The concept of installing a roof membrane without anchorage has been around for decades. As a contrast to conventional roof membranes, both the thermal insulation and membrane may be un-adhered to the substrate. When the concept was first introduced to the U.S. market, there were a number of obvious questions raised:
- What keeps the roof on? Theoretical uplift forces can reach in excess of 45 pounds per square foot (psf), so how can 10 pounds of gravel keep the membrane from billowing and ultimately tearing itself apart?
- What keeps the rocks from being wind-scoured, blowing off the roof and causing impact damage below?
- How do we test these systems at Factory Mutual (FM) or Underwriters Laboratories (UL) since they use pressure tests that reach 75 or more psf, while the ballast application is in the 10 to 20 psf range?
- Why would you use all that heavy rock in the first place?
The answers are documented and available. First, ballasted roofs have been used for decades and blow-offs are not a problem – the same can be said about wind scour. The most authoritative documentation is contained in ANSI/SPRI RP-4, available free from SPRI.
For clarification, David Roodvoets, former technical director at SPRI, explains the difference between common roofing gravel and ballast stone:
For the last few cycles of the IBC code-change process, the roofing industry has been trying to educate the structural committee on the differences between aggregate as used in asphalt built-up roofing (BUR) and stone and other materials used to ballast single-ply. Since the late 1970s, the roofing industry has separated the aggregate used in BUR from the stone or other ballast used in single-ply.
The aggregate used for BURs has been well defined in ASTM D1863 and incorporated into the code. This is relatively small in size usually less than 0.5-inch in diameter. It can perform well in high winds if it is adequately embedded into the asphalt matrix, or is on a roof with a high parapet. Built-up roofs are designed to be fully adhered to the underlying roof matrix, either the roof deck or insulation that is above the roof deck.
The stone used for (single-ply) ballast has been sized using ASTM D448. The wind performance of this material was extensively studied in wind tunnels and by field inspections during the 1970s and 1980s, resulting in the development of ANSI/SPRI/RP-4. The key reason for developing RP-4 was to eliminate improper use of stone ballast. The standard considers two common-sized stones, those of about 1.5 inches in diameter or No. 4 stone, and those of about 2.5 inches in diameter or No. 2 stone. The tables in RP-4 have a large safety factor, and there have been additional safeguards added to prevent systems being installed using small aggregate in hurricane-prone areas. See Paragraph 2.6 of RP-4.
RP-4 requirements were again validated during the RICOWI inspections following the 2004 Florida hurricanes and Hurricane Katrina.
Since round stone is roughly spherical in shape, the mass of it is proportional to the diameter cubed. The mass of an individual stone increases by a factor of eight when the diameter is doubled. Nominal 1.5-inch-diameter stone weighs approximately 27 times more than 0.5-inch-diameter stone.
The following general design considerations and definitions are excerpted from RP-4.
Conventional Ballasted Roof System – consists of membrane or membrane and substrate material loose-laid over a deck using ballast to hold the system in place.
Protected Membrane Ballasted Roof System – consists of a roof deck, with or without insulation, over which the membrane is installed. The membrane is either loosely laid, mechanically attached, or adhered to the substrate. Insulation is then installed over the membrane. The insulation is then covered with a water- and air-pervious fabric over which ballast is applied.
Lightweight interlocking pavers with integral cementitious coating – the panels shall be interlocking and weigh a minimum of 4.0 psf.
Other interlocking ballast pavers are available as well.
Slope design shall not exceed 2 inches in 12 inches.
Perimeter attachment shall be designed to withstand a minimum load of 100 pounds per linear foot.
RP-4 also explains several design options that include specific designs for the corners, perimeter, and field of the roof. For example, in Denver, it’s common to use No. 2 (larger) stone or concrete pavers at 20 psf in the corners and perimeters of the building, and then revert to smaller No. 4 stone at 10 psf for the area within the perimeter .
Parapet height is an important design parameter to prevent wind scour. RP-4 provides seven tables for heights as follows:
- 2 to 6 inches.
- 6 to 12 inches.
- 12 to 18 inches.
- 18 to 24 inches.
- 24 to 26 inches.
- 36 to 72 inches.
- 72 inches and above.
Each table includes design wind velocity for various building exposures and building (not parapet) height. See Table E for an example of what design information can be found in ANSI/RP-4.