Green Roof Drainage Issues

Drainage is an essential design component on any green roof.

Not enough drainage and the green roof can quickly fail via leaks, dead plants and even a collapsed ceiling.

Too much drainage and  the green roof plants may suffer from not enough irrigation.

Inspecting the roof before installation of the green roof system is very important.  Be sure to review the plans and find the elevation breaks, low spots and ridges.  Take the plans and try to determine how the rainwater and irrigation water, if irrigation water is used, will flow.

Inspect the roof for drainage breaks before installation of Green Roof System

Once the underlying roofing system is installed, inspect the entire roof.  Watch for issues with the flow of water, look for depressions formed into the surface during construction where water may pool.  Ensure seams appear to be properly attached.  Always insist on a flood test and conduct a metal sweep before commencing any work.

When you feel comfortable with understanding the direction of rainwater flow and other issues and requirements for the green roof and install the base system, be it trays, monolithic built-in-place or other system, the soil media will be one of the first components to be installed.

Soil media is usually premixed and possesses the same water or rainfall permeability no matter the location placed on the roof.  Filling the green roof system with soil media possessing the same drainage and water flow characteristics may be adequate, unless the roof pools water in places it shouldn't, or drains water away from areas where water needs to stay and provide irrigation to the green roof plants to be installed.

If your roof construction requires a change in water flow patterns across the roof surface you can easily accomplish the re-direction of water by substituting materials possessing different permeability and water flow qualities for the soil media in the affected area, or by placing a layer of material with increased or decreased water flow characteristics under the soil media.

This obviously requires solid knowledge of  the materials you are working with, their permeability, saturation rates and other physical and chemical qualities that may affect the roof drainage pattern.

Sand for instance can be employed to either slow down or speed up drainage depending upon the type of surrounding green roof soil media.

Peat and coir are other materials with varying drainage characteristics that may be successfully used to after drainage patterns to your requirements.

Adding peat to affect drainage flow

Adding peat under fast draining green roof soil media allows for additional rainwater adsorption and storage.  Peat moves water through wicking action, and can direct much needed irrigation towards the plants needing the water the most.

Natural and geo-synthetic fabrics can also be used to accomplish the same water direction exercise, or re-direction as is the case.

Though all of the above water distribution efforts should in reality be accomplished through proper design,  we all know that ultimately we must make adjustments after the fact sometimes to accomodate anomalies in the as-built system.

Understanding how to 'tweak' rainfall flow through the green roof soil media is important.

Doing so will allow your green roof to mature into a successful planting.

Mature Green Roof Planting, Florida (MetroVerde)