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Structure of Residential Floors

Certified Building Code Official

 

The description of structure implicitly offers an account of what a system is made of: a configuration of items, a collection of inter-related components or services. The combinations of systems and materials used to build the main elements of our homes: roof, walls and floor are ofter referred to as your homes structure. These systems are many and varied and each has advantages and disadvantages depending on use and requirements of design.

The current Building Regulations require floors and roofs to be tied or strapped to the walls to maximise the stability and robustness of the structures. While this occurs naturally in frame structures, it must be considered at the design stage for masonry structures. New structures can be built with this requirement in mind, but many older buildings, mainly domestic, do not meet it.

Span Tables

Picture of sample span tables for floor joists. Any modification or erection of floor joists would require a building permit. Check with your local buidling departemnt for required plans and permits prior to starting construction or renovation.

Structural engineers are responsible for engineering design and analysis. Entry-level structural engineers may design the individual structural elements of a structure, for example the beams, columns, and floors of a building. More experienced engineers would be responsible for the structural design and integrity of an entire system, such as a building.

IMPORTANT! When using an engineered product it can only be used for the purpose it was designed for and installed according to design. Any modifications or design changes must be made by a qualified designer or engineer who will product a "detail" which is stamped.

Engineered Floor Trusses

Often refered to as "Silent Floors" engineered floor joists are becoming very popular in new construction. Floor joists constructed with traditional framing lumber (2 x 8 and 2 x 10) is prone to splitting, warping, shrinking, bowing and twisting, often resulting in floors that squeak and twisting.

IMPORTANT! When using an engineered product it can only be used for the purpose it was designed for and installed according to design. Any modifications or design changes must be made by a qualified designer or engineer who will product a "detail" which is stamped.

Engineered I-Joists were developed to replace dimensional lumber in floor systems. Their advantages are they are stronger and span further, are straighter and uniform in depth and come in any length. They are also less susceptible to shrinkage. Shrinkage in dimensional lumber can cause floor joists to warp and twist leading to squeaks in the floor system. The unique design of floor I-Joists also allow for holes to be cut through them in certain locations for plumbing pipes and mechanical ducts.

One of the common defects found during floor joist inspecitons are missing nails in joist hangers. Every hole in joist hanger requires a nail and the proper nails are required to be used. The most common hangar nail is stamped with a T on the head. It is also important that the right size hanger and type for joist be used.

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Definitions


dead load:  This is the weight of everything that is permanent such as the floor joists, walls, piping, ductwork, floor tile, etc.

live load:  This is the weight of everything that you add to the house or apartment when you move in.  Furniture, bookshelves, people, appliances, and of course, your computer and your aquarium(s).

safety factor:  Your floor was designed to support loads without collapsing using a "safety factor".  The "safety factor" in most structures is usually somewhere between 1.5 and 2.0.  So, if I tell you that your floor can "safely" support 1000 pounds then that also means that your floor might theoretically fail  when it receives a load of 1500 to 2000 pounds.

bearing wall:  This is a wall in your house or apartment that was designed to support the weight of the floor, wall, ceiling or roof.  (Most or all of the concrete or masonry block walls in your basement are bearing walls.)

partition wall:  This is a wall in your home that acts only to separate rooms.  While it might be able to support some load, it was not designed as a part of the structural system that carries the weight of your floor or roof down to the foundation.

floor joists:  These are typically 2 x 8's or 2 x 10's at 16 inches on center that support your floor. Each end of the joists are supported by bearing walls or beams.

subfloor:  The sheet of wood (usually plywood) that is nailed to the top of the floor joists to form the floor itself before carpeting or tile etc.

beams:  The beams act to support the floor joists.  These beams might be constructed of wide-flange steel beams (commonly and incorrectly called an I-beam) or they might be wood triple 2 x 10's, etc.

column:  A vertical post that supports the floor beams.  In a home this is usually a round hollow pipe.