Most skylight installations on commercial buildings require a steel curb to mount the skylight on. Steel curbs require an insulation to be installed around it to prevent condensation. In some cases a wooden box curb can also be used. The most commonly used skylight for flat roofs is the domed curb mounted type.
Curb mounted skylights are more secure than flush mounted skylights. However in sloped applications these are quite common. However there are premium type skylights that can be used such as the Velux type for both sloped and flat roof applications.