The header for a door is much like a bridge transferring the weight above it to the floor and foundation below.
Header above sliding glass door.
For header spans see tables r502 5 1 and r502 5 2.
A 36 door has a rough opening width of 38 so your header is cut at 41.
However all the header sizing tables i have seen require 2 2x10 for this size opening.
A typical header width with single jack studs is cut 3 larger than the rough opening.
Window and door header requirements.
The existing header is 2 2x8 and is actually long enough for the new door.
We are installing a new 6 sliding glass door to replace an existing window.
Calculating window and door header requirements is complicated because they depend on a variety of factors including the size of the opening the weight of everything above it snow loads if it s an interior or exterior wall and more.
So if you re on the top floor you can use 2 2x4 s unless the building is 36 wide in which case you ll need 2 2x6 s.
The longer the distance a header spans and the heavier the load it supports the more substantial it needs to be.
So for piece of mind i am leaning toward replacing the header to avoid future problems.
Also be aware double or triple jack studs may be required based on the opening width and the load the header carries.
The header is usually made out of dimensional lumber installed on its edge.