What Are Joists & Rafters?

Joists

• Joists are the common term to describe the wooden frame for flooring construction. The joists are generally built out of sawn lumber between 2 to 4 inches in diameter. However, it can also be constructed from trusses or a wood composite. Plywood or particle board is laid over the joists to form sub-flooring.

Rafters

• When a builder uses the term rafters, they are describing a similar wooden frame used to support the roof of the structure, usually in steeper roofs. Rafters are built from wood ranging in size from 2x4 to 2x10, depending on the slope of the roof. The purpose of the rafters is to provide a frame for the roof and carry the load of the roofs structure.

Roof joists

• Roof joists are similar to rafters and perform many of the same function. However, roof joists are found on structures with a low slope roof. They carry the load of the ceiling like rafters. Many time roof joists are used with rafters to carry the weight of the roof to the walls.

Live load vs. Dead Load

• When builders frame a home, they take into consideration the live load and dead load of the structure. The load of the structure is the amount of stress and force applied to the structure. Live loads are unstable forces applied to the structure at inconsistent times. This refers to the structures ability to support the weight of people, the force of winds, or even earthquakes when encountered. Dead loads are static forces such as the weight of the structural elements such as the shingles, insulation, or flooring.