Yet it is not uncommon to find bridging not properly installed or missing all together.  Why does this happen?  There are a couple of reasons.

1. When cross bridging is used, wood or metal, the upper portion of the bridge is nailed into the top of the floor joist if it is metal and the upper top of the floor joist if it is wood before the floor or sub-floor is installed on the joists.  Attaching the bottom portion of  the cross bridging should be done after the flooring is installed.  Because the cross bridging is a function of the rough-in carpenters they have usually left the site before the finished flooring is installed.

It is not unusual, especially in new homes, to find that the cross bridging has not been nailed to the bottom of the floor joists.

2. Bridging is removed to allow for the installation of duct work, dryer vent pipes, plumbing drains, water pipes, and lighting.

Although not as good as solid or cross bridging, a 2 X 4 supporting the duct work would provide more support and load sharing than a light piece of metal.

Bridging should be installed between floor joists in both indoor and outdoor applications including decks.

 A common mistake when installing solid bridging is to offset the bridging.  The reason that this is done is to make it easier to nail the bridging to the floor joists.  However, it is not the nails that truly provide the transfer of the load and prevent the twisting and warping of the floor joist.  It is the strength of the bridging material itself.

When installing cross bridging, the bottoms and tops of the cross bridging lumber should be directly opposite themselves.