How to Install Floor Joist Bridging
For anyone who has had a squeaking or bouncing floor, you may already understand the importance of floor joist bridging and how it is a critical component to the structural strength of a floor. Proper bridging distributes the load on the floor to other joists and over time prevents floor joists twisting and warping. Unfortunately it is common to find bridging improperly installed or missing.
Potential Causes for Incorrect Bridging
1. When cross bridging is used the upper portion of the bridge is nailed into the top of the floor joist, if it is metal and with wood cross bridging it is nailed to the side of the floor joist near the top edge before the floor or sub-floor is installed on the joists.
Attaching the bottom portion of the cross bridging to the joists should be done after the flooring is installed. Cross bridging is installed by the framing carpenters and they typically do not return after the finishes have been installed.
In new homes it is not unusual to find that the cross bridging has not been nailed to the bottom of the floor joists.
Incorrect installation of cross bridging
2. Bridging is removed to allow for the installation of duct work, dryer vents, plumbing drains, water pipes, and lighting.
Bridging removed for duct work
2 X 4 installed below duct work
Where solid bridging has been removed by subsequent trades, a 2 X 4 should be installed at the bottom edge of the joist to distribute the load and support duct work or piping. Metal straps would not provide enough structural support, especially if they are not fastened to the top edge of the joists.
3. Solid bridging is offset.
The reason the bridging is installed in this manner is to allow for faster installation, especially by less experienced carpenters. A common misconception is that the nails of each piece of bridging supports the joists, but the strength is in bridging material itself.
4. Bridging that is the wrong size or poor cuts.
When installing solid or cross bridging it is important to have the dimensions correct and the cuts straight and true. Poor fabrication of bridging can increase the chance of floor squeaks by not having the bridging material come in contact with the floor joists.
The table below provides the correct quantity of bridging relative to the span of the floor joist.
In order to nail the solid bridging into position without offsetting every other piece it is necessary to drive the nails in on an angle referred to as “toe nailing.”
As shown below, the first piece of solid bridging is toe nailed into the farthest floor joist. The toe nailing technique would have to be used on the first piece of solid bridging even if you were to offset the solid bridging pieces.
Two nails should be installed on opposite sides on the top and on the bottom of the bridging toe nailed to the first floor joist.
Two nails should be installed through the second floor joist into the end of the solid bridging - one at the bottom and one at the top.
Nailing first piece of solid bridging
Once the first piece is in place the next pieces are nailed on an angle through the floor joist and into the solid bridging.
Two nails should be applied on an angle through the second floor joist and into the end of the solid bridging. One on each side top and bottom.
Two nails should be applied through the third floor joist into the end of the solid bridging - one at the bottom and one at the top.
Nailing subsequent pieces of solid bridging
Toe nailing will have to be utilized to secure the solid bridging to the last floor joint, in the same manner as nailing the solid bridging to the first floor joist.
When installing cross bridging, the bottom of the first piece must line up with the bottom of the next piece of bridging. The same goes for the top of the cross bridging, as seen below.
Correct installation of cross bridging (top view)
For engineered floor joists, such as I-joists or trusses, you must follow the manufacturers’ specifications for installing bridging. Typically I-joists are braced with metal strap cross bridging and truss style joists use continuous 2x4 lumber as bracing. I-joists can also be braced with dimensional lumber installed horizontally at the top and bottom edge.
Floor Truss Bracing
If you are uncomfortable or unfamiliar with basic carpentry concepts you should hire or consult with a trained professional or contractor before attempting any repairs yourself.