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Keys to Crickets

Requirements and tips to build better crickets for metal roofs

Cricket Oct18 6
Photo courtesy of Isaiah Industries Inc.

Roof crickets are a specialty flashing detail. In terms of preventing wind and water damage, it’s critical that crickets are high enough with a proper slope, made with the right materials, and sealed and installed correctly.

What is a cricket, anyway?

Specifically, crickets are pyramidal forms that divert water around the upside of chimneys and other roof penetrations. They can be used where mechanical units protrude from roofs, at wide decks and other locations.

For instance, Maciek Rupar, director of technical services at National Roofing Contractors Association (NRCA), says, “If water is being channeled to a valley that for some reason can’t handle the amount of runoff, then you use a cricket to split it or divide it into separate avenues for it to run. Or maybe [roofing contractors are] directing [water] to different drains or sections of gutter. Any wide penetration, not just a chimney, that gets in the way of runoff is where you would put a cricket.”

For projects required to follow it, the 2018 International Building Code (IBC) and 2018 International Residential Code for One- and Two-Family Dwellings (IRC) specifies crickets for protrusions 30 or more inches wide, parallel to a ridge.

The NRCA goes a step further. Rupar says, “In our manual, we say that if you have a penetration that’s 2 feet or wider across, and it’s on a roof on a curb that’s going to get in the way of water draining, you should probably put a cricket behind it. Many of NRCA’s recommendations are more conservative or more robust than the minimum requirements in codes.”

In describing crickets, it’s also useful to consider some terminology. Rupar says sometimes contractors use the terms cricket and saddle interchangeably, but they have slightly different meanings in “The NRCA Roofing Manual.” “A cricket is basically two sloping planes joined at a ridge,” he says. “And a saddle is just two crickets put back to back, or a pyramid-looking structure.”

Slope and height

One common problem with crickets is that they do not go high enough up on a chimney or whatever protrusion they’re diverting water around to adequately drain runoff. Insufficient cricket height is the first thing Joel Chesney, president at Tomball, Texas-based All Star Roof Systems Inc., identified when asked about common installation errors. “The biggest problem is [roofing contractors] make crickets that make a dead valley in the back of the chimney,” Chesney says. “They don’t build the cricket high enough or wide enough, with enough pitch, so when they put the roof down, the valley still dies into the back of the chimney. Now the water moves from the center to the corners, and the corners are the weakest parts.”

The IRC specifies cricket heights based on roof slopes (R1003.20 Chimney crickets). Generally, the steeper the roof slope is, the wider the cricket should be. A roof with a 12:12 roof slope should have a cricket that is at least half as tall as the width of the chimney or other protrusion. For a roof slope of 3:12, the cricket needs to be 1/8 the width of the protrusion.

To determine how high a cricket needs to be, Chesney says his company makes a mock-up with string. “We run chalk lines and string lines before we build [the cricket] so we know where the valleys are going to die out; there is a mathematical principle to it,” he says.

Similarly, Rupar says, “A good rule of thumb is that the slope [of the cricket] should be the same or slightly more than the slope of the roof that you’re installing it over.”

However, there are exceptions to the rule. The amount of runoff and other project-specific conditions should be considered when determining the height and size of a cricket, Rupar says. “Let’s say you’ve got a metal panel roof on a shopping center with a parapet roof in front because there’s an awning or some sort of a commercial display. You’ve got a blind valley where panels are draining into a wall. There you may have to consider putting a cricket up against the parapet and increasing the slope, depending on the size and the amount of water you expect to come down from that roof, so as not to have all of this runoff simply run into the wall and cause other problems. The more runoff you expect, you increase the slope, typically.”

Photo courtesy of Isaiah Industries Inc.

Matching assemblies, metals

Crickets vary in their assemblies based on what type of roof they are on, what materials are in the roof assembly and how large the crickets are. Some smaller crickets can be built with a small amount of framing, covered with sheet metal, and possibly skinned with the roof cladding. Some larger crickets have assemblies more similar to the main roof assemblies, and are, in a sense, miniature roofs.

Oftentimes, roofers use the same materials as main roofs to build crickets. Several factors account for this. Besides being a cost-saving measure, using the same metal materials can prevent galvanic corrosion. For example, Chesney says, “You always want to match up materials; you never want to mix aluminum and steel. The steel will actually eat the aluminum.”

Another reason to make sure metal materials used for crickets are the same as, or compatible with, those used in the main roof assembly is to allow for even expansion and contraction. This can be illustrated by aluminum and steel’s different expansion coefficients. Aluminum expands significantly more than steel. The linear thermal expansion coefficient ratio between aluminum and steel is approximately 2:1, Rupar says.

Indeed, Chesney says, “If you put aluminum tiles on steel purlin, and that aluminum expands more than that steel, you’ll literally be tearing the fasteners off the first time there’s a hot and cold spell.”

Shop fabrication and flashing pans

In addition to their various assemblies, Rupar says crickets on metal roofs also vary in how different metal materials are joined together. For example, crickets made with aluminum can be welded; crickets made with copper can be welded or soldered. Zinc panels can be soldered, but not welded. Some finished metal cannot be welded or soldered. To make crickets with metal as effective as possible, whenever possible, Rupar says weld or solder them, and complete as much of the fabrication process as possible in a shop, as opposed to at a job site. “Shop fabricating a shape for a specific application would be the optimal way to do it because then you can control the quality of the workmanship much better than you can in the field,” Rupar says.

Hence, All Star Roof Systems fabricates and installs crickets on roofs with a variety of materials including stamped steel and aluminum tiles, and standing seam panels. Instead of building crickets with separate pieces for the valleys, flashing and ridge caps, Chesney says his company fabricates most crickets from a single piece of material without any penetrations. Workers use a CNC slitter, CNC brake and mold to fabricate crickets from coil roll (for stamped tiles) and flat sheet (for standing seam roofs). The piece is called a flashing pan, Chesney says.

“Instead of worrying about making sure that each component [of a cricket assembly] is working together, we just make one component with no weak points and nothing to fail; it’s one solid metal piece. There’s no cuts in it, no breaks and no screws through it. We make a return hem, lock the panels to it, and there’s nothing for the water to bypass. I don’t ever have to worry about caulking failing, butyl tape failing, screws backing out, because it’s one, solid flashing pan.”