I am building a 10×12 shed on a slight slope. I was hoping to do an on grade foundation, but when using a string level, it looks like my slope is about a 2ft difference on its lowest corner.
What are the height limitations of using stacked paver blocks and deck piers with 4×4 post? The plans I have mentioned not to stack more than four 4inch tall blocks as it becomes unstable, but what if it's just one corner? also it didn't specify the height restrictions on the deck pier.
Is there anything wrong with doing a deck pier or a post in ground in the lowest spot, and paver blocks every where else?
Best Answer
How you approach the problem depends on what you propose to use as a foundation for the shed.
Option 1: On-grade paver foundation
Step 1: Build a retaining wall to support a flat grade. The simplest would be a gravity wall: bare wall blocks on a packed gravel base, no mortar. The 4" pavers you mention are not suitable but there are many blocks available for this task. For a small corner wall I suggest buying a few wall blocks at the local chain store. There are nicer blocks available from dedicated stone suppliers but you typically have to buy a full palette. You can also use concrete blocks (CMB) but the wall blocks look nicer and are designed to stack without re-bar or fill. I'm only glossing the surface here. Search for other questions on gravity retaining walls and you will learn more about their construction. The key to their long-term durability is to address any drainage issues so water doesn't build up behind them.
Step 2: Put down an on-grade foundation to support the shed. Sounds like you plan to use 4" tall pavers (e.g. on a sand base). If you want to set the pavers on top of the wall then you need a frame to retain the pavers so they don't slide out. It's probably easier to set the pavers flush with and behind the wall. In this case the shed will sit on the wall, rather than the pavers, in the one corner.
Option 2: Deck foundation
My shed sits on a frame built from pressure-treated 2x6 boards covered with pressure-treated plywood. A deck foundation of this style can be supported by a combination of methods: a continuous gravel base, concrete blocks set on gravel, or deck posts set into the ground with typical deck construction methods. I'm not a fan of the deck post solution because it looks a bit unfinished and tends to create animal habitat (see below). It's better to set the deck on a block retaining wall as above and elsewhere on individual blocks set on a gravel base.
If you intend to install the shed near flush to grade then some of the deck foundation must be buried. Typical pressure-treated 2x6 lumber and plywood is not treated for ground contact. It will not last as long as the shed that sits on it. The paver foundation will.
Building animal habitat
The deck foundation method tends to leave sheltered space under your shed. Once the dust settles your local mammal population is sure to move in. If you're not particularly fond of rodents then you should take steps to deny them this space. With deck construction you can't really fill the space in the corner so fence it off with steel hardware cloth (23 gauge galvanized material with 1/4" holes should do it). If the entire deck is above grade then you'll need to fence it off all around. On end of the fence attaches to the deck. The other end should be buried. The buried bits will rust away but by then hopefully the dirt is packed and the rodents steer clear. I like to cover the buried cloth with gravel as a further deterrence.