Many people confuse a pier-supported house with a house that sits on pilings. Pilings are timber, metal or concrete columns driven many feet into the ground to support the house on bedrock, while piers sit above ground. Building a house on pilings requires the assistance of an engineer to calculate the load and the type, length and diameter of the pilings. Houses are built on pilings where the land won’t support traditional foundations, such as flood zones, coastal areas and mountain sides. They also are used in freeway construction, where their long columns descend beneath soil.
Crews use special equipment to insert pilings deep into the ground. They set a large pile-driving crane on tracks on level ground that is or near the construction site. Pile drivers often count the number of blows it takes to drive a pile into the ground because it often translates to the pile's load-bearing capabilities.
Several piling types are available, and an engineer usually recommends which kind to use based on soil type, piling strength and the weight of the structure the pilings will support. Piling types include timber, steel, masonry and a combination of steel and concrete. A region may influence the kind of piling used, too. For example, timber pilings are used frequently along coasts.
Houses in flood zones must be built above grade on pile or masonry foundations, depending on the location's building codes. The design allows the area underneath the houses to flood without damaging the structures. Because pilings are driven deeply into earth, often to bedrock, flood waters do not affect pile-driven foundations.
Driving a pile into the ground involves more than just choosing a location. Engineers take core soil samples to determine the soil structure and how far down a pile must go. Most piles are driven 20 to 25 feet into the ground for deep rock embedding, and that much piling may be above ground. The more exposed a piling is above ground, the deeper below ground it must go to support the house adequately.