Sheds and outbuildings have many different foundation options when placed on level ground. However, the methods for building on uneven ground can become quite involved and expensive. Pier foundations offer a good compromise. They are relatively cheap, simple to produce, and allow flexibility when deciding the final level and height of the floor.
Lay out the corner post positions and mark with a small stake. Pull a measurement line diagonally across the foundation from corner to corner to check it for square. Hold the tape at the level of the highest corner to get an accurate corner-to-corner measurement.
Dig a post hole at each marked corner about 2 feet deep and 10 to 12 inches in diameter. Dig the holes vertically, not perpendicular to the surface of the sloped ground. Stand a 4x4 post in each hole and check again for square. Plumb the posts with a level.
Stand each post in its hole and pour dry concrete premix into the hole around it. Tamp the premix down to pack it around the post. Tamping more on one side than the other will make small adjustments to the post's position and can be used as a fine-tuning method to achieve the final position and plumb. Water the ground surrounding the hole once the posts have been set in position and bedded well with dry premix. Let the premix set overnight.
Determine which of the four posts is on the highest point of the slope. Mark your intended floor height on this post. Measure down from this mark to allow for the width of your bandboard and make a second mark.
Cut all the posts completely off, level with the top mark made in step 1. Measure down from the top on the other three posts to allow for the width of your bandboard and make a second mark.
Notch the posts 1 1/2 inches deep from the bottom marks to the top to form the shoulder that will support the weight transferred to the bandboard by the floor joists. Cut the notches deep enough that the face of the bandboard comes flush with the face of the 4x4 post.
Stand a bandboard up in the notches and drill two 1/2-inch pilot holes in the bandboard in the centre of the corner post. Bolt the bandboard to the post with the 5/8-inch lagbolts and draw them up tight. Lap the next bandboard across the end of the first at the corner and repeat the process until the perimeter of the foundation is banded.
Cut the floor joists to fit between the bandboards. Lay out the joists on 24-inch centres starting at the inside edge of the side bandboard. Hang the floor joists by nailing through the bandboard into the end of the joists, then install your joist hangers underneath the joist. Drive a hanger nail in every hole in the hanger to properly support the loads.
Repeat the above process to hang all of the joists starting at one side of the foundation and working to the other until all joists are hung.
Be sure to allow the concrete to set before you try to notch the posts to receive the bandboards, or else you run the risk of weakening the foundation and causing level and racking problems.
Use caution when using the skilsaw to notch the posts. It is an unsupported saw operation and should be done by persons highly experienced in handling power tools in non-standard applications.