Water is a threat to the the foundations of all houses and buildings. Water can cause the soil on which foundations are built, and that surround foundations, to expand, to weaken, or to be displaced by freezing. When the soil is weakened or expanded, the foundation loses support and may shift. This shifting will be translated throughout the structure’s frame. It can cause walls to crack, studs and joists to realign, and affect the entire stability of the structure. For this reason, all foundations are build with a foundation drainage system.
Foundations for structures come in two types, deep or shallow foundations. A deep foundation is exactly that, deep. It is selected as a foundation for extremely heavy structures, but also if the soil on which the building is to be built is loose or the climate is extremely rainy.
In most residential buildings, houses are built on shallow foundations. They have the same purpose as deep foundations, to bear the structure’s load, but these foundations are only set a little ways into the earth. There are five kinds of shallow foundations: mat-slab, slab on grade, rubble on trench, spread-footing, and earthbag foundations.
Slab-on-grade and earthbag foundations are even with the ground. These do not have basements, so these do not require a foundation drainage system beneath the structure. A foundation drainage system for a slab-on-grade or earthbag foundation will embrace the sides of the house. Gutters surround the house to capture runoff and surface water, and to guide the water captured to sewers or dispersing fields. This foundation drainage system is also a sub-system of the other three foundations types.
The rubble on trench foundation is particularly adapted to provide both support and drainage of water. A narrow trench is dug beneath the frost line and sloped towards an outlet. The trench is bedded with washed stone and a layer of protective fabric. Rubble and stones are laid on top of the bed and then, on top of this base, foam board for insulation. Finally, reinforced concrete is spread as a slab over the trenches, to bear the structure’s weight. The rubble trench foundation drainage system, like the slab-on-grade and earthbag foundations, do not have a basement.
Both the spread-footing foundation and the mat-slab foundations typically have basements sitting atop a shallow bed of concrete. A spread-footing foundation is constructed by digging and setting into the earth a mold into which concrete is poured. The mat-slab foundation is constructed in a similar manner, deeper than spread footing, with mats of heavily reinforced cement. The foundation drainage system for these types of foundations are installed before the foundation concrete has been poured. The intake drain captures water on the basement floor which must be sloped, in order to force water to flow down the outlet pipe and into the main sewage and on to the structure’s main sewage outlet. The drain trap which collects the basement floor water is connected to a ‘P-trap’, which is a curved pipe that holds water, thereby preventing sewage gases from rising up through the drain. Floor trap drains can be removed in order to remove obstructions in the pipes.
A foundation drainage system is an essential part of any house. Without it, basements would flood and the foundation eventually destroyed. A sump pump can be installed in the basement to pump out water when the drainage system overflows. Recently, internal gutters have been introduced to capture water from leaking and spreading walls. Be mindful of your home’s foundation drainage system. Your house depends on it!