Here in lovely rural upstate New York I live on a large plot of blue clay. Solid blue clay as far down as you can dig, and everywhere you go. It ranges from a muddy quagmire after a drizzle to a dry, cracking wasteland after a few days without rain. It’s horribly unsteady and causes all sorts of building related headaches. That is to say that planning the foundation for my shed wasn’t straightforward. Here is my lovely building site, smack next to the existing raised beds and pallet compost bins
I considered concrete piers, but was not looking forward to digging them out. I would have needed 12″ holes for the sonotube, and even with a rented auger, 12″ holes 4′ deep is no fun. None.
I considered just doing concrete blocks with a treated deck resting directly on them. Other people online had built sheds successfully this way, and it even looked like the timber frame that inspired me may have been built this way.
But I was too worried about the blocks settling in the clay after the first decent rain.
There was a whole trenching out for skids method that held some promise. I was really close to going with this method, but again wasn’t excited about the digging, nor was I sure that the gravel wouldn’t just sink into the clay.
And finally I hit on the solution that looked like it offered my best shot at a solid foundation (short of doing a real poured slab / footer foundation, which I had no intention of paying for). Basically I build a sandbox for my shed.
I built the frame out of pressure treated 4 x 6s and filled it with 3/4″ gravel. For those who don’t know 3/4″ gravel just means stones up to 3/4″ in size, it’s like the maximum size stone you should find in the mix. The shed would be 10 x 14 in the main part and 6 x 8 for the coop “addition”. I built the frame to give about a foot perimeter around the whole shed for drainage. Before the gravel was poured though, I timber spiked the 4 x 6s together, and drove rebar every 2′ into the ground at least 12″ to try to keep the frame in place. I then laid down landscaping fabric in an attempt to stop the clay from consuming my gravel base like some sort of pit of sedentary sedimentary m&ms. (Mhhmmm m&ms). I calculated that the base would take about 4 yards of gravel. Roughly 6″ high, so taking one half of the area of both rectangles was ((.5 x 12 x 16) + (.5 x 8 x 9)) = 94 and then dividing by 27 (one cubic yard in feet) gave me 4.8, rounded up to 5. Yay math! In fact it took a bit more like 5 and half yards but I think that had more to do with some rough estimation of pickup beds and tractor scoops, which is fine.
My helpers (two of which pictured above) helped me get it spread out in an afternoon (including two pickup truck fulls unloaded by hand) and soon it looked more like this.
So now that I had a solid place to build that would hopefully drain away water without shifting or sinking straight into the clay, I was ready to get going building this shed thing.