Horizontal Supports
Horizontal supports carry the loads of your floors, ceilings and roof system.
The parts that make up your horizontal support system include: Joists, Beams, Headers, Trusts and Rafters, and decking used for flooring and roofing.
Joists are used to span the distance between two vertical supports.
They are usually 16 foot long 2×12″ lumber that will require a mid-span steel beam to span the full width of a building but some contractors may use engineered I-beams that can span the full length without a center beam.
Joists are laid out 16 inches on center and the studs used in walls above should be located directly above the joist so that loads will pass through correctly.
On top of the joists that are used for floors Plywood Subfloor is installed. Subfloor is 3/4 inch plywood with a tongue and grove on the edges so they will fit together securely. This thick plywood will distribute loads of furniture and other items evenly across the joists.
When you need to put a hole through the subfloor and joist system to provide for stairs you will need to support the ends of the cut joists with headers. Today architects like to use a glue laminated wood product for the ledger that is fastened to the joists with steel hangers.
Larger openings may require a beam and post system.
The Attic
On the top most floor of the building you will have standard floor joists but the ceiling will be part of the attic and roof system.
If you have installed a premade truss system the bottom of the truss will act as the nailing area for your ceiling drywall.
If you stick build your attic / roof system you will use smaller conventional 2×6 or larger lumber to create a ceiling joist / rafter system. The load of drywall used for the ceiling can be heavy but because you will not have living area in the attic smaller lumber is used to save money. Even with lighter loads 2x4s are not enough unless they are part of a truss system.
There are many products that can be used for both the Joists and the Beams and the selection is made by an architect who will calculates loads and the materials that need to be used.
Final Note
Horizontal Supports are designed to transfer all floor loads out to the perimeter stud walls. The studwalls on the outside of the building transfer that load down into the foundation and into the ground.
It is always important to remember the direction the load will travel and where its final destination is.
Remember that every ounce of weight of every shingle on your roof is supported by your foundation and footings.
Design your support systems to get that load to the footings efficiently.