A satisfactory slab on ground can be built without a subbase, but a subbase is frequently placed on the subgrade as a leveling course to equalize minor surface irregularities, enhance uniformity of support, bring the site to the desired grade, and serve as a capillary break between the slab and the subgrade. Where a subbase is used, the concrete contractor should place and compact to near maximum density a 100 mm-thick layer of granular material such as sand, gravel, crushed stone, or slag. If a thicker subbase or subgrade is needed for achieving the desired grade, the material should be compacted in thin layers about 100 mm deep. Subgrades and subbases can be compacted with small portable vibrators, rollers, and hand tampers. Unless the subbase is well compacted, it is better to leave the subgrade uncovered and undisturbed.