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I am building a shed from a kit. The floor kit has a pretty standard 8x12 layout, but the instructions state that it should be leveled by setting it on 3 parallel treated 4x4s (runners), and that the 4x4s should be leveled by putting supports under the 4x4s about 4' apart.

The sample they show illustrates supporting the 4x4s setting on dirt, and they suggest stone blocks on the ground, 4 inch wood blocks above any blocks, then a bit of asphalt shingle, and finally the runners sit on the shingles.

I'm setting the deck on the concrete patio, shown here. The low point is the top left, high point bottom right. We do get water pooling on the patio in the rain but it does drain off within 24 hours.

When setting the treated runners on concrete should I still lift them off the concrete with blocks? Can I just put pieces of shingle under them at the high point and use wood blocks + shingle to lift the other points to be level?

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  • Do you know the difference between high and low? – longneck Jun 12 '18 at 15:09
  • About the height of a single course of bricks so ~2". – The Evil Greebo Jun 12 '18 at 15:21
  • No tell a lie, its more like a course and a half. – The Evil Greebo Jun 12 '18 at 15:21
  • Make sure the 4x4s are labeled for ground contact. Given that the shed is small I wouldn’t overthink this. If one rots out in 15 yrs they are fairly easy to replace. Following the instructions your concrete patio is equivalent to stone blocks so add enough wood to level and then shingles as suggested. – Stanwood Jun 13 '18 at 20:43
  • Exactly what I ended up doing. 4x4 wood blocks to level + shingles. – The Evil Greebo Jun 14 '18 at 10:26
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+200

I would want to keep the shed as low as possible so you don't have a tall step up into it. Instead of pressure treated 4x4s I would consider composite lumber, which you can reasonably expect to last as long as the shed. It's more expensive of course, but you only need three pieces.

I would pull drylines at level and measure every 16 inches between the line and the existing concrete. Transfer those dimensions to your footing lumber and cut the tapers into them. I think I'd shoot for about an inch at the high corner, just enough to allow water to flow underneath the deck without soaking it.

This approach gives you high durability, full contact support of the footings, plenty of support for the shed floor framing, and a low step-in height.

  • "pull drylines"? – The Evil Greebo Jun 15 '18 at 15:47
  • Chalkless carpenter's string for establishing straight lines. – isherwood Jun 16 '18 at 16:31
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It's really all about water, and what will happen in the future. Just leaving the blocks on the patio you'll have in short order debris against each 4x4, which over time will rot out even pressure treated.

I'd put down 1x concrete pavers to make some space, then the wood block/shingle to level it out.

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