why do ineed to install a header when i install french doors and take out the window that is 72" with

Headers 101:

The header is the structural part that carries the load of all the weight of the house that is above your door or window. A typical load bearing wall has solid framing studs every 16". The 16" spacing is short enough so that the load above it will be fully supported. Any opening that is larger than this 16" will need a header. Trying to build without a header will result in either immediate failure or a slow and steady deformation of all the structure above the opening. Don't be fooled if you open up a section of a load bearing wall and it doesn't fall down immediately - it will soon fail.

Headers are sized to carry the load of each different opening. The larger dimension (in height) that the header is, the more load it can carry. Typically headers are 4x8, 4x10 or 4x12 but 4x6 can work for small openings. The headers are supported by jack studs that transfer the load on the header downwards to more solid framing in the floor structure.

That's the basics. Since you are asking I would strongly recommend that you hire a pro to do this or work with someone who is familiar with residential house structure. The exception to this in your case would be if the new door can fit within the space of the old window. As long as the door will fit without widening the rough opening left by the window, you can install the door without any major structural changes. You will still need a header, but in this case it's the same header that is currently supporting the window.

This message has been edited. Last edited by: Jaybee,

thanks jaybee .I will let the pros handle this project