Sill and Parapet Flashing

The result of poor flashings is buildings with stains under windows and flashings, and at worst case with water penetration.

In Impact of Sealant on the Water Shedding Performance of Metal Flashing

By: Afarin Maleki, Ramani Ramakrishnan and Hitesh Doshi

Department of Architectural Science, Ryerson University, Toronto, Ontario

They found

"Profiles with horizontal projection of 15 mm or less have efficiencies less than 50%. Good detail therefore should have horizontal projections more than 15 mm.

..metal flashing, to perform efficiently, must have horizontal projections of 25 mm or more and the drip-edge angle must be between 30˚ and 60˚.

The application of sealant can reduce the efficiency of the flashing. Where sealant is to be applied behind the flashing, the profiles having a drip-edge length of 30 mm or greater can be expected to perform more efficiently.

In summary, metal flashing without sealant, must have a horizontal projection of 25 mm to shed the water efficiently. If sealant is applied, metal flashing must have a drip length of 30 mm to shed water efficiently. The drip angle, however, is not a dominant parameter. As long as the horizontal projection is 25 mm and the drip length is 30 mm or more, the flashing sheds water efficiently."

Source: http://www.buildingenvelopeforum.com/watershedding.htm
In: Building Envelope Forum
Published by: York Communications/MMPI

This finding indicates that in general architects who design flashings to be small for initial appearance sake may end up with buildings that may not wear well, and are not durable.