How tapered insulation can eliminate ponding on low-slope roofs

by maz_atta | April 14, 2021 12:00 pm

By Marcin Pazera, PhD

Photo courtesy Hunter Panels

A well-functioning roof system is essential to protect a building from the elements. Across Canada, where many regions cope with substantial winter snowfall and demanding weather throughout the year, a tapered insulation system can reduce water ponding by effectively managing rainwater runoff into gutters and drains. This simplifies maintenance and yields cost savings, while offering a thermally efficient roof. This is increasingly important as the continued effects of climate change place greater demands on the built environment. Given the large footprint of low-slope roofs on typical commercial buildings, designing a system that manages rainwater in a timely and effective manner is critical for both new construction and roof replacements because it can improve longevity of a system and decrease maintenance costs over the life of the structure.

Figure 1: Two-way tapered insulation system. Images courtesy Johns Manville

Efficient drainage is the most important element in the maintenance of a low-slope roof. This reduces loads and stresses on the membrane and the supporting roof structure. Often what determines a roof’s ability to drain effectively has less to do with the roof membrane itself and more to do with what lies beneath the visible roof surface. Designing the foundation for a roof membrane with a slope provided either through structural elements or a tapered insulation system ensures the roofing system drains well and is easier to maintain.

Tapered insulation systems can help reduce or eliminate the amount of ponding water on the roof when the structural roof deck does not provide adequate slope to drain. Due to its wide use in low-slope roofing application, tapered polyisocyanurate (ISO) insulation systems offer a number of benefits in addition to providing positive drainage: high R-value, versatility and customization to accommodate project-by-project complexity, and ease of installation.

Slope and drainage requirements in building codes

Figure 2: Four-way tapered insulation system.

Notwithstanding the absence of prescriptive strategies, subsection of the National Building Code of Canada (NBC) 2015 does provide general guidance requiring mandatory drainage provisions for horizontal or sloped assemblies that may be subject to accumulation of water, snow, or ice. Conformance with any relevant provincial, territorial, or municipal regulations is also required. Further, the Appendix Notes in NBC recognize a perfectly sealed, watertight cover on any surface of a building is likely unachievable, and as such, drainage is specifically acknowledged as a likely and overarching principal strategy to divert water to the building exterior.

Again, while no specific acceptable solutions are cited, the relevant mandatory code language is tied to the identified functionalities of providing resistance to the ingress of water and to the facilitation of its dissipation.

Figure 3: 6.5” 4.5” Q4 2.5” Q2 0.5”Q2″ FILL 2″ FILL Cross-sections of a single panel tapered insulation system.  Image courtesy ATLAS Roofing

In the case of re-roofing, the Canadian Roofing Contractors Association’s (CRCA’s) Technical Bulletin, Reroofing Design Considerations (June, 2018), also acknowledges roofs must, via strategies of deck design—or insulation—be properly sloped, together with an adequate number of drains and scuppers.

When reviewing the options available for achieving the required slope in a roof system, designers have a number of choices. According to the National Roofing Contractors Association (NRCA), the slope can be achieved by sloping the structural framing or deck, designing a tapered insulation system, using an insulating fill that can be sloped to drain, proper location of roof drains, scuppers, and gutters or a combination of the above (Consult the National Roofing Contractors Association’s (NRCA’s) Roofing Manual: Membrane Roof Systems, 2019).

Design considerations for tapered insulation systems

Figure 4: Cross-section of four-panel tapered insulation system. Image courtesy Soprema.

Proper design and installation are critical to the effective performance of tapered ISO insulation systems, and this is true for any product or system. Tapered ISO is manufactured in 1.2 x 1.2 m (4 x 4 ft) or 1.2 x 2.4-m (4 x 8-ft) panels that change thicknesses over the 1.2-m distance from the low edge to the high edge on the opposing sides of the panel. The standard slopes for tapered insulation are 3, 6, and 13 mm (1/8, 1/4, and 1/2 in.) per foot to accommodate specific project requirements. However, tapered insulation panels with slopes as low as 1.5 mm (1/16 in.) and other alternative slopes (4.7 and 9.5 mm [3/16 and 3/8 in.] per foot) can be ordered to accommodate unique field conditions.

Figure 5: Cross-sections of an eight-panel tapered insulation system. Photo courtesy Johns Manville

The common minimum manufactured thickness of tapered ISO insulation board at its low edge is 13 mm and the common maximum thickness at the high edge is 114 mm (4 ½ in.). Other product thicknesses (both minimum and maximum) may be available from certain ISO manufacturers.

The design of the tapered insulation system will be governed by the footprint and complexity of the roof under consideration, slope of the roof deck, presence and configuration of roof drains (primary and secondary), scuppers, gutters, or drip edges. Additionally, roof structures, height of parapet walls, expansion joints, curbs and through-wall flashings, and any other elements that may obstruct water management also need to be considered in the design phase. The tapered insulation system will be lowest at internal drains, scuppers, gutters, and drip edges, and will slope upward away from these features.

