Ceiling Joists are structural framing members that are used to support anything attached or mounted to the ceiling, including drywall or other finishes, light fixtures, air vents, smoke alarms, security cameras, and fire sprinkler systems, etc. As like other types of joists, ceiling joists have to be designed to carry their own weight too.
View the Joist Span Tables
How to Determine Ceiling Joist Spans
On the attic level, the size and length of joists, used for framing the ceiling, depends on the design load (measured in lbs/ft2), which is the dead load (weight of the ceiling's building materials) plus the weight of items stored in the attic (the live load). Typically, span tables for these types of joists are calculated based on a combination of live load (20 lbs/ft2 plus a dead load of 10 lbs/ft2). See the ceiling joist span tables page for details.
On a typical two-story residential structure, the floor joists on the second level also serve as framing for the ceiling on the first level. The design of the second level floor joists needs to include the first floor's ceiling load requirements.
Ceiling Framing - Using a Beam to Break up the Span
If you have a ceiling span that is too long for the materials available, you can reduce the span by placing a beam between the long span, or by making use of interior load-bearing walls for vertical support. Note: If the interior wall has an opening, such as a doorway, a properly sized header should be installed above the opening.
If a beam is used to break up the span, it must be designed to carry the required ceiling/floor load of the structure. Joist hangers must also be used where the joists butt-attach to the beam.
Framing a Raised Ceiling
In gable-type roof framing, ceiling joists are positioned parallel to the rafters so that the base of the rafters can be attached to the ends of the joist. This helps prevent the weight of the rafters from pushing out the outside walls.
You can raise the height of a typically placed ceiling joist, so long as the height above the outer walls (A) does not exceed 1/3 the height from the top of the outer walls, to the ridge beam (B). See diagram below. As you raise the height of the ceiling joists, the rafter span needs to be adjusted at every step (1/6, 1/5, 1/4, 1/3). See the 2021 IRC - Rafter Span Adjustment Factor for specific details.
Framing a Ceiling
There are many ways to frame ceilings, just as there are for framing floors, walls, and roofs. For example, the diagram below shows how you can create a slant on the edge of a ceiling by adding some simple framing. This would be a good use for scrap material.
For building code information related to ceilings and rafters, check out the International Residential Code (IRC) website.
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