The term pile cap may describe a reinforced concrete slab constructed on top of a group of foundation piles to evenly displace or spread the load they are to carry. It can also refer to plastic or glass fiber caps placed over the tops of wooden pilings to protect them against rot and to stop birds nesting on the pilings. A pile cap most commonly, however, refers to the concrete slab that is cast on concrete or steel pilings once they have been suitably trimmed and prepared. These slabs offer a larger area for the construction of the columns they support and also help spread the weight of the structure over all pilings in a group allowing them to better support the load.

Pilings are used as foundations where the size of the building, poor soil quality at shallow depths, and intense site gradients precludes the use of conventional or raft foundations. Pilings are long lengths of solid or hollow steel, wood or concrete either driven into the ground by a pile driver, placed in ready excavated cavities, or cast in situ in specially drilled holes. In the case of driven pile foundations, several piles are typically placed together in a group. A reinforced concrete slab or pile cap is cast over the tops of the group to form a single structural element. This allows for the even distribution of the weight the pile group will carry so no individual member is over stressed.

To construct a pile cap, pilings are first trimmed to ensure that the individual columns are all the same length and their tops are flush. An area around the piling group is then excavated to accommodate the timberwork which will form the boxing or mold for the concrete. The reinforcement for the pile cap will typically be built in the form of a steel cage which is then inserted into the cast box and fasten to the piling group, typically with a timber frame or cross member. At this point, the concrete can be poured into the mold and allowed to cure.

Once the concrete has cured correctly, the timber cast frame may be removed leaving a slab of reinforced concrete joining the pilings. Now the rest the buildings structural work may commence using these slabs as foundation points. The term pile cap may also refer to fiberglass or acrylic caps that are fitted onto the exposed ends of wooden piles particularly in marine applications. These protect the raw ends of the pilings from moisture ingress and resultant rotting of the wood. These pile cap inserts often have pointed ends to prevent water fowl from nesting on top of the piles.