How do I draw moment diagrams?

Snow is 20lbs per cubic foot and last night it snowed 6in. The roof only holds the snow (ignore the weight of the beam and other loads for this thought-exercise). Horizontal beams, simply supported, 30ft long, 10’ O.C..  What is the maximum bending moment in the beam? Click here to watch my video answer.--Michael Ermann, Amber Book