Tile Talk: Tributary Water Design

Responsibilities for the Design Professional

by Richard K. Olson, president & technical director, Tile Roofing Industry Alliance

(Editor’s Note:  Richard K. Olson is president and technical director for the Tile Roofing Industry Alliance.  The association represents industry professionals involved in the manufacturing and installation of concrete and clay tile roofs in the United States and Canada, and works with national, state, and local building officials to develop installation techniques, codes, and standards for better roofing systems.  Olson can be reached at rolson@tileroofing.org.)

For the design professional, meeting the challenges of tributary water flows is becoming more of a responsibility than a consideration.  When a roof system leaks, the roofing contractor has been called upon to correct the issue as it is presumed an installation problem.  But what if the root cause was really a design issue of poor water transitioning?  When we review a roof design, we need to follow that principle and do our analysis from the ridge to the eave for all our roof planes to determine the amount of water that each roof plane will generate.  Our goal is always to avoid breaching the roof cladding and allowing water to migrate below.  What are some of features that might be considered?

Roof Slope

         As the roof slope increases, so will the speed of water as it travels downslope.  This can help reduce aspects of water pooling seen on minimum roof slopes in high rain events but can also create velocity of water that could breach the design in areas around valley and wall flashings by overflowing them.  When looking at roof slopes over 5:12 consider where the major routes of water will travel to determine any challenges in high rain events. 

Chimneys & Skylights

         When reviewing tributary water accumulation, consider the placement of roof penetrations such as chimneys, skylights, and even roof jacks.  For instance, there will be substantially less water flow when these are located towards the ridge, versus at the lower areas of a roof plane.  Increasing the height of the cricket and even the width can help redirect the water around these obstacles.  Increasing the curb wall height on a skylight or a wider pan flashing can also reduce the chance of water penetrations.

Adjacent Roof Planes

         This is probably the greatest challenge for a design professional, as the use of multi plane roof system is in high demand in residential markets.  For such designs, we need to consider the amount of water and velocity that will flow from each roof plane.  One should consider a revised roof slope, additional valley, dormers to help reduce roof plane areas, boxed soffits, or increased flashing details.  Where roof slopes intersect, consider transition flashings and cladding overlaps to reduce the potential breach from the formation of a head of water that can breach the cladding.  We have seen this issue where multiple steep side planes intersect at the valley or drop onto a lower pitch roof plane down slope such as a carport, garage, or patio area. 

Gutter Designs

         Having gutter downspouts adjacent to valleys or wall flashing that already have significant upslope tributary water may exceed the capacity of those areas to properly transition the water.  The ability to help balance where the water will flow can significantly reduce the overflow challenges.  The use of a transition flashing under the downspout may help reduce the impact on the roof cladding in some designs.  Working with the gutter manufacturer for selection of the proper gutter size can reduce overshooting of water when steeper slope roofs are designed. 

Cladding Profile

         For certain roof claddings such as concrete and clay roof tiles, the use of a profiled tile can help direct the water in the channels.  The S-tile has deep channels that help direct the water downslope in high wind driven rain areas.  Flat tiles with striations or rough surface designs can also help reduce water velocity for better tributary water control. 

Solar Panels

            When solar panels are integrated into the roof system, the design professional needs to work closely with the solar manufacturer and the roofing contractor to ensure that the mounting systems are properly flashed and sealed.  Since solar panels reduce visibility of water flow around the supports, extra precautions are needed during installation to safeguard the proper flow of the tributary water anticipated for those roof areas.