IRC does not provide guidance distinguishing whether a member is a joist or a beam.
How do you determine which term to use - joist or beam?
Joists are generally considered to be members that are a nominal 2 inches maximum in thickness and placed not more than 24 inches on center.
There are many terms that differ from area to area but to keep it simple a joist is a type of beam but a beam is not a joist. A beam is a generic term that covers many structural elements. If you double up some joists you can call that a beam. Beams generally hold up several joists or other beams or point loads.
I know it when I see it!
A joist, in architecture and engineering, is one of the horizontal supporting members that run from wall to wall, wall to beam or beam to beam, to support a ceiling, roof (or floor).
It may be made of wood, steel or concrete. Typically a beam is bigger than a joist and thus is distinguished from a joist.
Joists will often be supported by beams. Joists support the sub-floor (floor deck) directly.
The wider the span between the supporting structures the deeper the joist will need to be if it is not to deflect under load.
Lateral support also increases its strength .
There are approved formulas for calculating the depth required, however, a rule of thumb for calculating the depth of a floor joist for a residential property is; half the span in feet plus two inches.
For example the joist depth required for a 14 foot span is 9 inches.
Engineered wood products like an I-Joist gain strength from depth of the floor or the height of each joist.
A common saying in this industry is deeper is cheaper referring to the lower quality cost effective joist 14 inches and above.
Also; A horizontal member in the framing of a floor, roof, or ceiling plane.
Beam;
Horizontal or inclined structural member spanning a distance between one or more supports, and carrying vertical loads across (transverse to) its longitudinal axis, as a girder, joist, purlin, or rafter.
Three basic types of beams are: (1) Simple span, supported at both ends, (2) Continuous, supported at more than two points, and (3) Cantilever, supported at one end with the other end overhanging and free.
Also, A horizontal, weight-supporting member of a structural frame.
:)
What Marcel said…
I’ll add; a beam may also be a joist, but a joist can not be a beam. :shock:
Joist a minute there Ben while I add ice to my Beam & Coke…
Brian, two fingers of Beam and one Coke.
And don’t forget to measure using the index finger and the little finger for the Beam. :mrgreen:
Sometimes a beam is referred to as a girder.
That’s right Peter,
Girder Truss used in Hip Roof framing, Girder Truss used to support steel Bar Joist. Etc. 
Good point.
Girder=usually refered to as a beam that supports floor joists or rafter joists.
Joist=framing members such as cieling, roof, and floor joists. There are usually more than one.
Beam=A framing or structural member that is oversized. Is typically bigger than a joist. Is used in “bearing” or “carrying” a load.
If you remove a Beam the joists may fall,but if you remove a joist the Beam will not.
Beam is the King and Joists are the pawns.
The Beam often determines where the joists will lie.
A structure can have one Beam ,but I never see just one joist.
Surely you joists Bob
Oh I thought he was talking Whiskey and pot.
Never mind.
So where is the Queen Bob.?:mrgreen:
Girders support beams, beams support joists and joists support decks.
And the Queen post is flanking the King post of the truss. :mrgreen:
Jeff said the joists support decks,so doesn’t every deck have 4 queens.
Below that is the Joker…:):)
Yup! the joker is usually under the deck with my luck. :mrgreen:
Framing members are determined by the architect and approved by an engineer.