FLOOR & ROOF STRUCTURE
Floor & Roof structure INTRO:
We ask a lot of the floor and roof systems of our buildings. They handle all the loads we impose on them with our activities and possessions, and all the loads nature sends their way. The floors and roofs of a home use a lot of structural material to handle these loads, and therefore represent a good deal of the resources that go into making a building.
The structural elements of our floors and roofs are typically hidden behind finishes of many kinds, so we are often unaware of how they are made and from what materials. There are not as many choices as with walls and foundations, but the choices are important ones as they represent a lot of material and cost and play a structurally significant role.
Though floors and roofs will have very different sheathings and finishes, the materials used to create them are often very similar and are combined in this chapter to avoid repetition in addressing the various options.
Building Science Basics for floor structures
Structurally, a floor transfers live and dead loads from the building, the occupants and the furnishings to the foundation or the ground. The floor system will often play a role in providing structural support to the walls and/or foundation by bracing these vertical elements and providing diaphragm resistance to racking and twisting.
Floor structures can be divided into two basic categories
Many floor designs are intended to transfer their loads directly to the ground. While the floor itself will typically be separated from the ground by a vapor control layer, a thermal control layer and compacted fill, there is no open air space under a grade-based floor. The barriers, insulation and fill used to create grade-based floors will vary depending on climate and local codes and conventions. The focus in this chapter will be on the material used to create the finished floor and not the layers beneath or the surface finishes above, unless the material itself can be the finished floor.
Grade-based floors are the main level floor or the basement floor. If there are multiple floors, the higher floors will be suspended floors of some type.
These systems create a floor that is suspended between the exterior walls and may also bear on partition walls and leave open, inhabitable space beneath and above. Suspended floors will usually have a ceiling finish on the bottom side and a floor finish on the top side. They are not typically insulated as the entire suspended floor is within the insulated building enclosure.
Suspended floors on pier foundations are the exception to this rule, as they are exposed to the exterior and must be insulated to the same degree as the walls of a building.
Mechanical services like plumbing, wiring and ductwork for heating and ventilation are often run in suspended floors.
In residential, low-rise construction both types of floors are viable, and multi-story buildings can combine the two.
Floor structural properties for grade-based floors involve compressive strengths. Fully supported by the ground below, a grade-based floor only needs to resist compression or crushing from the loads imposed on it. Many materials offer the relatively low compressive strengths required to be a grade-based floor.
Suspended floors are typically composed of floor joists, which are structural members carrying the floor loads to the exterior — and sometimes interior — walls. These joists are spaced at regular intervals and are often tied to one another with blocking or bridging at regular intervals. Flooring and ceiling finishes can also provide structural integrity to the system.
Suspended floors must resist bending or deflection when loads are imposed. The amount of structural strength required depends on the spans between points of support. The longer the span, the more strength will be required to resist bending under load. A home can be designed so that the exterior walls are the only supports for the floor system, or interior walls and/or post and beam systems can be used to shorten spans. The depth of joist required gets higher as spans get longer and the spacing between joists may get smaller.
Building codes include span charts that specify the loads a particular floor system must be able to withstand, and offer correlated sizes for floor joists. For proprietary floor joist systems, manufacturers will provide span charts to meet code requirements.
Floor thermal properties are different for grade-based floors than for suspended floors on pier foundations. Grade-based floors are not exposed to outdoor temperatures or wind and the ground below is a constant and fairly moderate temperature; exposed suspended floors are exposed to conditions ranging from extreme heat to extreme cold, as well as wind and varying levels of humidity. The insulation materials and strategies will be very different for these approaches, as will the air and vapor control layers. All the options for insulating and sealing an exposed, suspended floor are covered in Chapter 4: Sheathing and Cladding Materials.
Building Science Basics for Roof structures
A roof structure provides a platform for a weather-resistant sheathing on the outside and a ceiling finish on the inside. A roof must be strong enough to resist the considerable loads of wind, rain and snow and effectively transfer those loads into the walls and foundation. While the roof sheathing provides the protection against water penetration, the roof structure must supply the geometry, support and capacity for the sheathing to be effective.
An almost endless variety of roof shapes are possible, and the structure of the roof will vary depending on the shape chosen. In some scenarios the roof rafters (the sloped members that define the shape of the roof) will act alone, while in other designs they act in conjunction with collar ties or ceiling joists as complementary structural elements.
Roof structural properties are similar to those of suspended floors. Individual rafters are spaced at regular intervals, and are sized to able to handle all expected loads for the given span between points of support. Joist sizing is determined by loads, spans and geometry, and can be found in building codes or supplied by manufacturers of rafter products.
Fasteners, hangers and bracing
To be structurally sound, all suspended floor and roof systems require particular types of fasteners, hangers and/or bracing. We will not address these components in this book, as they will vary between systems and between different designs and installations, but do not change performance parameters significantly.