Directing water with a built-up inside roof corner
Where roofs have parts that are actually back graded or flat, there will be areas of ponding. Areas of ponding hold or retain puddles of water for more than a certain amount of time following rainfall or precipitation. Several of the most common building codes and the National Roofing Contractors Association (NRCA) generally say that the yardstick or measurement of time should be 48 hours after precipitation.
In areas where ponding occurs, it can lead to a variety of different problems:
- Ponding water leads to excessive weight in points or areas of the roof which causes an undue structural load.
- Ponding water leads to accelerated deterioration of substrate materials. Many types of roofing materials are somewhat impervious and resistant to the effects of water, but most materials are susceptible when exposed for long periods of time.
- Water that is built up or ponding on a roof where it doesn’t drain or evaporate quickly, because of the back grading, that we described, leads to growth of algae and bio colonization. Those types of organic material growths lead to cycles of higher retention of moisture in the future which turns into a compounding vicious cycle.
- Water ponding areas areas coincidence with seams in the membrane, can lead to relatively quick deterioration of the bonding or binding connection between those membranes, which leads to leakage.
In the pictures above and below, we’re looking at a modified bitumen smooth roof surface. This is a torch applied type of roll roofing that is laid in succession starting at the low end of the roof and working up towards the higher end of the roof. Each seam has an overlap where the upper portion slightly lays on top of the lower portion and is welded to create a connection between those individual layers of roofing materials.
At these inside corners of the regular roof configuration, where the horizontal or near horizontal low-slope plane meets the external parapets, there is an intentional buildup that increases the slope drastically where the parapet walls meet and create those corners. This intentional slope helps so that those critical areas where ponding is most common avoid both build up of tree materials and debris and shed water more efficiently.
The next picture below shows the upper side of a skylight curb. Curbs are built into the low slope plane so that a skylight can be set higher than the remainder of the adjacent plane of the roof. This difference in height helps during typical precipitation events and rainfall, but it particularly is helpful during conditions of ice damming where snow and ice are built up on top of the roof as well.
The next picture below, you can see the area of ponding at the upper side of this curb has created an area where dust has laid down from the typical rainwater where that water has washed the dirt off of other areas and then settled into that low area. Low areas like this could be avoided by installing a cricket if the remainder of the adjacent context of the roof does not allow for a grade or unlevelness at the edge of the curb to drain water away.
Any ponding areas are particularly problematic when coincident with a seam in the roof membrane. At the upper edge of this skylake, there’s not only a ponding area, but there’s a seam directly in the middle of that low area which is doubly problematic. Not only is there ponding which can cause the other problems that we mentioned at the top of this article, but when coincident with the seam, it also has a higher likelihood of causing water infiltration through that seam area.
We provide this information here on our blog, and our website, to help our customers and future clients, and we recommend every building owner in DC who values the longevity of their roof (and their investments) and building use a contractor who values the simple and important principles of proper roof construction like Dupont Roofing DC. Our company specializes in flat roofing here in Washington DC and we’re happy to help building owners of almost all types.
Learn more about our company and the proper techniques of working with roofing on historic buildings in Washington DC here on our blog at DupontRoofingDC.com, and you can call us at (202) 840-8698 and email us at dupontroofingdc@gmail.com. We are happy to help and at least talk through options.
