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Building a foundation for a yard shed

So you want to build a shed in the backyard? Unless you plan to construct a mini-mansion or one with fancy gingerbread, you are better off buying a prefab shed or one that the manufacturer will build for you. Prefabs are a good buy, offering several styles and many sizes with a price that compares favorably with the cost of materials if you build it yourself.

The main issue when you buy a prefab shed is to make the foundation (the structure the shed will sit on) the same size as the shed. Such a shed does not sit on the ground.

There are three types of foundations: 1. Timbers on concrete blocks. 2. A four-sided border of joists. 3. Concrete slab. Hand and power tools are needed for these projects. You will need masonry tools for the concrete slab. The most important tool is a spirit level at least 4 feet long, because getting the foundation level is crucial.

Most sheds have joists under the floor, such as 2-by-6s and 2-by-8s for bigger sheds.

1. Start with the easiest foundation: Timbers on concrete blocks. Place one concrete block in each corner of the foundation, holes side up. You can bury the blocks a bit in the ground, but it is not necessary; a shed generally does not have to have a foundation 4 feet into the ground (below the frost line).

Make sure the blocks are level in two directions. To do this, use a long 2-by-4 or other-sized joist, lay this on two blocks and put the level on top. Raise or lower one block until the level shows the joist is, well, level. Do the same along a second dimension. If two dimensions are level, the others will be (or should be) too, but put the level on them to make sure.

Place a timber (4-by-6 or larger) on its narrow side to connect the blocks on the long length. Two timbers are needed, one for each long length. You can drill holes in the timbers and drive in a reinforcing rod so that it goes through the timber and 2 feet into the ground. Place rods every 4 feet.

Now you can have the prefab delivered and put on the foundation, toenailing through a joist into the timbers with galvanized nails.

With the timbers on top of the concrete blocks, there will be gaps between the blocks and other sections. This is good for ventilation, but the gaps should be screened to keep out rodents and other vermin. You can make a framework to hold quarter-inch steel mesh. Half-inch mesh will also do nicely. The ventilation will prevent buildup of moisture.

2. Build a four-sided border of joists. This is easy; just nail 2-by-6s or larger joists in a rectangular shape to match the size of the shed. Getting everything level is another matter. Doable, but not easy. Place polyethylene plastic on the ground to keep moisture in the ground. Now the shed goes on top.

To vent this space, drill 1-inch holes in the joists, and staple screening behind them.

3. Concrete slab. Excavate 6 to 12 inches and pour in 6 inches of crushed stone. Build a four-sided border of joists or other boards to make forms for pouring concrete. The slab can be 4 to 6 inches thick, with reinforcing rods for extra strength. If you are pouring 2 yards or more, buy it ready-made, delivered by truck. Anything less can be mixed on-site.

The shed then goes on the slab, which is floating; that is, it is not connected to a foundation. It will rise and fall when the ground freezes. The hope is that the rise and fall will be even, without the slab tilting. The crushed stone under the slab will help reduce movement.

Then the shed gets bolted through the floor plates of the walls into the concrete.

Finally, to ventilate the slab, drill 1-inch holes in the perimeter joists and put screening on the backs of the joists.

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