What are joists and rafters?

In civil engineering and building construction, joists and rafters are the major components including beams and columns. Almost every structural construction including buildings, skyscrapers, bridges, commercial complexes, and so on makes use of joists and rafters to provide structural integrity to the building. Joists are primarily thick long pieces of metals, concrete, or wood that are used to support the floor and ceiling. On the other hand, rafters form the major component of pitched roofs, which are a series of sloped long pieces extending from a ridged area that makes contact with the plane area of the wall. They account for the load-bearing of the building along with the beams.

In this article, a detailed explanation has been provided regarding joists and rafters, and a brief comparison has been made between beams that act as the primary load-bearing structure of a building.

Joists vs beams

Joists can be spotted between the spacings of foundations of a building or between the spacings of walls. Joists are the horizontal members which connect the foundations of a building, walls, and beams. Linking multiple joists together can provide structural integrity to the building by acting as load-bearing structures to support ceilings and floors. Compared to the beam, joists typically have shorter spans and are made up of timber and composite materials. Timber-based joists are more precisely known as wood joists. For longer spanning structures, composite joists are used such as engineered timber joists, metal web joists, concrete joists, and so on. Sometimes to avoid twisting of joists under the action of external loads, a series of parallel joists are provided in between perpendicular joists herringbones and noggins.

Beams are the structural members that form the prime build pillars of a structure. They can carry a wide variety of loads including axial, shear, twisting, and bending moments. Beams are responsible for the entire building support. The allowable span in the case of beams is large as compared to the joists. For example, a cantilever beam supports large horizontal concrete protrusions of a building, which are precisely known as slabs. They carry shear loads and resist huge bending moments. Other beams such as simply supported beams form the skeleton of a building, that are intended to carry the entire building load, and with a high modulus of rigidity, they provide excellent compressive and tensile strength.

Types of Joists

There are two kinds of joists used in a building and for structural constructions, they are outlined below.

Floor joists

Floors joists are horizontal members often made up of lumber to connect the spacing between two horizontal beams. Floor joists form the main foundation of a building floor. Floor joists intend to transfer the loads falling on them vertically to columns, which supports the beams. In modern construction, floor joists are made with concrete, lumber, composites, etc. Lumbers are products of timbers that are processed into planks, and beams. The timber industry is responsible for the conversion of timber to lumber for constructional purposes based on construction data and design values. Including pulpwood, lumbers are also used in many woodworking and furniture making applications

Strictness in adherence to the design values is essential because it eliminates the deflection caused to the floor joists during its operation. Also, floor joists operate according to their permissible load values that specify their deflection limits, beyond which there is a possibility of floor joists failing significantly. Floor Joists are laid in a grid pattern or a series of networks over the beams. They are securely fastened at a place by bolts and nuts. These grid patterns and network patterns help to distribute the load uniformly by the floor joists.

Ceiling joists

Unlike floor joists, ceiling joists are horizontal members that intend to support the ceiling of a building. Ceiling joists support the ceiling along with diagonal rafters and provide shape to the roof of a building. The bottom chords of modern ceiling joists are attached to plane or space trusses. The plaster sheeting is attached to the undersides of joists and battens. The battens are fastened to trusses.

A ceiling joist
CC BY-SA 3.0 | Image credits: https://en.wikipedia.org | Visitor7


Rafters are a series of sloped structural members, typically of wood. They extend from the roof ridge to the building wall plate. Rafters are used to support roof shingles and roof decks. The space between the rafters is filled with webs or horizontal joists. Rafters form the primary structural materials for usually limited storage or volume buildings as large buildings will require advanced structural components to distribute and support the loads including the building itself. Modern architecture and construction use exposed rafters for constructional and aesthetical purposes. These rafters generally use larches. Larches contain resins naturally and resins protect them from decay.

Joists sizing and span tables

Knowledge of mechanical properties and structural principles behind the usage of joists leads to the efficient selection of joists while referring to span tables. For instance, the majority of joists make use of wood as the primary material. Wood is derived from wood-bearing plants and timber and has complex mechanical properties such as compressive strength, bending resistance, twisting resistance, and so on. A basic understanding of the properties of wood is crucial in referring to span tables to decide an appropriate design value for the joists kipping in mind the external loading conditions.

The joists, columns, rafters, and beams are the critical components that aid in resisting loads in the building, including dead loads and live loads. The resistance that these members provide is restricted by design and deflection limits set by building codes. These codes suggest the maximum permissible dead load values and other values of external loads that a building and related building material can handle. Span tables include all these values of loads and sizing related to the material to be used for constructions. For instance, a clay-type roof system has a live load value of 20 psf (pound-force per square foot) and a dead load value of 15 psf.

Context and Applications

This topic is taught in different undergraduate and postgraduate degree courses like:

  • Bachelors of Technology in Civil engineering
  • Masters of Technology in Civil engineering
  • Master of Science in Building Construction

Practice Problems

Q 1. Which of the following is a grade of composite joist used to support building and ceiling?

  1. No. 2 grade
  2. No. 1 grade
  3. Type II joist
  4. The engineered timber I joist

Answer: Option d

Explanation: Engineered timber 1 joist is used as a composite joist to support buildings and ceilings.

Q 2. With which of the following component is a rafter connected between a roof ridge?

  1. gable
  2. wall plate
  3. girder
  4. none of these

Answer: Option b

Explanation: A rafter is a diagonal connection between a roof ridge and a wall plate.

Q 3. In which of the following construction processes, a floor joist is used?

  1. In laying the floor foundation
  2. AWC construction service
  3. AWC construction LLC
  4. Sizing of floors

Answer: Option a

Explanation: Floor joists are used in laying the foundation of floors and uniformly distribute the loads that fall on them.

Q 4. Which of the following documentations include the load and sizing information regarding building construction and materials?

  1. Span tables
  2. Building codes
  3. Both a and b
  4. None of these

Answer: Option c

Explanation: The loads and sizing information regarding building construction; and materials are included in building codes and span tables.

Q 5. What is the full form of psf?

  1. Pounds per square force
  2. Pounds square foot
  3. Pounds-force per square foot
  4. None of these

Answer: Option c

Explanation: The full form of psf is pound-force per square foot.

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