۲٫ Ballasted: The membrane is loose-laid over the substrate and then covered with ballast to resist wind uplift. Ballast can either be large aggregate (for example, 1-1/2 or 2-1/2 inches nominal diameter, depending upon design wind speed), concrete pavers weighing 18 to 25 pounds per square foot (psf), or specially designed lightweight interlocking concrete pavers weighing approximately 10 psf [49 kg/m²]. Ballasted systems are limited to a maximum slope of 2:12. Ballasted systems should conform to ANSI/SPRI RP-4. Finally, ballasted systems should not be used in high wind or hurricane areas because the ballast tends to become airborne, causing massive damage to adjacent buildings.
If crushed aggregate is specified, a stone-protection mat between the membrane and aggregate should be specified to avoid puncturing the membrane. A stone-protection mat is also recommended when smooth aggregate is used because some sharp fragments are often among the smooth aggregates. Also, aggregates sometimes fracture into very sharp pieces after they have been installed. It is also a conservative practice to specify a mat to protect against abrasion and puncture from fragments during paver installation. A somewhat thinner mat is normally sufficient for paver-ballasted jobs.
NRCA recommends designers consult specific membrane manufacturers’ recommendations for their acceptable aggregate types and ballast application rates.
Generally, loose-laid, ballasted roofing systems are not recommended, or at least, discouraged, for three reasons: 1) a leak is very difficult to locate because the water runs freely in numerous directions and for longer distances under the loose-laid membrane; 2) the membranes tend to pull at their perimeter ties, raising and stretching the membrane around the parapet copings and base flashing areas where the membrane then thins and punctures easily; and 3) the ballast can puncture the roof membrane. For these reasons, fully adhered systems or mechanically attached systems are preferred over loose-laid.