All About Wood Floor Framing and Construction
If you are renovating an older home, you may peel back carpet to reveal wood planks and decide to leave this structural floor exposed. Concrete and wood are the most widely used floor structures. Typically wood joists are covered in plywood. Concrete floors, such as a garage floor, may be painted and new concrete can be dyed.
A platform-framed wood floor is the most common method of construction above ground-floor level. The floor is constructed using wood joists resting on double plates. The method of securing these in place is dependent on a floor’s age and local building codes.
Floor joists are supported by exterior walls (the interior section in cavity walls) as well as interior load-bearing walls. Smaller sleeper walls are often used to add support below suspended wooden floors at ground level. The floor surface is constructed by laying wooden boards or chipboard sheets across the joists.
Since the 1930s, the most common type of floor framing in this country has been platform framing, also called "western" framing. Joists, studs, rafters, and plates are the members that are used to construct platform framing. Each floor is constructed as its own unit, helping to prevent fire from spreading between floors. Wall sections for each floor are also constructed as one-story units. Settling does occur after a house is framed, but most shrinkage occurs uniformly over a structure. Typical construction methods for platform framing are shown below.
Platform frames are started with a sill plate attached to the foundation, and joists run perpendicular from the sill plate. A joist header attaches to the end of each joist. The studs are then attached to the joists, running the ceiling height of the floor. The subflooring is installed on top of the joists. A sole plate is then installed on top of the subflooring material.
The details in upper floor framing are similar to the ground floor. No firestopping is required, as the construction method provides a joist hanger, which is a built-in firestopping and structural system. The height of the studs for each floor are determined by the desired ceiling height for that particular floor.
Introduced during the mid-1800s, balloon framing is no longer a popular framing system. Balloon framing uses the same type of members as platform framing, but with more substantial sized material for studs. Studs in a balloon frame run continuously from the sill all the way to the rafter. The second floor joists are supported on a ribbon (see below) instead of a joist header. Firestopping is added to the spaces between the studs, which provides a space for mechanical system installation. Typical construction methods for balloon framing are shown below.
The foundation for a balloon frame is constructed the same as in the platform framing method, shown above. There is a sill plate attached to the foundation, and joists run perpendicular from the sill plate. The studs are then attached to the joists, and run the entire height of the house. The subflooring is installed on top of the joists.
The details of upper floor construction are different in balloon framing. Each floor's joists are run off of the same stud members that continue through the height of the house. In order to hang the joists, ribbon members are installed to the studs. This type of construction provides great stability. Firestopping is included in the stud bays.