Joist vs. Rafter — What's the Difference?
By Tayyaba Rehman & Fiza Rafique — Updated on September 8, 2023
A joist is a horizontal support member in floors and ceilings, while a rafter is an inclined structural element that supports a roof.
Difference Between Joist and Rafter
Table of Contents
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Key Differences
A joist is a horizontal structural element that is used to support floors and ceilings. It generally runs between walls, beams, or other structural elements and is an integral part of floor and ceiling framing. Rafters, on the other hand, are inclined members that extend from the peak of a roof to its outer edge, essentially forming the "skeleton" of a roof.
Joists are most commonly made of wood, steel, or engineered wood and lie flat, providing a surface upon which flooring or ceiling materials can be laid. Rafters, in contrast, are usually made of wood and are set at an angle to support roofing materials such as shingles, tiles, or metal.
When it comes to load distribution, joists bear the weight of the flooring or ceiling and distribute it evenly across the horizontal plane. Rafters carry the weight of the roof and any additional loads, such as snow, and channel it downwards to the building's walls or supporting structures.
Although both joists and rafters serve to support and stabilize a building, they differ in orientation, purpose, and where they are installed. Joists are essential for creating level floors and ceilings, while rafters are crucial for constructing stable and durable roofs.
Comparison Chart
Orientation
Horizontal
Inclined
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Supports
Floors/Ceilings
Roof
Common Materials
Wood, Steel
Usually Wood
Load Distribution
Even, horizontal
Downwards, angular
Location
Inside building
Under roof
Compare with Definitions
Joist
A joist is a horizontal beam used in construction to support floors.
The joist under the living room floor needed replacing.
Rafter
A rafter is an inclined beam that supports a roof.
One of the rafters was damaged in the storm.
Joist
Joists are commonly made from wood or steel.
The steel joists in the warehouse can support heavy machinery.
Rafter
Rafters form the internal framework of a roof.
The rafters give the roof its sloped shape.
Joist
Joists run parallel to each other to distribute weight.
The joists are spaced 16 inches apart to evenly distribute the load.
Rafter
Rafters are often visible in open-ceiling designs.
The exposed rafters add a unique aesthetic to the room.
Joist
Joists are structural elements that support ceilings.
We had to add extra joists to support the new ceiling fan.
Rafter
Rafters are primarily made of wood.
The wooden rafters matched the rustic interior of the cabin.
Joist
Joists often connect to beams or walls.
We attached the joists to the central beam for additional support.
Rafter
Rafters distribute the roof's weight to the walls.
The rafters channel rainwater and snow load towards the walls.
Joist
A joist is a horizontal structural member used in framing to span an open space, often between beams that subsequently transfer loads to vertical members. When incorporated into a floor framing system, joists serve to provide stiffness to the subfloor sheathing, allowing it to function as a horizontal diaphragm.
Rafter
A rafter is one of a series of sloped structural members such as wooden beams that extend from the ridge or hip to the wall plate, downslope perimeter or eave, and that are designed to support the roof shingles, roof deck and its associated loads. A pair of rafters is called a couple.
Joist
Any of the wood, steel, or concrete beams set parallel from wall to wall or across or abutting girders to support a floor or ceiling.
Rafter
One who travels by raft.
Joist
To construct with joists.
Rafter
One of the sloping beams that supports a pitched roof.
Joist
A piece of timber or steel laid horizontally, or nearly so, to which the planks of the floor, or the laths or furring strips of a ceiling, are nailed.
Rafter
A group or flock, especially of wild turkeys.
Joist
(transitive) To fit or furnish with joists.
Rafter
(architecture) One of a series of sloped beams that extend from the ridge or hip to the downslope perimeter or eave, designed to support the roof deck and its associated loads.
Joist
To fit or furnish with joists.
Rafter
(collective) A flock of turkeys.
Joist
Beam used to support floors or roofs
Rafter
A raftsman.
Rafter
(transitive) To make (timber, etc.) into rafters.
Rafter
(transitive) To furnish (a building) with rafters.
Rafter
To plough so as to turn the grass side of each furrow upon an unploughed ridge; to ridge.
Rafter
A raftsman.
Rafter
Originally, any rough and somewhat heavy piece of timber. Now, commonly, one of the timbers of a roof which are put on sloping, according to the inclination of the roof. See Illust. of Queen-post.
[Courtesy] oft is sooner found in lowly sheds,With smoky rafters, than in tapestry halls.
Rafter
To make into rafters, as timber.
Rafter
To furnish with rafters, as a house.
Rafter
To plow so as to turn the grass side of each furrow upon an unplowed ridge; to ridge.
Rafter
One of several parallel sloping beams that support a roof
Rafter
Someone who travels by raft
Rafter
Provide (a ceiling) with rafters
Common Curiosities
How do joists and rafters differ in orientation?
Joists are horizontal; rafters are inclined.
What materials are commonly used for joists?
Wood, steel, or engineered wood.
What is the basic function of a rafter?
A rafter supports the roof.
What is the basic function of a joist?
A joist supports floors and ceilings.
How do rafters distribute load?
Downwards and angularly to walls or supports.
Are joists only used in residential buildings?
No, they are used in various types of structures, including commercial buildings.
What materials are commonly used for rafters?
Usually wood.
Where are joists located in a building?
Inside, supporting floors and ceilings.
Do joists require additional support?
They often connect to beams or walls for support.
Where are rafters located in a building?
Under the roof.
Can rafters be visible in a room's design?
Yes, especially in open-ceiling designs.
Do rafters require additional support?
They usually rest on wall plates or beams.
How do joists distribute load?
Evenly across the horizontal plane.
Are rafters only used in certain types of roofs?
They are common in pitched or sloped roofs but not usually found in flat roofs.
Can joists be visible in a room's design?
Rarely, as they are usually hidden by flooring or ceiling materials.
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Written by
Tayyaba RehmanTayyaba Rehman is a distinguished writer, currently serving as a primary contributor to askdifference.com. As a researcher in semantics and etymology, Tayyaba's passion for the complexity of languages and their distinctions has found a perfect home on the platform. Tayyaba delves into the intricacies of language, distinguishing between commonly confused words and phrases, thereby providing clarity for readers worldwide.
Co-written by
Fiza RafiqueFiza Rafique is a skilled content writer at AskDifference.com, where she meticulously refines and enhances written pieces. Drawing from her vast editorial expertise, Fiza ensures clarity, accuracy, and precision in every article. Passionate about language, she continually seeks to elevate the quality of content for readers worldwide.
