Good roofing practice allows penetration movement independent of the roof. This requires a flexible seal at the penetration and proper attachment to the roof. With the complexity and assortment of roofing systems available today, a variety of designs is required to work with all conditions.A good flashing system must be versatile, economical, maintenance-free, simple to install and adapt to multiple penetrations and angle irons, yet have uniformity allowing fast, easy installation and specification.All components must be weather resistant, approved by the industry, and, above all, eliminate leaks.