Lessons from experience

Understanding common single-ply roof system installation mistakes will help you avoid costly repairs

  • Figure 1: Nearly all single-ply roofing manufacturers require base attachment at angle changes of 2 inches per foot or greater.
  • Photo 1: Missing base attachment on a fully adhered system
  • Photo 2: Structural damage caused by unrestrained membrane
  • Photo 3: Example of a raised edge with base attachment but without a tapered edge
  • Photo 4: Properly designed crickets
  • Figure 2: How to correctly build a raised roof edge
  • Figure 3: Anatomy of a cricket
  • Figure 4: Calculations of cricket valley slope based on distance between drainage points and width of cricket, assuming an underlying roof slope of 1/4 of an inch per foot. The shaded cells represent properly designed crickets.
  • Figure 5: Calculations of cricket valley slope based on distance between drainage points and width of cricket, assuming an underlying roof slope of 1/8 of an inch per foot. The shaded cells represent properly designed crickets.
  • Photo 5: The crickets on this roof are too small for the conditions, leading to ponding water.
  • Photo 6: A termination bar should not be installed flat on horizontal surfaces.
  • Figure 6: Typical termination bar detail
  • Photo 7: A termination bar bent at an inside corner
  • Photo 8: A termination bar bent at an outside corner
  • Photo 9: Vertical termination bars should be sealed on both sides.

Editor’s note: Following are the author’s opinions about common installation mistakes. Manufacturer instructions will vary. Views expressed are not necessarily those of Professional Roofing or NRCA.

Everyone makes mistakes, and roofing contractors are no exception. But though you may expect to find a slip-up or two in the work of a novice, you’d be surprised by the number of seasoned veterans who repeatedly make errors when working on low-slope roofing projects.

To prevent the pitfalls of subpar single-ply roof system installations, be sure you know how to avoid typical mistakes.

Common mistakes

Some of the most common mistakes made when installing single-ply roof systems involve base attachment, cricket design and termination bars.

Missing base attachment

Simply stated, base attachment is installing a secondary attachment at an angle change to prevent a field membrane from moving. Nearly all single-ply roofing manufacturers require base attachment at angle changes of 2 inches per foot or greater (see Figure 1). Two areas where many contractors become confused are base attachment on fully adhered projects and at raised roof edges.

Many contractors believe base attachment is not required when the field membrane is fully adhered. The membrane is adhered to the roof and it should not move much, if at all, but what happens if it does move? Base attachment provides an important second line of defense to keep a roof system in place if the adhesive bond becomes weakened or broken (see Photo 1). Base attachment also helps prevent serious structural damage caused by unrestrained membrane, such as edge metal pulling off or masonry parapet walls toppling over (see Photo 2).

Raised roof edges happen when metal edges, typically installed flush with a roof’s surface, are raised slightly and installed on top of wood blocking (see Photo 3). They fall somewhere between a standard drip edge and a short parapet wall, and they occur quite often on the rake edges of sloped roofs to create a constant elevation at the perimeter.

When working with a raised roof edge, many contractors misinterpret the base attachment requirement of any angle change of 2 inches per foot or greater to mean any elevation change of more than 2 inches. Figure 2 illustrates the difference and why the best way to address a raised roof edge is to install a tapered edge that will eliminate the need for base attachment.

Poor cricket design

Crickets, also known as saddles, are structures built between roof drainage points to move water laterally across a roof to the drainage points. Properly designed, they virtually eliminate ponding (see Photo 4). 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 insulation panels. The anatomy of a cricket is shown in Figure 3. If the drainage points are next to a wall, this configuration would be used. If the drainage points rest in the middle of a roof as seen in Photo 4, a cricket is built on both sides of the low point on the existing slope, creating a diamond-shaped pattern.

Cricket performance has little to do with the slope of the cricket panels used and everything to do with how far a cricket extends up the underlying slope. A cricket moves water laterally across a roof by creating a sloped valley along its outside edges. Because slope equals rise over run, the only way to increase the slope of a cricket valley is to extend the point up the underlying slope, essentially increasing the rise.

The one constant in cricket design is the existing slope, and the major variable is the distance between the drainage points, or the cricket length. The farther drainage points move apart, the more a cricket needs to extend up the existing slope to create the slope needed at the cricket valley.

Figure 4 demonstrates the relationship between existing slope and distance between drainage points. Calculations were made to determine cricket valley slope based on existing underlying slope, distance between drainage points and cricket width. A properly designed cricket valley has a slope of a nominal 1/8 of an inch per foot.

