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I am building a 10x12 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 4x4 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?

  • It would be helpful to know soil type (clay, sand, mix), slope pitch, is the surface stabilized (e.g., covered with sod), climate (what state are you in?). – Stanwood May 15 '18 at 12:46
  • @Stanwood It has about 5-6 inches of black dirt added for sod, and below that is clay soil freshly tilled last year with added new sod. I am not sure of the pitch (won't be able to get that as I won't be home until late tonight). I am located south of Chicago IL. – eaglei22 May 15 '18 at 12:53
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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.

  • Thanks for the response! never thought of using a retaining wall. I do have a pallet left over of retaining wall blocks from my last year swimming pool install. I was considering putting the retaining wall around the shed to hide the exposed areas having gone the 4x4 post route. So since you didn't mention the deck peir blocks, do you advocate against it then in this situation? I was starting to think possibly using the deck piers, and hiding the "openness" with a retaining wall. However I was only planning on using the retaining wall to fill with dirt, which I was going to save for.. – eaglei22 May 15 '18 at 14:07
  • a different question.. because I was trying to think of a good approach of what to use to put the dirt up against near the shed (in between the retaining wall), so I wouldn't have it sinking and going under the shed. I was thinking of this approach so I could add flowers/landscape to it. Btw, I do have a wood fence around my yard, which will help with nesting animals. – eaglei22 May 15 '18 at 14:08
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    Deck pier blocks sound like a good solution. You could mount the deck joists directly on the blocks in the flat locations and to a 4x4 post where the slope demands it. You could screen with a retaining wall and back-fill to make a planting bed. If the wall doesn't support the shed then it could be built less stringently. In particular, if the wall starts to fail from water heaving it's not such a big deal to reset a few blocks. You could take steps to reduce this by using gravel under and behind the wall. If the wall is all around the shed consider putting in a drain pipe. – Stanwood May 15 '18 at 15:17
  • Not to shift this too far off topic, but for the areas that sit higher off the ground, is there anything I can tac against it the supports you recommend to keep the dirt from going under the shed? Holding it just between the shed and retaining wall? I was thinking of just getting pressure treated plywood, and throwing roofing tar over it. This way I won't have to back fill too much dirt, and can keep under the shed with landscape fabric and rock to prevent weeds growing through. – eaglei22 May 15 '18 at 15:24
  • I understand the question but it's hard to comment without a sense of the overall solution. How high is the deck over grade, does the wall go all around, etc. Maybe try to draw this up and submit as a distinct question to get feedback on it? – Stanwood May 15 '18 at 15:35

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