Yes, ceiling joists are typically load-bearing. However, their primary function is to support the weight of the ceiling and loads applied to it, like lighting fixtures and ceiling fans.
Joists are the actual framing of the house, and they're very strong. You can hang really heavy weight weights from a ceiling joist with the right hardware—over 100 lbs. in many cases.
Look at the Direction of the Ceiling Joists
If the ceiling joists run perpendicular (90 degrees) to the wall, the wall is load-bearing, as it's bearing the weight of the ceiling joists. If the joists run parallel to the wall, the wall may not be load-bearing.
In general one does not cut ceiling joists holding up an existing ceiling without risk and consequence of partial or substantial ceiling collapse if not done with appropriate precaution and procedure.
Floor joists are horizontal structural members that span an open space, often between beams, which subsequently transfer the load to vertical structural members. These joists, part of the floor system, carry the weight of everything inside a room, including walls, furniture, appliances, and even people.
The loads carried by floor joists, ceiling joists, and rafters are transferred through their end points to supporting walls and beams. The ends of these members must be able to resist these loads without crushing.
When builders need to increase the load-bearing capacity of floor joists, they often use a process known as “sistering.” This process involves attaching a new joist alongside an existing joist to increase its strength.
Ceiling Joists are structural framing members that support anything attached or mounted to the ceiling, including drywall or other finishes, light fixtures, air vents, smoke alarms, security cameras, fire sprinkler systems, etc.
They will either rest on top of a beam or be connected to the side of a beam. In the case of ceiling joists, they help to transfer loads from the roof to vertical members, prevent walls from crumbling and also provide a structure to attach the ceiling to.
An attic ladder typically requires a space between two joists behind the drywall on the ceiling. It should be at least 25 inches by 55 inches.
The weight we place upon a floor by way of bathroom suites, beds, wardrobes etc, is known as the “imposed load”. It is again accepted that, for normal household requirements, the imposed load will not exceed 1.5kN/sq. m. Unless C24 timber is specified, the more generally used timbers are less expensive C16.
Load-bearing structure is the load taken by the wall and transferred to the soil and the framed structure refers to the load taken by the beams, and columns and transferred to the footing and foundation of the structure. I will share with you the difference between load bearing structure and framed structure .
The ends of each rafter or ceiling joist shall have not less than 11/2 inches (38 mm) of bearing on wood or metal and not less than 3 inches (76 mm) on masonry or concrete.
The 2x6 joist can be considered a really thick single ply I-beam web. It won't take much by way of stiffening by sistering with real plywood, with proper glue and fasteners (lots of staples), to make the span super strong, as in 10x stronger than a solid wood joist.
Ceilings are another matter, though, thanks to the direct downward pull of gravity. The average 2×4 ceiling joists can't safely hold more than around 15 to 20 pounds regardless of the hanging hardware you use. A 5-inch flower pot with the combined weight of soil, water, and a plant can easily reach that weight.
Blocking required every 4' - 6'. Blocking must be equally spaced in rows, maximum of 4 to 6 feet apart. WHY IS BLOCKING NEEDED? Solid wood blocking will help reduce up or down movement and/or twisting of joists.
In ceiling joists, they help to transfer loads from the roof to vertical members, prevent walls from crumbling and also provide a structure to attach the ceiling to. Floor joists are built to support the floor and any live/dead loads that will be placed on them.
How much weight ceiling joists hold depends on their size and spacing and what they're made of. Wooden joists can support 50 to 100 pounds per square foot. Steel joists are more common in commercial buildings and can handle higher loads. Joists are spaced at regular intervals of 16 or 24 inches apart.
The most direct load path would be to always align a stud (or studs if necessary) directly below each truss/joist.
An actual floor joist is only used on a two story structure and homes that have pier and beam foundations. If you have a one story home or building, you would refer to that particular joist as a ceiling joist or rafter because there is no floor to support, just a ceiling.
Sagging ceilings can be caused by a variety of factors including the age of the property, minimal glue and fixings at ceiling board installation, leaking roofs causing insulation inside the roof too be heavy, movement in the structure and foundation of the home and termite activity to the ceiling joists.
Most building codes, including the most used, the IRC, do not directly address sistering a joist, but they do address girders, which are similar.
Floor Joist Spacing Code Requirements
The maximum live load is 30 psf. For living areas, the maximum dead load is also 20 psf. However, the maximum live load is 40 psf. 'Dead load' refers to the long-term furniture, appliances and other fixtures that are expected to remain in the building over many years.