To protect the floor from wet, concrete bases require an underlay with a moisture barrier, a damp proof membrane (DPM), or a separate barrier. If the subfloor is made of wood, the underlayment chosen will be determined by what is installed on top. If it is to be finished, as is most common, then a smooth surface-wood flooring or vinyl planks-should be used. If the subfloor is not going to be finished, as is usually the case with dirt or other non-slip surfaces, a thin layer of sand or gravel is used as an underlay. This allows water to drain away from the house and prevents soil from entering through air gaps around windows and doors. Concrete foundations are heavy so extra care should be taken not to damage the substrate during installation. The type of underlayment used will depend on how much protection the floor needs. If you want to use a concrete floor but don't want to pour a new foundation, consider these options: A install a DPM B install a concrete slab with an optional vapor barrier C install a concrete slab with a standard foam insulation package D install a dirt floor Your local home improvement store can help you decide which method is right for your situation. Be sure to get advice from a professional if you have any doubts about whether or not concrete should be poured as a foundation.