I want to install a closet door under my stairway in my basement. The stairway is in the center of the basement and the main support beam is split. There is neither a support beam nor a header currently where i want to separate the 2x6 studs and install this door. There is only a double top plate over this approximately 4 foot span. I pieced together a header using osb and 2x10s. Now im realizing there is not enough space for both the header and the door. Since there was never a header there to begin with am i ok trimming the height of the header about 3 inches to make my door fit? Or does the door necessitate a full 2x10 header?
I am assuming that the main support beam was properly supported on either side of the staircase before you began this project. If not, you have a bigger problem that may need an engineer, or at least a serious framing contractor. If is is properly supported, read on.
A simple closet door by itself in a partition wall (non-loadbearing) does not need a true structural header. Headers are used to carry and transfer weight where a door interrupts a load bearing structure.
If there was no header in the wall before and it was sound, the only weight being carried is the small amount of framing over the door and the surface covering, such as a small area of drywall. The top plates should be enough to carry this weight.
If you added a 2x10 inch header, you have given it more support than it really needs and trimming 3 inches off that header should be no problem.
If you added the header yourself to replace the strength of the wall when you removed the studs from a 4 ft wall, yes you can easily.
The header for a 4 ft wall section only needs to be a 2X6 at best to carry the load of a staircase, although a sketch showing the locations of the original configuration would certainly help.
The stairs where, most likely the only thing the wall was supporting if that. Typically the stairs are supported by the floor level that the stair head is attached to, then a short wall is added mid span to reduce deflection, if there are no walls on either side. If there are walls, which sounds like there is for the closet, there is no need for an intermediate wall for mid support. Of course the stringer are attached to these wall for added support.
There should be short wall there to keep items in the closet from "getting lost" at the very narrow end where the back of the closet would be. A person would be on their hands and knees to get what may go back there.