MythBusters

MYTH 3:  Fabrics can make good moisture storage layers

Fabrics offer an effective method for intercepting and distributing water, but distributing water and storing water are two very different things.  By their very nature, fabrics can never be very effective as water storage components. 

Logic tells us that a ¼-inch thick fabric cannot hold more than ¼ inch of water.  In fact, it must hold less.  Most fabrics used for moisture management are thinner than ¼ inch. Moreover, a water reservoir that holds only ¼ inch of water cannot stave off green roof desiccation for very long (days at most).  Don’t be misled by claims of moisture retention fabrics holding many times their weight in water.  Fabrics are very light and water is heavy.  You can never fill the volume of a fabric by more than 100%!  Compare the water retention properties of FLL-compliant green roof media that holds 35% to 40% of its volume in water: A 3-inch thick layer of media will capture and hold over 1 inch of water. 

Note that synthetic retention sheets (e.g., the egg-carton style sheets) are rarely, if ever, more efficient that 20%. That is, when installed according to the manufacturer’s directions, the water captured will not exceed 20% of the thickness of the synthetic retention sheet.  A 2.3 inch thick retention sheet (60 mm) can hold, at most, 1/2 inch of water.     

Fabrics also have low capillary potentials relative to most components of green roof media.  As a result, water captured in fabrics 1) will be rapidly consumed by the plants and 2) will drain out of the fabrics over time under the influence of gravity. 

What then are fabrics good for?  Roofscapes, Inc. uses fabrics in the following ways:

• Distribute moisture evenly across the bottom of an expanse of green roof.   For moisture distribution, we use fabrics with comparatively high capillary potentials (i.e., 0.015 – 0.030 bars, or 6-12 inches of water).  These “capillary fabrics” make base drip irrigation more effective. 
• Protect the underlying waterproofing.  Dense fabrics provide excellent protection against abrasion and the accidental puncture by spades or shovels.
• Prevent migration of soil materials.  This is the most common use for fabrics.