Last year, I started working on building a shed. Building a shed is a great way to learn a lot about construction. It allows you to create a miniature version of a lot of things that would need to be done to build a house. I learned a lot doing this project, but I had trouble finding good resources online, so I decided to document some of the things I did.
The shed I am building is a 12x16 approximately 200 square feet shed. I used a poured concrete pier foundation and a gable roof.
Most guides that I found online discussed building a shed on one of several foundation types. The common types I saw are:
I could not build a concrete pad for several reasons. 1. The ground was very sloped where I wanted to build the shed, and 2. There is no way to get a concrete truck back to where I wanted to build.
Additionally, I did not want to build on a gravel or dirt foundation because I wanted to build a quality building that would last a long time.
This led me to decide to build the foundation for the shed using concrete piers with runners to support a subfloor. This method is much less well documented on the internet and so it took me quite a while to figure out exactly what to do.
I started by creating a model in fusion 360 of the shed that I wanted to build. I also looked up plans and videos of sheds.
Here are some of the resources that I found to be helpful:
Thankfully, I had a friend who is a structural engineer. He was able to recommend the sizing/spacing for the concrete piers and runners.
After discussions with my friend I came up with 3 rows of 4 piers (12 piers) each 12 inches in diameter with a 6x6 runner on top of each pier.
I planned the pier locations so that the outside of the pier would be even with the outside of the building and then calculated the midpoint of each pier.
Next, I create batter boards with nails on top to hold two strings.
These boards were pounded securely into the ground and I zip tied another board to them at an angle for even more stability.
I used the batter boards to create a string square that marked out the 4 corners of the shed and then I placed 4 more so that I would have strings crossing each other where each pier was supposed to be.
Additionally, I made sure that all the strings were perfectly level. To do this I used a line level that clips onto the line.
Now that I know how I want the shed foundation to be placed I marked the locations of the piers with spray paint and moved the strings out of the way so that I could start digging.
So, in order to be structural sound these piers need to extend to at least 6 inches below the frost line. Where I live that means that they need to go down at least 36 inches. I considered digging these holes by hand, but thankfully thought better of it. Instead I rented the HD99 tow-behind posthole auger from a local equipment rental company. This was a very good decision.
This auger made it much easier to dig the holes, it was still very tiring to use since you have to hold it still and pull it out of the whole after digging. Even though it is counter-weighted with the engine, it is still very hard to pull it up when the auger bit has lots of dirt on it.
After we had dug all the holes, I put the strings back to check that they were correct. I had a few holes where roots or rocks had knocked the auger off course, so I had to fix a couple of the holes by digging out to the side.
The next step is to set up the cylindrical forms to pour concrete into. I used a small section of one of the tubes to mark a line for the correct depth on each tube and then I cut them with the circular saw.
I had to measure how long to cut each tube. Since the lines that I set up are perfectly level I simply measured from the line down to the bottom of the hole. Then I subtracted a few inches so that the forms wouldn't interfere with the string.
Then I placed the tubes into the holes and moved them up and down until they were all level with each other. I did this by placing one pier first (I selected where the ground was highest) then I measured the distance from the top of the form to my string and then made sure all the forms had the same distance to the string.
Next, I placed a few inches of gravel into the bottom of each hole so that water will drain away from the bottoms of the piers.
Finally, I wanted to reinforce these piers with just a bit of rebar. So I cut a few pieces to go in each hole and placed them nearby.
I used a small concrete mixer to mix one bag of concrete at a time. This took forever and was incredibly tiring. In the future, I would probably try to rent a small tow-behind trailer that can carry concrete. Many concrete companies have these trailers that the rent and include the concrete as part of the rental.
As we poured the concrete we put in the rebar. Then when we got to the top, I smoothed the concrete and placed a j-bolt into the concrete so that we would have something to secure the runners to later. I would probably switch out these wet-set j-bolts for something else since it was difficult to get the bolts precisely in the middle before the concrete set up. I have read that you can drill a hole afterwords and use epoxy to set a bolt and that it is just as strong. I think that would be easier and less stressful than trying to use the wet-set bolts.
Another issue that came up is that the concrete would settle down from the top of the form. I didn't know how to prevent this. So I just hoped that it would settle the same for each form and they would all come out level. I did use a long pieces of rebar to try and eliminate any air pockets in the concrete by vibrating it as much as I could while pouring.
After we poured all the piers, I covered them with plastic to prevent them from drying too fast.
After letting the concrete dry I used a level to check the height of all the piers. They were pretty close, but there were a couple that were a little off (about 1/2 inch). So I used an angle grinder with a diamond blade to grind them down a bit. This was difficult due to the bolt being in the way. Which is another reason I wish I had waited to place the bolts until after the concrete dried.
After ensuring that all the piers were level with each other I bolted Simpson strong-tie post bases to the piers.
Because I knew the grass was going to die underneath the shed once it was built, I decided to go ahead and put gravel down to ensure that the dirt underneath was not washed away.
Creating a pier base for a shed was much more work than I anticipated it being, however I think that it was worth it. This foundation ensures that no part of the shed with touch the ground which will help the shed last a long time. In the next entry I will talk about building the subfloor.