Joist Vs Beam | Compare and Contrast
For the uninitiated, roofing terminology, can be confusing, like learning a whole new language, especially if you have no previous experience in construction. Here at RenoCompare we help to demystify the difference between different roofing elements and in this article we’re going to focus on the difference between joists and beams. We will define both of these roofing terms, explain their construction, costs, durability and maintenance.
WHAT IS THE MAIN DIFFERENCE BETWEEN JOISTS AND BEAMS?
If you’ve got a construction project on the board, then you’ll use both joists and beams, but there are important differences to be aware of as you work with an architect or builder to plan construction.
Beams are the main structural component of any wooden structure. They carry the main loads for floors, ceilings and the roof. Both are important elements in residential and commercial construction.
The main differences between beams and joists are that joists provide secondary support for the loads rather than primary support. And they are many, while beams are few.
And beams support joists.
WHAT ARE BEAMS?
Beams are indispensable.
As the main load-bearing member of residential buildings, beams transfer loads downward to posts, also called columns, or to walls or the foundation including slab foundations. When placed in a foundation with a basement or crawlspace below, the beam is often supported by posts called jack posts.
Terminology: The central beam, the largest beam in a home, is called a girder.
Depth = Strength: Wood beams have strength based on their depth. For example, a beam with a depth of 8 inches is significantly stronger and can carry more load than one that is 4 inches deep. Stability is added as their width increases. Typically, an architect or structural engineer is employed to determine the load all the beams in a structure are going to carry and how long they are going to be. This determines the strength and stiffness required. And then options can be decided upon for material – and the depth of the beams based on those factors.
WHAT ARE JOISTS?
If you’re inside right now, there are likely joists beneath and above you.
Joists are smaller members and can rest on beams (typical) or hang from beams with the help of steel hangers fixed to the beam’s side. They can also be supported by wall structures.
Joists are used for subfloor and ceiling structures, providing support for flooring or a board to attach drywall to. They can be visible from the basement or attic. Roof joist systems are an essential component of roofing structures with a wide span that are built with rafters. Wood joists are commonly used for decks, and if you have a deck, that might be the easiest way to see how beams and joists are used together to support a load – in that case, the deck boards.
Clarifying terminology: It’s worth noting that joists are a type of beam, though they are usually called joists to avoid confusion with main beams that carry the home’s dead loads.
Trusses Vs Rafters
OK, we’ve covered the definitions and the main differences between beams and joists.
But how do these elements work together when framing a home or constructing a commercial building?
Beams can be pieces of dimensional lumber, steel or made with engineered or laminated (layered) wood such as plywood to produce what are called LVL or build-up beams. These beams are lighter and stronger than solid wood beams.
Joists for residential use are produced from dimensional lumber, while steel joists are often used in commercial buildings. Joists are less commonly laminated materials.
Both structural components are made in various lengths, though main beams of a home can be longer than most joists.
Joists as structural elements usually run the opposite direction as beams – that is, they are arranged across the beam rather than parallel to it. And they run parallel to one another and bridge the open space between walls.
There are differences between beams and joists when considering installation.
Support: Main beams, as noted, rest on the foundation walls of a home including a poured concrete or block foundation or posts/columns supported by the foundation. Smaller beams rest on the main beam or on one of the foundation components.
Joists usually rest on beams and support elements like the roof deck or flooring. Overhead, drywall might be attached to the roof joists or the joists forming an upper floor.
Single vs multiple: Beams are usually a single member extending across an entire span. When the span is exceptionally wide, two or more beams are used. But each must rest on the foundation or posts.
When joists, including floor joists and ceiling joists, cover a wide span, usually more than 16’, two joists can be secured together, though they often rest on a post or beam where they are joined.
Non-horizontal orientation of beams: While usually used as a horizontal structural member, beams can be positioned as necessary for their purpose structural members. In roofing systems that do not use rafters or trusses, for example, beams can be installed with a slight angle.
Joists always provide horizontal support in floor applications and most ceiling/roof use. In some roof applications, they are installed with just enough slope to allow for water to run off the roof deck.
Beam slots: In a poured or block foundation, beam slots are provided to the right depth, so that the top of the beam will be level with the top of the foundation. In other words, it could be said that the beam rests “in” the foundation vs “on” the foundation.
Headers: Beams span any opening in a load-bearing wall. Common openings include windows and doors, and the beams over them are called headers. Header beams are used in non-load bearing walls too, but in those applications, the beams do not have to be as large. All beams used as headers rest on posts or columns at the sides of the opening.
We should give you fair warning on prices. The cost of building materials has ranged from two times to four times the cost of those same materials pre-Covid 19. Costs remain high with little relief in sight due to inflation, supply chain problems and high demand. As a result, the actual current price of beams and joists is subject to change.
Prepared for sticker shock?
Beams: The material cost only of wood and laminate LVL beams is $4.00 to $14.00 per linear foot based on the depth and width of the beams. On the cost spectrum, LVL beams are priced higher per linear foot. But they are lighter and stronger than wood beams. Steel beams fall in the middle of the price range – about $8.00 to $10.00 per foot, but steel prices are volatile too.
A total beam package for a 2,000 square foot home is $2,100 to $3,600 installed. These beams include the main beam, or girder, or the home and all additional beams including headers in load-bearing walls.
Joists: Joists are dimensional lumber or plywood, usually 2x6s to 2x12s, in lengths from 10 feet to 16 feet. Joist cost for just the material is $1.35 to $2.50 per linear foot. A typical single-story 2,000 square foot home uses between 175 and 215 joists based on the home’s design. These are floor joists and roof joists. The material cost then is around $3,400 to $6,400. Installation labor boosts total cost to $8,1000-$12,500, which includes transportation, unloading and the actual construction of the floor and roof systems.
200 years plus?
When your roofing and roof deck are well-maintained and your home doesn’t experience significant flooding or other catastrophic event, your home’s framing including beams and joists will last indefinitely.
These structural elements don’t require any maintenance – but your home’s roofing and siding does. When the exterior is maintained, as noted, the framing will last for generations.
Now you know the differences between beams and joists, but it isn’t an either/or proposition. Both of these structural elements are important to the construction of residential and commercial buildings – beams as the primary load-bearing support and joists as secondary support for floors and the roof.