Headers are double 2 by 4 boards that get installed above a doorway to help absorb some of the weight from above. Headers can also be used above windows for the same purpose.
How to Build Headers
If you are building headers for above the door you must remember to make the header the same thickness as the door frame. You can use plywood or drywall in between the header boards to achieve this.
The span of the door and if the door is in a load bearing wall will help you decide the size of the headers. You have to make sure that you have a species of wood that is at least as strong as number 2 Douglas fir for a load bearing wall.
For example, if your door span is 48 inches you would have to have 2-2x4s for it to be an adequate load bearing header. If the door span is greater than this your headers size will increase as well.
It’s important to keep in mind that when you calculate the load bearing weight that is going to be applied to the header that you account for changing conditions like snow that can add a lot of extra weight to the headers.