The primary goal of a tapered insulation system is to move water to the specified drainage points. Two-way (two-directional slope) or four-way (four-directional slope) systems are the most common designs. A two-way tapered insulation system is commonly used on roofs where multiple drains are in straight lines. In this scenario, there is a continuous low-point between the drains, and it often extends to the parapet walls. Crickets are installed in between the drains and between the building or parapet walls and the drains (Figure 1).

Tapered polyisocyanurate (ISO) insulation systems offer a number of benefits, such as high R-value, versatility and customization to accommodate project-by-project complexity, and ease of installation. 

Many buildings are not designed with a straight line of drains, and benefit from a four-way tapered insulation system to move water off the roof. This approach is the most efficient way to manage roof drainage, and is recommended by industry professionals. In this scenario with a drain located in the centre, water funnels in from the higher perimeter edges on all four sides (Figure 2). Variations of two- and four-way systems exist to accommodate complexities in the field. One-directional slope and three-directional slope tapered systems can also be used to move water to gutter, drip edges, and scuppers.

Since a tapered system is more expensive than one constructed with standard flat insulation only, some might look at the former as a target for ‘value engineering.’ This can compromise the drainage intent of the design professional, architect, or roof consultant to lower the installed cost of the roof system by changing the specified slope or redesigning the configuration of the tapered panels. However, any savings upfront will certainly be lost when the roof design does not drain as it should, resulting in higher long-term costs for roof maintenance and premature roof system failure.

Figure 6: A diamond shaped cricket design. Images courtesy Soprema

Most tapered insulation systems incorporate flat ISO board stock (referred to as “fill panels” or “tapered fill panels”) beneath continuing, repeating tapered panels. The tapered panels can be a single panel (or “one panel repeat”) system meaning the taper is provided by a single repeating panel in conjunction with fill panels (Figure 3). In a four-panel (or “four panel repeat”) tapered insulation system (Figure 4), the slope is developed using four repeating panels. This system utilizes 50-mm (2-in.) and 100-mm (4-in.) fill panels.

Non-typical designs can feature up to an eight-panel (or “eight panel repeat”) system with eight tapered panels making up the sloped section prior to incorporating the first fill panels. Figure 5 shows an example of four panel repeat system with a 25-mm and 50-mm fill panels and 1.5-mm per ft slope.

Figure 7: Examples of diamond-shaped crickets with 1:1, 2:1, and 3:1 ratios. Two half-diamond shaped cricket configurations.

The various configurations, slopes, and fill panel thicknesses in ISO tapered insulation systems demonstrate its versatility to accommodate a range of roof designs and rooftop complexities.

A tapered insulation system can reduce water ponding by managing rainwater runoff into gutters and drains. Photo courtesy Hunter Panels

Crickets are an integral part of a tapered insulation system and are commonly used in two-way systems. Crickets can divert water toward drains and away from curbs, perimeter walls, and roof valleys. The two factors that must be considered in the design and installation of crickets are slope and configuration. The general rule of thumb is that for a full diamond cricket, the total width should be between 1/3 to 1/2 of the total length (Figure 6). The wider the design of the cricket the more slope is utilized in the field of the roof. This improves drainage efficiency. The greater the slope of the tapered insulation, the narrower the cricket can be (to a certain limit).

Figure 8a: Examples of drain sumps. Images courtesy Soprema

Crickets typically have diamond or half-diamond shapes (Figure 7). However, kite-shaped and snub nose crickets can also be configured to accommodate specific roof designs. To keep water from remaining on the cricket surface, the design needs to have sufficient slope (generally, twice the slope in the adjacent field of the roof). NRCA provides guidance regarding cricket geometry(Consult the National Roofing Contractors Association’s (NRCA’s) Roofing Manual: Membrane Roof Systems, 2019).

Figure 8b: Examples of drain sumps. Images courtesy Soprema

More often than not, the drains will not be located at the lowest structural points due to physical limitations (e.g. beams and columns). A proper sloped roof should have sumps at the drains to increase efficiency. Sumps can be used with both structural slope roofs and with fully tapered systems to promote positive slope to the most critical location on a roof. This helps ensure the drains are the actual low points of the roof (Figure 8).


Tapered insulation systems offer a cost-effective solution to achieving positive slope and improved drainage in new roof systems and roof replacement applications. Occasionally, structural slopes have limitations that can be overcome with the help of custom tapered insulation designs. They will complement each other to achieve the optimal drainage solution. An adequate rainwater management strategy that includes both proper drainage and elimination of ponding water is critical to the long-term performance and durability of a roof system. Additionally, proper design, detailing, and installation of products must be an integral part of a tapered roof system design.

[13]Marcin Pazera, PhD, is the technical director for Polyisocyanurate Insulation Manufacturers Association (PIMA). Pazera co-ordinates all technical-related activities at PIMA and serves as the primary technical liaison to organizations involved in the development of building standards. He holds a doctorate in mechanical engineering from Syracuse University.

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