Floor joists often require reinforcement either by building code requirements, or to solve a subfloor framing issue. Floor joists (and floor trusses) make up most of the floor system of a structure, along with band joists, sill plates, and subfloor.
Floor joists are sized to the job they perform and are spaced evenly across a foundation to provide support for the entire building.
Today, we will discuss why sometimes floor joists need to be reinforced and the methods of reinforcing floor joists.
What Is a Floor Joist (I-beam/Floor Truss)
A floor joist is a horizontal board placed on its edge used to provide structural support for a subfloor. Floor joists are limited in their structural span by the species of wood, as well as the width and thickness of the board. Floor joists can be overlapped, sistered, and placed closer together to increase the overall strength of the floor system. For clarity, floor trusses perform the same function but are constructed from many individual components, whereas a floor joist can be made from one board.
Floor joists can also be constructed from I-beams, GluLams, LVLs (laminated veneer lumber), and engineered trusses. Here is a description of common floor joists to help discern which type you have:
Standard 2” x 8”-12” lumber is included in this category. If the floor joists consist of a single board, or two boards nailed together it is considered a solid lumber joist.
I-beams are much like the steel versions we’ve seen used to build skyscrapers. In residential construction, these I-beams are formed from oriented strand board (OSB), which reside in a groove cut into a 2” x 3”-4” board on both sides, forming a profile that looks like a capital “I”. In this configuration, an I-beam can be used when solid lumber is too narrow. I-beams can be virtually any size, making them very popular with today’s modern architecture.
Laminated veneer lumber (LVL) is a manufactured board, built from layers of laminated wood and resin. LVLs are used somewhere in virtually all new construction to build floor joists, beams, and girders because they are custom-made for the job. LVLs are extremely stable and will not bow or twist due to the grain crossing used in manufacturing. LVLs are also usually coated in wax or other water repellent to prevent water damage during construction.
Why Would a Floor Joist Require Reinforcement?
As mentioned previously, floor joists are sized to the job based on local building codes and span tables. These authorities have determined through scientific testing which species and sizes work best in a given application. However, these requirements evolve so what passed inspection in the past may not do so again. Additionally, as home designs become larger, these floor joists must be assembled in such a way as to provide lateral support for the structure.
What Are the Different Types Of Floor Joist Reinforcement?
Floor joists are usually made from 2” x 8”-12” spruce lumber and along with pressure treated sill plates, establish the height of the floor. HVAC ductwork and other mechanical parts require space in the crawlspace or basement for routing, so depending on the design and slope of the lot, floor joists can be interconnected in several ways. Beams, boxes, and other structural support members can be constructed using floor joists. Floor joists can also span a partial distance within a structure, or the entire span.
Because floor joists are so versatile, they can be attached and connected together using joinery, mechanical fasteners (like nail or screws), or both. Floor joists can also be connected to form a beam or girder, providing support where direct bearing is not possible. For example, certain walls within a structure will be weight bearing, meaning they carry the weight of the building. Weight bearing walls require support all the way to the ground, so when a post, pylon, or perimeter wall is unavailable, beams and girders can be used to transfer this force to the ground.
Blocking refers to the addition of wooden blocks to reinforce a butt joint. Butt joints occur when two or more floor joists are joined end to end. The joint can be parallel or perpendicular, but unless the joint is reinforced mechanical leverage can unintentionally pull the two apart. For this reason, solid blocking is cut to the inside dimension between two adjacent floor joists and face nailed to both joists. Using this method also provides for additional connection points to the subfloor between the joists, adding strength to the whole system.
Before I-beams and engineered floor trusses were widely available, solid lumber floor joists were standard. However, being solid wood meant that these joists were subject to grain pattern and moisture induced warping. In fact, the wider the floor joist, the greater the chances the joist will twist. Bridging involves the use of small dimension lumber,(often 1”x material) to install angle braces between the joists to resist this movement.
Sistering a floor joist describes joining two or more floor joists face to face to add strength and rigidity. For example, beams and girders are built using the sistering technique. Building codes however, usually dictate that a pier must provide direct bearing support under any sister joints. Professionals use appropriate sized lumber to avoid installing more piers than necessary.
Drop girders provide the most additional support to floor joists because they provide a mounting location for every joist and provide consistent bearing from joist to joist. By adding support to every joist simultaneously, a drop girder can adjust an entire floor system at once. Drop girders are usually built from the same lumber as the joists, but if they are interior floor joists they can also be built with laminated veneer lumber. Exterior drop girders, such as would be used in a deck project, will be built from pressure treated or naturally rot resistant lumber.
Drop girders are installed from below the joist system and usually incorporate one or more hydraulic bottle jacks. These jacks are often capable of lifting sixty tons or more and will support the drop girder until a pier can be installed for permanent support. For example, if a home is sagging in the center it is often due to a lack of bearing support, so a drop girder can be installed perpendicular to the joists to flatten the floor. If the process is done slowly enough (about ¼” per week), much of the collateral damage, like cracked drywall, can be avoided.
Do I Need Floor Joist Reinforcement In a New Home?
Floor joists used in the construction of a new home should include any necessary bracing as part of adherence to local building codes. Normally, this will require solid blocking every 48” between the first four joists on opposing ends of the building. For example, if the building is a rectangle, the two short sides will be reinforced. In general, any floor joist more than 2” wide (like an I-beam or floor truss) and 12” tall will require this blocking to prevent twisting and reduce noisy floors.
However, some municipalities do not require adherence to any building code, so additional blocking may not be present. If this is the case, blocking can be installed retroactively to prevent future sagging floors and joints. In areas where no adherence to building codes is required, professionals strongly recommend hiring a qualified home inspector prior to closing. In most situations, any additional structural reinforcement required will be noted, allowing the buyer to negotiate the expense of adding any support with the seller.