Mule-Hide Blog

Poor Cricket Design

Posted: 7/30/2019

Crickets, also known as saddles, are structures built between roof drainage points to move water laterally across the roof, diverting it to the drainage points. Properly designed, they virtually eliminate ponding (see Photo 1). But many contractors -- and some design professionals -- do not understand the mechanics of proper cricket design.

Crickets can be constructed of wood, concrete, lightweight concrete or gypsum, but the most commonly used material is tapered polyisocyanurate (ISO) insulation panels. The anatomy of a cricket is shown in Figure 1. If the drainage points are next to a wall, the configuration shown would be used. If the drainage points rest in the middle of the roof, as seen in Photo 1, a cricket is built on both sides of the low point on the existing slope, creating a diamond-shaped pattern. 

To demonstrate the relationship between existing slope and distance between drainage points, we have run calculations to determine cricket valley slope based on existing underlying slope, distance between drainage points, and cricket width. We defined a "properly designed" cricket valley as having a slope of 1/8 inch per foot, plus or minus 10 percent. Figure 2 shows our calculations based on an underlying slope of 1/4 inch per foot, which is the minimum allowable slope for new low-slope construction under the 2018 International Building Code (IBC). The shaded cells show the calculated cricket slope that meets our criteria of 1/8 inch per foot, plus or minus 10 percent. As the graphic shows, it doesn't seem too difficult to build a proper cricket on 1/4 inch per foot slope. Pay attention to the 4-foot and 8-foot cricket width columns, though; we will address that a bit later.

Section 1511 Reroofing of the 2018 IBC states the following:

1511.1 General

Exceptions:

1. Roof replacement or roof recover of existing low-slope roof coverings shall not be required to meet the minimum design slope requirement of one-quarter unit vertical in 12 units horizontal (2-percent slope) in Section 1507 for roofs that provide positive roof drainage.

"Positive drainage" is such a subjective term. What does it really mean? It means the roofer had better understand cricket design when the existing slope is less than 1/4 inch per foot and he or she wants to meet the building code. Based on our experience, and for the sake of argument, we believe positive drainage will generally occur when the underlying slope is 1/8 inch per foot.

Figure 3 shows cricket valley slope calculations to acheive that 1/8" per foot standard. The shaded cells once again represent a properly designed cricket based on our previously stated criteria -- providing a slope of 1/8 inch per foot, plus or minus 10 percent. Clearly, it is more difficult to build a good cricket in recover situations where there is less underlying roof slope. See anything different in those 4-foot and 8-foot cricket width columns?

Roofers and designers get into trouble when they don't understand the relationship between existing slope and the distance between drainage points. Far too often, we find that the contractor installed a cricket that was too small for the conditions. Photo 2 is a prime example of the extensive ponding water that can result.

Why do even tenured pros get this wrong so often? As mentioned earlier, most crickets are field-fabricated of 4-by-4-foot tapered ISO panels. Nearly all ISO manufacturers provide a two-panel system for crickets. The two panels are installed next to each other, front to back, to build the taper. To continue with the slope, a layer of flat fill is added, and then the process is repeated until the desired width is reached. In our experience, roofing contractors seldom opt to install a cricket that is more than 8 feet wide, either because they don’t want to install the 2 inches of flat fill and additional tapered panels, or because they don’t understand that a wider cricket is required. Contractors and designers should verify existing slope and then calculate the cricket valley slope before installing the cricket. More often than not, a cricket more than 8 feet wide will be required.

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