I have a load bearing wall in my house that a previous occupant decided to cut a hole in to make things feel more open. They did not add a header, much to my chagrin. I'm planning to put an exposed beam in to bear the load. What information will I need to size the beam? I've found many beam sizing calculators, but none of them seem to be able to work with hewn or sawn timbers; they all require the use of dimensional or engineered lumber.
It depends on what kind of wood you're using. Different woods have different strength characteristics. You may want to swing by the local library and grab a copy of a "Timberframing: Design, Construction, Finishing" by Ted Benson, which has tables of wood characteristics.
Note: If you live in a town where a permit is required to do the work you're planning, you're going to have to get sign-off by a structural engineer - in which case, you might as well ask the engineer what dimensions you need for the wood you're planning to use.
In order for calculations to mean anything, you need to know what the actual dimensions of a beam are. A beam that has a single spot where it is undersized will be considerably weaker than a uniform beam. So at best, you should assume the beam is no stronger than the minimum dimensions of the beam. The problem is, a hewed beam is non-uniform. So how rough is it? Exactly how non-uniform is it?
Worse, mechanics studies will tell you that a corners and sharp edges are stress concentrators. A strong beam will have continuous parallel fibers along its length. In fact, this is a one of the reasons why a glue-lam beam is strong. So what happens in a beam that is hewn? Broken fibers along the edge in tension will be points of possible future failure.
All of this means you want to use a hewn beam that is a bit oversized for the application. How much oversized is something that only a structural engineer will know. Therefore, take the advice from liantics, and have someone size the beam for you.
The rule of thumb framing carpenters use for headers is "one inch of (doubled or tripled 2 inch) header height for every foot of span". So an 8 foot opening would call for an 8 inch tall header. Also, the ends of the header must rest on posts (typically doubled 2x4's) which transfer the ceiling load to the foundation. The easiest way to build install the header is to: 1) build a temporary support wall a couple feet back from the header location. A 2x4 on the floor and ceiling with doubled vertical 2x4's every two feet should do. This structure should be toe-nailed together. 2) Buy an engineered beam (gluelam or equivalent) or construct a built up beam from dimensional lumber ( 8 foot long 2x8 in the example). Fit the first 2x into the opening (resting on temporary support blocks, and then sandwich a second 2x8 and glue and screw it to the first. Make sure the inevitable bow of the wood faces up. Add a third plank or scale up to 2 or three 2x10's if you want to overbuild the header. A 10 inch plank on edge is stiffer than an 8 inch plank on edge. More strength is always good. 3) Jack your new beam up into place. A hydraulic jack with doubled 2x4 post works well, and fit doubled studs under the ends. This step will also allow you to correct any sag which might have occurred. 4) Now remove the temporary wall and trim out your new header.
Be careful! Remember that gravity sucks and these chunks of wood are heavy and can cause injury when they fall. It is always best to get approval from you local structural engineer/permitting agency. Good luck.