Floor joists provide structural support for floors in buildings. They need to be strong enough to carry the weight of people, furniture, and anything else placed on the floor above. Bridging is an important component that helps stabilize floor joists and prevents them from twisting or moving laterally.
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What is bridging?
Bridging refers to short, diagonal pieces of lumber that are installed between adjacent floor joists. Bridging is typically made from the same size lumber as the joists themselves, often 2×8 or 2×10 boards. The purpose of bridging is to brace the joists and keep them properly spaced apart from each other.
When installed correctly, bridging forms an “X” pattern between joists. The ends of the bridging boards are toenailed or strapped to the joists on either side. This creates a rigid connection that prevents the joists from moving in a side-to-side direction.
How does bridging work?
Bridging works by transferring loads laterally between joists. When weight is applied to the floor above, it causes the joists to bend slightly and push outward. Bridging resists this force and shares the load across multiple joists.
Bridging also stiffens the joists and helps them act as a unified system. This allows them to better handle loads without excessive deflection or bounce. Properly installed bridging makes the floor feel significantly more solid and prevents squeaking noises.
Why is bridging important for floor joists?
There are several key reasons why bridging is an important component in floor framing:
- Prevents joists from twisting or buckling under load
- Maintains proper joist spacing for consistent support
- Minimizes uneven floors and sagging over time
- Stiffens joists and reduces floor bounce
- Shares loads laterally across multiple joists
- Reduces floor squeaks and noises
In general, bridging provides stability and helps the joists work together as a unified system. This results in a sturdier floor that will provide many years of reliable service.
Bridging requirements
Most building codes and standards contain specifications for where and how bridging must be installed in a floor system. The general requirements are:
Bridging spacing
Bridging is typically required at intervals between 4 to 8 feet along the joist length. The exact spacing may depend on factors like:
- Joist size and grade
- Joist spacing
- Load requirements
- Floor span
For example, a common bridging spacing for a room with 16″ on-center floor joists would be every 8 feet.
Number of bridging rows
Most floors require a minimum of two rows of bridging. One row near the center and one row near the supports. More rows may be needed for longer spans over 12 feet.
Extending to supports
The end joists next to all supports must be bridged. This includes walls, beams, and internal supports. Extending the bridging to the supports provides maximum stability.
Notching bridging lumber
The bridging boards must be notched to fit around any utilities or ventilation running through the joist bays. This maintains continuity of the bridging.
When can bridging be omitted?
There are some cases where building codes allow omitting bridging in floor joist construction:
- Joist spans less than 8 feet
- Rooms with very light loading conditions
- Certain engineered wood products that resist lateral movement
However, it is usually recommended to install bridging even when not strictly required. Bridging is inexpensive but provides valuable strength and stiffness.
Materials for bridging
Floor joist bridging is typically made from wood, metal, or engineered wood products:
Wood
Solid sawn lumber is most commonly used, such as 2x pine boards. These match the depth of the joists and install easily.
Metal
Light gauge steel strapping can also be used. Metal bridging offers rigidity but requires special fasteners.
Engineered wood
Products like laminated veneer lumber (LVL) can make effective bridging. LVL is strong and dimensionally stable.
| Bridging Material | Pros | Cons |
|---|---|---|
| Solid sawn lumber |
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| Metal strapping |
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| Engineered wood |
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Installing bridging
Proper installation techniques are important when fitting bridging between floor joists. Here are some tips:
Cut bridging to length
Bridging lumber should be cut to neatly fit between the joists. Allow 1/4″ clearance at each end for expansion. The length will match the on-center spacing of the joists.
Notch for obstructions
If there are any ducts, wires, or pipes running through the joist bay, notch the bridging boards to maintain continuity.
Toenail ends to joists
Use 10d or 16d galvanized nails and toenail into the adjacent joists at a 30-45 degree angle. This creates a strong connection.
Install both rows
Fully bridge both designated rows before moving on. This helps keep everything square.
Stagger overlaps
When using multiple bridging boards, stagger the butt joint overlaps so they don’t all occur in one place.
Bridge to supports
Make sure the first and last joist bays are bridged next to the supports or walls.
Special considerations
Certain situations introduce complications when bridging floor joists. Here are some special cases:
Above basements
It can be challenging to bridge over open basement spaces. Using long diagonal bridging is an option. Or, installing permanent lateral bracing along the basement ceiling.
Insulation
Insulation in the joist cavities can get in the way during bridging. Either the insulation needs to be split and replaced around the bridging, or thin rigid foam boards can be used.
HVAC ducts
Ducts for heating and air conditioning that run perpendicular to the joists interrupt the bridging lines. Notching the ends of the bridging to fit around ducts maintains the needed continuity.
Plumbing pipes
Water supply pipes and drain lines also pass through joist spaces. Plan notches in the bridging boards ahead of time to accommodate any plumbing.
Irregular joist spacing
Bridging is more difficult if the joists are not at regular intervals. Using scab lumber to box the joists to a consistent spacing helps.
Cost to install bridging
Bridging only adds a small amount to the total cost of a floor framing project. The materials are relatively inexpensive compared to the joists themselves.
A typical price for bridging labor and materials is around $0.50 to $1 per square foot of floor space. So for a 500 square foot room, expect $250 to $500 total. Having an experienced carpenter install the bridging helps ensure proper spacing and nail patterns.
Not bridging floors
It is usually not advisable to completely omit bridging from a floor system. Without bridging, floor performance will suffer in several ways:
- Joists can rotate and move out of position
- Excessive bounce and vertical deflection
- Inconsistent feeling floors, prone to sagging
- Joist creep under heavy loads
- Increased noise like creaking and popping
Neglecting bridging may save a small amount upfront, but often leads to problems down the road. It also violates code requirements in most cases.
Cost of retrofitting bridging
If a floor was originally built without proper bridging, it can be retrofitted after the fact. However, this involves working in the tight space between the joists.
Typical range for retrofitting bridging is $3 to $8 per square foot. So for a 500 square foot room expect $1,500 to $4,000 in labor and materials. Extensive finishes like drywall may also need patching.
Conclusion
Bridging is an important structural component in any floor system with joists. It stiffens the joists, resists lateral movement, shares loads, reduces bounce, and prevents twisting. By following the required spacing and spans outlined in building codes, bridging helps create a sturdy and quiet floor. Omitting bridging can lead to performance issues and may violate code. Incorporating proper bridging during construction is highly recommended for a durable and quality floor assembly.