Multiply the load per square foot by the roof's entire area. If a roof is 10 feet by 20 feet, the total area is 200 square feet, multiplied by 40 pounds per square foot = 8,000 pounds. This figure shows the overall weight that your roof must support. A beam that can support this much weight will be strong enough to support your porch. The best way to find out how big a beam should be is to look at other beams in buildings like houses or churches and calculate their size from there. For example, if the other beams are 2x4s, then you would need 2x4s too. Beams are usually specified by their diameter, which is found by multiplying the height of the beam by 4. Thus, a 2x6 beam is 6 inches wide, and a 4x4 beam is 16 inches wide. Beams are usually cut from single pieces of lumber and therefore have regular dimensions. However, people sometimes make their own beams using double-duty framing members like 2x4s or 2x6s. These parts can be used for the top and bottom of the beam as well as for side supports. If you're not sure how big to make your beam, we recommend using a formula called "rule of thumb". The rule of thumb is simply another name for an old tool known as a "beam calculator".