The calculations used in Figure 4 are based on an underlying slope of 1/4 of an inch per foot, which is the minimum allowable slope for new low-slope construction in the International Building Code,® 2018 Edition. The shaded cells show the calculated cricket slope to meet the criteria of a nominal 1/8 of an inch per foot.

Things get significantly more complicated with roof re-cover projects. IBC 2018 clearly states minimum slope requirements for new construction but is not as definitive when it comes to re-covering existing roofs. In Section 1511-Reroofing, the 2018 IBC states the following exception in 1511.1-General:

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.”

To meet the building code requirement for positive roof drainage, a roofing contractor must understand cricket design when the existing slope is less than 1/4 of an inch per foot. Figure 5 shows calculations of a cricket valley slope over an existing 1/8-inch-per-foot slope. The shaded cells represent a properly designed cricket that provides a slope of a nominal 1/8 of an inch per foot. Clearly, it is more difficult to build a good cricket in re-cover situations where there is less underlying roof slope.

Roofing contractors and designers get into trouble when they don’t understand the relationship between existing slope and distance between drainage points. Far too often, we find a contractor installed a cricket that was too small for the conditions. Photo 5 is a prime example of extensive ponding water that can result.

Why do seasoned pros get it wrong so often? As previously mentioned, most crickets are field-fabricated with 4- by 4-foot tapered polyisocyanurate insulation panels. Nearly all polyisocyanurate 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.

Roofing contractors seldom opt to install a cricket that is more than 8 feet wide because they don’t want to install the 2 inches of flat fill and additional tapered panels or don’t understand a wider cricket is required. Roofing 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.

Termination bar installation

A termination bar is used to secure and seal single-ply membranes at parapet walls and other penetrations. Usually made of extruded 1-inch-tall by 1/8-inch-thick aluminum with pre-punched holes for fastening, termination bars are sold in 10-foot-long sticks.

A termination bar detail is simple (see Figure 6). The bar is placed at the top of flashing and fastened in place. The standard detail requires a bead of water block behind the membrane and a bead of one-part urethane sealant on top of the bar.

When used in the appropriate applications and installed properly, a termination bar is a tried-and-true solution for finishing off flashings. It is designed to be used a certain way—installed horizontally on a vertical surface. Unfortunately, roofing contractors often use it outside of those conditions, and the results can be disastrous.

A termination bar should not be installed flat on a horizontal surface as seen in Photo 6. Doing so will inevitably result in leaks. A termination bar detail allows the termination bar to be hydrokinetic, or water-shedding. But mounting a termination bar flat on horizontal surfaces creates a hydrostatic condition in which water could periodically cover the bar, allowing water in. There are better solutions to address these scenarios, such as coping caps or drip edges or simply moving the termination point to a nearby vertical surface.

Another common error roofing contractors make when installing a termination bar is bending it around or into a corner. Photos 7 and 8 show the result of this error—gaps. Most corners on a roof are a sharp 90 degrees (or nearly so). A termination bar is too thick to be bent and then properly fit into a square corner—particularly inside corners. It always should be cut and tucked tight to a corner with a fastener installed within 1 1/2 to 2 inches from each end, enabling it to provide constant compression on the flashing membrane all the way to the corner.

Although a termination bar primarily is installed horizontally on vertical surfaces, it can be installed vertically on vertical surfaces. However, when doing so, roofing contractors often neglect an important step, leading to leaks. When installing a termination bar vertically on a vertical surface, both sides of the termination bar must be sealed (see Photo 9). When a termination bar is installed horizontally on a wall, the top edge is sealed to prevent water from going behind it. When it’s turned 90 degrees, there now are two sides where water can enter, so both sides must be sealed.

The accompanying photos show several types of fasteners being used—lead hits, threaded fasteners and nylon hits. Mule-Hide Products recommends using threaded fasteners so if a termination bar must someday be removed or repositioned, it can be removed without causing damage to the termination bar or wall.

Avoid costly repairs

These common mistakes are made by inexperienced and seasoned professionals alike. All are fixable, but the repairs can be quite expensive. It’s far less costly (and easier and better for your reputation) to invest the time to understand what to do and what not to do—and, equally important, why to do it or not do it—when installing single-ply roof systems. As Benjamin Franklin once wisely said: “An ounce of prevention is worth a pound of cure.”

Darran K. Sellers, RRC, RRO, is technical training manager for Mule-Hide Products Co. Inc., Beloit, Wis.


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