Framing Types old and new
There are two common styles of wood framing: balloon and platform. The main difference between balloon and platform framing is apparent at the floor lines. The balloon wall studs extend from the sill of the foundation all the way to the top plate of the second floor. The platform framed wall, on the other hand, is independent for each floor.
Balloon framing is an older method of wood framing that utilizes long, continuous framing members (studs) that run from sill to eaves, with intermediate floor platforms nailed to them. Once popular when long lumber was plentiful, balloon framing has been largely replaced by platform framing. However, balloon framing is growing in popularity again in light-gauge, steel stud construction. For light-gauge steel, long framing members are not as much of an issue. Some electricians prefer working in balloon frame buildings because the lack of fire blocking makes it much easier to add circuits.
Home inspectors should be able to explain that, in balloon framing, there exist chases for fire to quickly travel from floor to floor. This hazard can be mitigated by the use of firestops at each floor level, but firestops can't always be confirmed by home inspectors. Balloon framing has been outlawed by building codes in many areas because of the fire danger that it poses. Again, this can be mitigated by adding firestops.
The home inspector might notice a down-slope in the floor toward central walls caused by the differential shrinkage of the wood framing members at the perimeter.
In platform framing, the joists comprise any number of individual floors or platforms that wall framing components are constructed on top of--hence, the term platform framing.
Platform framing is the most common method of frame construction. The floor, or platform, is made up of joists that sit on supporting walls, beams or girders and covered with a plywood or OSB sub-floor. In the past, 1x planks set at 45 degrees to the joists were used for the sub-floor.
Floor joists can be engineered lumber trusses or I-beams that have increased rigidity and longer spans, with the added benefit of conserving natural resources. They allow easier access for runs of plumbing, HVAC, etc.www.nachi.org