Architecture is a three-dimensional discipline. However, architects, engineers and contractors rely on drawings to depict building forms. In order to surmount the two-dimensional nature of drawings, especially orthogonal drawings, such as plans, sections and elevations, architects use shading and shadow. Shading and shadow are conventions appropriated from art drawing during the Beaux-Arts period, and they are highly effective at giving an orthogonal drawing depth. Shading is used to model form, and shadow provides the depth of elements shown in light.
Find the depth of elements represented in the orthogonal drawings. You will need to look at plans and sections of the view you are to shadow. Measuring with the architectural scale, note the depth of building elements on the facade or section on an overlay or lightly on the orthogonal drawing.
Draw 45-degree diagonal lines in one direction from the profile lines of any building elements that have depth in the facade, i.e., any elements that recede into or protrude from the elevation. For example, windows, pilasters and pediments will all have depth. You should choose one direction for the diagonals to be drawn. Most front elevations will cast a shadow from top left to bottom right to show depth. So, draw the diagonals from top left to bottom right at 45 degrees. Although this is not a formal convention, most elevation shadows are drawn in this direction.
Draw the diagonals from top left to bottom right on the other elevations besides the front or model the angle of the sun on the other facades, relative to the initial position on the front. For the latter option, draw the shadows from top right to bottom left on the facade to the left of the front facade, and both of the other elevations will be completely in shadow.
Mark the terminus of the shadow on each diagonal line. The 45-degree line should measure exactly the depth of the corresponding feature of the facade. So, a roof eave that steps out two feet from a building wall will have a 45-degree line extending from its profile measuring 2 feet at the corresponding architectural scale.
Connect the endpoints of the 45 degree lines and shade in the areas above the shadow line boundaries. Draw lightly initially and add heavier values as you become confident about the appropriate shadow value.