One of my goals for this project was to tread lightly on the land.
The high spot of the site was a boulder maybe 16 ft. x 8 ft. and protruding 8 ft. above grade at one end. I decided that a nice approach would be to set the structure on the boulder. I reasoned that the boulder probably extended several feet below ground and was probably not going to move much over my lifetime.
Here I laid out the structure with some 2×4s to figure out how to orient it relative to the boulder. The boulder is covered with moss and lots of wet organic stuff as is typical of this part of Vermont.
I used an angle grinder (I’m quite happy with this Bosch model) and a diamond blade to cut some flat spots in the boulder for the six points where I planned to support the structure. (So much for treading lightly some of you might observe…but these little cuts are nothing like blasting a big hole in the ground and pouring a foundation.)
The structure is basically two 3 1/2″ x 10″ x 16′ beams supported in three places along their length and then tied together with some cross members. There are some diagonal cross braces to keep everything rigid. The beams were cut at the local saw mill and hauled (dragged really) down the trail a few feet at a time. The beams were wet, wet, wet…cut from green hemlock. But, wood does not shrink much axially when it dries (in contrast to shrinkage across the grain, which is significant) so I didn’t worry about building with the green lumber. The only real problem with green lumber is that it is very heavy, maybe 300 lbs. for each of these beams.
One of the those six support points was a soft patch of organic soil. I did not want to dig a deep hole for a concrete pier, so I just dug out a decent sized hole and filled it with gravel collected from the stream. I set a concrete lintel on the gravel and set the post on the lintel. It hasn’t moved in over two years, so I think I’m going to be ok. Here is the basic support structure in place. You can see the concrete lintel at the nearest support point.
In retrospect, I would have used 6×6 posts instead of 4×4 posts to support the structure. A 4×4 can support over 20,000 lbs. in compression and my entire building with snow and contents only weighs about 20,000 lbs., so they won’t fail. Still, they seem a little spindly now that the structure is completed.
Finally, I placed five sheets of 3/4″ plywood on the deck and nailed every few inches to form a (hopefully) very rigid deck on which the rest of the structure would be placed. I had planned for the deck to be exactly 16′ long, and it was, so the plywood went in place quite nicely: 2 sheets end to end laid out in two rows, with one sheet cut into two 2′x8′ strips and laid down the middle. This is only place where my careful planning back at my desk actually worked out perfectly.