A garage door strut is an extruded piece of metal to form a C-channel. And is cut slightly shorter than garage door width. It is used for reinforcing the garage door and to prevent the panels on the door from bending or sagging, as the opener pulls on the top panel to open the garage door. Struts will strengthen the door and help prevent damage from wind, however if you live in an area susceptible to high winds, reinforce with extra struts.

Most manufactures do not require a strut on any door up to 14ft (in width). Anything above 14ft must have at least one strut. The more struts you have on the garage door the stronger it will be. Any garage door that has an opener is required to have a strut on the top panel. The top panel is the weakest point. If you do not reinforce the door, it will damage the top section in a short period of time. The struts also help to prevent wind damage, the larger the door the more susceptible the door is to wind damage.