As previously mentioned, common practice is to use tapered materials integrated into the roof during construction.
In situations where the rooftop equipment blocks drainage paths, crickets created by sloped insulation can help direct water around the equipment to allow for proper drainage. For blockages caused by concrete pavers with insulation bases that are used for walkways or as setting surfaces, drainage slots can be cut into insulation, or using smaller pieces of insulation, to let water flow below the pavers and toward the drains. There is also the option of having walkways with pedestals to allow water to flow beneath.
Other types of surface retrofits use material infills with cementitious materials, rubber polymer, or additional asphalt flood coats with gravel to build up the surfaces of bituminous conventional roof assemblies at problematic low points of the roof. Where pooling water is desired to be reduced prior to the next roofing replacement cycle, these materials can be placed at the low spots to displace the water and promote better drainage. The long-term performance of these products will vary on the materials used and the harshness of the climate.
Mechanical pump drains
Another option to remove water from roof areas is by using a pump. A power-operated or solar-powered pump can be used to siphon water off a roof and direct it into the drains or over the edge. While power-operated pumps are typically only used to remove water from a roof in emergencies, passive pumps, including solar-powered, can be permanently placed at poor draining roof locations.
Conclusion
Standing water can be detrimental to the health and longevity of a building’s roof and its occupants. Water remaining on a roof 48 hours after rainfall can cause many complications, from increased maintenance, reduced roof service life, and an increased risk of leaks. Following the governing plumbing code and using smart and practical drainage design for a new roof are all methods to having quick a drying roof. A roof that is already constructed and struggling with standing water still has a number of retrofit options to help with problem drainage as well.
For best results, building owners should obtain the advice of experienced professionals who will assist with determining the best solution. Do not get sucked into reusing a poorly functioning drainage system or being sold ideas that will not hold water—or rather—end up holding too much water. A structural engineer should be engaged when the roof design load is unknown, and an experienced roofing professional can help navigate serviceability and durability concerns to ensure the most appropriate drainage design for a building and the owner’s budget.
Authors
Allen Lyte, B.Tech., C.E.T., RRO, is a principal partner in the building science consulting firm, W. Allen Partners, and specializes in roofing materials, and their application, with more than 27 years of consulting experience and more than 10 years of hands-on construction experience. Lyte is a two-term past president of the Ontario chapter of the International Institute of Building Enclosure Consultants (IIBEC). He can be reached at alyte@wapeng.ca.
Barb Kemp, B.Tech., C.E.T., RRO, is a principal in the building science consulting firm W. Allen Partners, and has experience working from the east to west coast of Canada. With more than 24 years of experience in the industry, Kemp has worked in various construction sectors ranging from residential, commercial/industrial, to institutional. She can be reached at bkemp@wapeng.ca.
