How to Prepare a Building Site for a Slab Foundation

Written by erin moseley
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Proper building site preparation is essential for project success when it comes to slab foundations. Uneven building surfaces present the basis for uneven foundations and subsequently weakened structures. Measures need to be taken to ensure an even surface, so problems can be avoided when the slab is poured. Later may be too late to try to fix it. Best to make sure the site is free of roots and debris, filled, levelled and compacted. So plan to do it right, get the assistance you need and sleep well knowing that your prep work is responsible for another great foundation.

Skill level:


  1. 1

    Look over your building site to determine where your slab will sit, and what orientation you plan to build. Check to see that your slab can fit between any large boulders and if you need to reinforce a hanging wall.

  2. 2

    Verify the type of soil and the soil depth and determine its stability. Check with a soil engineer to be sure what you're dealing with. Clay soils can expand and shrink and sandy soils can be unstable. Soil type will determine what type and depth of subbase to use before your pour the slab foundation.

  3. 3

    Decide what trees or shrubs you want to preserve. Install stakes and tack on plastic screening around any vegetation so it won't be damaged later.

  4. 4

    Clear your building site area so that it's free of unwanted trees and roots, rocks, debris and any other vegetation. Use shovels or heavy machinery to remove larger rocks and tree stumps that can later settle and shift your foundation. The building site includes the area within the building perimeter and any other adjacent areas where work will be done.

  5. 5

    Augment the height of the slab area if you anticipate drainage problems. Add fill like gravel, sand or the same material as your building site to increase the height to your desired level.

  6. 6

    Level the building site. Use a grader or hire someone who can operate one to flatten and level the building area. Compact the entire area to encompass your slab plus footing. Use a level to check that you don't have depressions.

  7. 7

    Slope the surrounding ground away from the building site using a ratio of 1/8 inch per 1 foot to prevent accumulation of water. This measure is most important for soils that don't drain well, like clay.

Tips and warnings

  • Preparing the site for the foundation is a critical step leading up to construction. Be sure that the site is levelled inside the slab perimeter and sloped slightly outside of the footing trench for proper drainage.

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