Legend

 Fnet = net force m = mass a = acceleration Fg = force of gravity FN = normal force Ff = force of friction
The roof of this building is constructed as a strongly tilted inclined plane to allow snow to slide off rather than pile up and load the structure.

Inclined Plane

Although the force of gravity acts straight down, when an object is supported by an inclined plane, components of its weight exist both parallel () and perpendicular () to the surface.   It is only possible for motion to take place on the incline at a inclined plane, and it is that drives that motion.  influences friction on an inclined plane.  The angle between the line-of-action of the weight vector (Fg) and the normal to the incline equals the incline angle.   Consequently,

Fg
sin pushes the object down the incline, and Fg cos pushes it into the surface.   The Second Law applies independently in each perpendicular direction.
 As shown in the free-body diagram, there is a force that will accelerate the object down the slope.  There is no acceleration perpendicular to the slope; net = maperpendicular =0, and must be balanced by an equal and oppositely directed reaction force FN.
 Friction Interactive Lab