Houses built low on a hillside will always face drainage problems. Water follows the path of least resistance, which typically means that the water will flow into the basement or beneath the house's foundation. Ignoring a drainage issue will eat away at the foundation and the ground, degrading the overall stability of the structure. Several options exist to address the issue.
Mold and Mildew
Any occurrence of standing water mixed with heat creates the ideal environment for the development of molds and mildew. A house constructed onto a hill without the proper drainage channels will find that the walls readily develop mold and mildew. The largest problem with this situation occurs when the mold and mildew develop within the walls. Water flowing down the hill into the house is siphoned into the walls, which allows the mold or mildew to grow. A resident will not notice the situation until the mold or mildew is present inside the house, which will require a lot of demolition and repair to remove.
Concrete slabs are often used in houses. Building a house on a hill using a concrete slab foundation requires the installation of moisture and vapor barriers. The builders need to evaluate the soil for proper drainage and type of soil before erecting the concrete slab otherwise drainage from the hill will seep up through the concrete. The moisture weakens the concrete causing cracks and sagging over time. Flooring adhered with some form of adhesive to the cement will loosen due to the moisture. Without the proper barriers and drainage, water flowing down a hill will find its way up through the concrete slab easily.
Basements are built into the ground. A house on a hill will have at least three sides of the basement almost fully submerged into the ground. Poor drainage around a house on a hill means that water will be standing against the exterior wall facing the hill and leaking into the basement. Severe rainstorms could easily flood the basement once cracks in the wall open up due to the presence of excessive moisture. The walls, which are load-bearing walls, will weaken allowing the house to start to sag and settle. The walls can be protected during the construction phase of the basement by sealing the outer edges with a moisture barrier or sealant paint.
Flooding, mold and mildew and moisture raise many health concerns. Poor drainage around a house built on a hill causes all three issues, which can lead to dangerous situations. Some varieties of mold are considered to be highly toxic. Several people have natural allergies to mold and mildew, which can create an unpleasant living environment if the presence of the toxin is not known. Flooding and moisture allows for standing water around the house, which creates an ideal breeding ground for a large number of insects, such as mosquitoes. The key to preventing such health concerns is to erect a drainage channel of some form around the structure to remove excess moisture from the ground. A lot of the concerns can be prevented by following proper construction techniques during the building of the house, such as properly sealing exterior walls and installing moisture or vapor barriers beneath the foundation.