Understanding how loads are transferred through a structure and act on structural members is the first step to sizing headers and beams. The beam must be strong enough so it doesn't break and stiff enough so that it doesn't deflect excessively under the load, however, the process for sizing these structural elements can be complicated if you are not an engineer. Here is a simplified approach that
. will help you specify the appropriate material for many applications. Add all the loads acting on the header or beam, translate this load into terms of how much load each lineal foot of header or beam will be. For more you can go to http://chestofbooks.com/architecture/American-House-Carpenter/126-Safe-Load-At-Any-Point.html
