16.  Why does stucco crack?  What are
      the worst kinds of cracks, and how
      should they be treated?

My father told me…..”Kid…there are only two kinds of stucco……stucco that has cracked and stucco that is going to crack”…in other words all stucco cracks.  That has been proven true in my almost 50 years of experience in the industry.  However I have rarely seen an envelope harmed by a crack. 
Stucco is a very strong, durable but brittle finish.  It is natural for wood frame buildings to move, shrink, expand and contract…and these movements will cause cracks.  However if the stucco is cement based finish coat and if the concealed barrier is intact and properly lapped….these cracks are not much of a concern.  If the finish is acrylic or painted, then sealing of these cracks becomes more of a priority.  Especially horizontal cracks are worrisome…because they intercept water running down the wall.  In the case of acrylic or painted stucco…all the water impacting the stucco finish runs down the wall and so can be turned to run in a horizontal crack….especially one that has opened significantly…not hairline.  The other cracks vertical or angular are not as much of a concern.
What causes the cracks….
Vertical cracks are usually caused by fast warming of the wall…..usually found on the east facing wall….especially if the sun can directly hit it in the early morning when the wall is still cool/cold.  Or the west wall where the hot afternoon sun can hit it.  Usually they are spaced around the stud locations…and may even be counted with studs in extreame cases.
Horizontal cracks…..this is usually caused by breaks in the framing…plywood sheets. Or some kind of framing defect or fault…on floor joist that don’t have a compression joint….or have squeezed shut the compression joint over the years.
Angle cracks…..these are usually found off the upper and lower corners of windows and doors…especially sliding doors.  The reason the thermal expansion contraction of the wall.  The wall gets warm….as the sun heats it….a dark colour can even be hot to the touch on  the sunny wall on a summer day….now touch the window…it is never hot…it is always cool….so this provides a black hole in the wall.  It creates uneven movement and the wall twists…torques..at that location….and the crack runs off the corner and out usually  6 inches to 3 feet.
There are also check cracks….these are small cracks that are in the form of a small line or check in the stucco.  This is from the hydration of the lime and cement in the stucco….causing shrinking…they do not expand nor cause any problems.
On stucco that is sealed….you must maintain the face seal….or the water may get in those cracks and build up moisture and cause rot….on conventional stucco they will rarely cause problems and are very unsightly to repair and are usually better left as they are.