Preparations: Foundations of Small Buildings (DIY Small Buildings)

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Rarely do you erect a building directly on the ground. Usually you have to prepare a base on which it stands and to support it. You might want to use a sectional building temporarily, such as a shed for garden tools during the summer months only, or you will use the building only briefly and then move it to another position. A dirt floor then might be acceptable, but you still must level it. Even a temporary building that's obviously not level looks wrong to you and all other observers. If you erect walls out of true, you will have difficulty fitting the roof.

Compacted soil might take the weight of a small building, but you must ram it or roll it hard before you put the building on it. If soil settles under the weight of a wall, you might have difficulty lifting and supporting it to make it level.

Even if you are satisfied with a dirt floor, it's usually better to arrange more solid supports under the walls. You could dig out a shallow trench and fill it with sand and small stones, rammed tight (figr. 1-4A). It would be better to use concrete for the top few inches (figr. 1-4B). You can embed anchor bolts as shown in figr. 1-4C. With stones only, you will have to drive spikes through them into the ground.

It is possible to use bricks or concrete blocks. It might be satisfactory to put them in line (figr. 1-4D), but you can spread the load better if you place them crosswise (figr. 1-4E). Wood as a foundation material might rot. Some woods, however, have a good resistance to rot. You can expect other wood, which is soaked in preservative, to have a reasonable life. However, painting preservative on your wood does not achieve much penetration and would have little effect. You can use old railroad ties, since they are saturated with preservative and should be immune to rot.

figr. 1-4. A wooden building needs firm foundations.

It is more usual to use an all-over concrete pad as a foundation for a wood en building. You can extend it outside to form a path or patio. You also can use it as the floor of the building if that will suit its use. A floor made of wood or other materials is also a good seal against moisture and rodents. The thickness and the way you form the foundation will depend on the state of the ground. On a hard soil, you could cover a few inches of rammed, small stones with 3 inches of concrete, but if the soil is loose and sandy, you would need to increase both thicknesses.

Most ground is not as level as you would wish. If you want to put the building on the side of a hill, the slope is obvious. You possibly will have to make a foundation pad partly into the slope at the back and build it up at the front.

On apparently level ground you might find a few inches difference in the length of the building, and you must allow for that. You must consider the circumstances and decide if you want one end above the surrounding ground or if you should go deeper at the other end. A compromise might be more appropriate. This decision applies whether you are putting down foundations under the walls only or laying an over-all pad.

For leveling, use the longest possible level, but supplement it with a long, straight board, preferably to span the whole foundation diagonally. It would be unwise to use the level alone to check a distance much greater than its own length.

figr. 1-5. Check foundation levels in all directions.

Start with one side. If you will be puffing down blocks, drive in several pegs as guides so their tops are level (figr. 1-5A). You can use this procedure for a concrete foundation, or you might put down the shuttering strip, making sure its top is level with what will be the top surface of the concrete (figr. 1-5B). Use pegs on both sides of the strip and pack it so it can't move once you have it level and straight.

Next, work square to the first side, setting out the angle as described earlier. Level the pegs or shuttering in the same way. As a final check on that side, put your leveling strip across diagonally (figr. 1-5C). If your leveling strip does not show a true level, test the other ways again—you do not want a twist in the foundation.

Level the remaining two sides in the same way. Mark them parallel to the first sides. Level each in the same way as the first side (figr. 1-5D). Also, check both diagonals. If you will be putting down a complete pad of concrete, put a few pegs in so that their tops are level with the shuttering in the body of the base (Ill. 1-5E). Use them as guides to leveling or laying the stones over which you will put the concrete. Pull them out as you lay the concrete.

If you are only laying concrete under the walls, put in the inner shuttering (figr. 1-5F), making sure it's level as you progress. If you are putting down bricks, blocks, railroad ties, or other sectional-foundation material, lay it with the pegs as guides, but check frequently with your level. You can do little to correct unlevel once the mixture has set.

This guide is not a guide on concrete work, and you should follow the supplier’s recommendations. If the foundation is in a position where you can have ready-mixed concrete delivered and shot, that might be the best way of preparing a foundation. You probably will settle for more than adequate thickness. If you mix the concrete yourself, do not be tempted to lay only a thin layer, for the sake of economy of materials and labor. Have a good, consolidated base of sand and stones, with about 3 inches of concrete, even if the only weight on it will be light storage and standing people. For storage of yard machinery or a car, use an increased thickness of concrete. Thin concrete might crack, even if you do not load it heavily.

An alternative to laying concrete as a one-piece foundation, especially where it also will be the floor, is to put down precast concrete slabs. You can obtain them with decorated or stone-like surfaces, as well as plain tops. You can spread slabs about 24 inches square over a large area quickly. Bed them in sand and small stones, making sure they are level as you progress. They need not have anything between them, although you can seal spaces with concrete. If you seal the spaces with concrete, make sure it's more than ½ inch thick, as very thin concrete mortar tends to crack and break away. Open spaces filled with soil are appropriate to the floors of summer houses or sun lounges, where you might consider grass or small plants taking root there attractive.

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Updated: Monday, April 27, 2020 9:33