. That helps us a little with setting the precast concrete pier foundations to the correct height.
Add some rebar to the footing; it will make it stronger. If there is no instruction, the rebar should be 1-2 inches deep from the sides so it has some concrete covering it. It should go in the x and y directions.
Finally, we mix concrete and shovel it into the forms and holes. We move the shovel up and down in the concrete to help eliminate air pockets or use a vibrating tool for the same purpose. With a straight edge, scrape the concrete even with the top of the form.
3. Attach concrete pier foundations on the footings
Next, it is time to install the precast concrete pier foundation on the footings; we need to do this before the concrete cures so we can have one solid piece. This way, the cement in concrete adheres to the precast concrete piers and the aggregates in it.
We can align the piers by placing a straight board across them with the board lying horizontally. Then put the straight board on the edge next and sit a level on top to level the anchors from end to end.
You can use a laser or any other tool to ensure all piers are at the correct height. We do not want to adjust them once everything is done.
We should let the concrete set for at least 24 hours before removing the forms. Depending on the concrete, it can be even 2-3 days.
After we remove them, we can shower the footings with little water so they will harden some more. It will also prevent possible cracks.
If necessary, we should place landscape fabric and gravel for drainage. If the drainage is already good, this is unnecessary. When in doubt, it’s better to be safe than sorry.
I’d compact the gravel around concrete piers well and under the shed. It will help against frost-thawing.
At last, we place 4×6 beams into the anchors on the piers. Beam sizes can vary depending on the shed size; I like to imagine this size shed.
We can level the beams from end to end, side to side, and with each other by slipping shims under them if needed. Next, we can attach the anchors to the beams with nails or screws.
Now we have the precast pier foundation ready for the shed. This is an excellent way to do a shed foundation; it’s stable and will work in most applications. It’s suitable for wet, uneven ground, frost-heaving ground, etc.
Precast concrete pier foundations have their place in building houses and sheds. It’s an excellent way to get the building off the ground if there’s a problem with ground moisture.
It’s also an excellent way to build on uneven ground, and you might have seen some precast concrete piers on hill houses, etc. There is more here if you want to read more about concrete foundations.
I would lift most of the storage and other buildings of the ground I have at home. The floor gets to breathe when it is lifted off the ground, and you might even plan for that quality.
Raise the structure enough, and you don’t have to worry about sinking anymore. I had a kid’s playhouse built on piers, and when it started to sink, all I did was raise it back up with some leverage and concrete pads. In my defense, I have to say that I didn’t build it.
If you’re interested in ways to support concrete foundations, here is an article about precast concrete piles supporting underground foundations.