The type and size of the pilings will be determined based upon the environment in which they will be exposed. It is recommended that treated trunks of trees be used for pilings due to their strength versus cost. As shown by Building Products Plus (2010), these are typically made from the trunks of southern yellow pine trees and are treated to prevent premature decay. The length and diameter requirements will be based upon availability and the environment of use. Typically, a 10- to 12-inch diameter will be sufficient to provide the support required. It is a good practice to find out the typical size of pilings used in the general area of construction, if possible. The length of the piling will depend upon the expected depth of penetration into the earth, the final height of the dock surface and the desired height of the pilings over the dock surface. Since the driving of pilings requires specialized equipment, it is suggested that you contract this portion of construction to an experienced contractor, who should also be in a position to estimate the length of piling required for the job.
The substructure of a wood dock consists of headers and stringers, both of which need to be made from treated lumber. Pilings are set into the ground in pairs. The area between two pairs of pilings makes up one section of substructure. The headers connect these pairs of pilings together and serve to support the stringers, and a minimum of 2- by 8-inch lumber should be used. It is recommended that two headers per pair of pilings be used to ensure the support required for the stringers. Make sure that either stainless steel or hot dipped galvanized steel hardware is used to prevent premature corrosion. The stringers lie on top of the headers and serve two purposes. First, they connect the pairs of pilings together, thus creating an interconnected substructure. Second, they provide the base for securing the deck boards. Typically, there will be three stringers per section, and a minimum of 2 by 10-inch lumber should be used. The two outside stringers are attached to both the headers and pilings. The third stringer is fastened down the center to the headers and serves as support for the foot traffic on the deck boards.
The deck boards also need to be made of treated lumber. A common thickness is 5/8 inch, which will be satisfactory for foot traffic. The boards should be cut to the length allowed on your permit and centered upon the stringers so that they are in alignment with the pilings. The boards are best installed using approved outdoor wood screws with a minimum length of 2 ¾ inches. In order to prevent splitting the deck boards, pre-drill the holes with a countersink bit. Ideally, the head of the screw should lie just below the deck surface. A minimum of two screws per stringer is recommended (six total per deck board for a three-stringer substructure). The application of an outdoor deck sealant is highly recommended to protect the boards from the elements.