Positive Low Slope Roof Drainage Extends Membrane Life

November 8th, 2012

There is a considerable effort in low slope roof projects to create designs which optimize the flow of water across roof surface. Getting rid of water on a low slope commercial roof is a priority that impacts the roof’s effectiveness throughout its service life, helping to avoid problems which actually shorten the roof system’s usefulness. Water and sunlight are the two most common variables of low slope roof impact, but it’s water that can ultimately create the most headaches.

One of the easiest ways to avoid water problems associated with a low slope commercial roof is to remove excess water in a timely manner from the roof’s surface. Most of the problems caused by water on a low-slope roof system are the result of ponding, i.e. water staying too long in one place and not being evacuated or evaporated.

Ponding Causes Problems

Ponding water on the roof surface causes multiple problems to the roof membrane, all of which threaten the longevity of the entire system. Ponding water simply shortens the life of the membrane. Even though membrane components are manufactured to be durable and weatherproof, water sitting on the roof membrane for a long period of time can weaken it significantly. This additional moisture can also lead to bacterial growth which eventually causes physical damage as well as cosmetic.

Sometimes debris and dirt deposits find their way to where the water is ponding. This leads to moss or other vegetation growth that also needs to be removed in order not to shorten the roof life or cause further contamination. Finally, if ponding water is frequent and not avoided, it could also serve to void the roof’s warranty by causing weakness in the system and violating manufacturers‘ design and installation guidelines.

Positive Drainage Prevents Ponding

The most established way to ensure constant water evacuation on a low slope roof is via positive drainage. Positive drainage in commercial building design is considered to be sufficient slope to facilitate drainage that removes water from the roof surface within 48 hours after rain, during ambient drying conditions. Positive drainage is part of any well-planned roof design.

Positive drainage conditions are created by routing water around deflections, to lower points on the roof via tapered systems where the water can be evacuated. Tapered systems can be part of the roof’s insulation system, and are key components of both BUR and single membrane systems. In previous years tapered systems were created on site by cutting and shaping regular roof insulation components. Today many manufacturers offer tapered system components pre-cut and pre-packaged which can be combined to achieve tapered positive drainage designs. Both manufacturers and building product dealers offer extensive design services to assist project teams and contractors in designing, estimating and installing tapered systems.

Tapered Benefits

Tapered insulation systems are used to move water to a desired drainage point, where it can be evacuated via the structure’s drainage system. When tapered systems and components are used to create positive drainage on a commercial low slope roof ponding water can be eliminated. When water is drained in a timely manner, this allows for proper drying of the membrane and other system components. This also helps prevent leaks and weakness around flashing and joints as well as other penetrations of the building envelope.

The dry conditions created by positive drainage ultimately extend membrane life. By extending the life of the roof membrane and other components, the usable life of the low slope commercial roof is ultimately prolonged, saving money and effort for building owners and managers, and putting less strain on the building envelope.