In any kind of construction, a strong foundation is key. Whether that foundation is the basement of a building or the insulation under a roof system, the foundation sets the groundwork for success. When designing a low-slope roof system over a substrate that is less than ¼:12, a properly designed tapered insulation system becomes the foundation for a sound roofing system that will provide long-term protection.
Considering that the majority of commercial buildings have low-slope roofs, it’s no surprise that 40 percent of all building-related problems are due to water intrusion. Even worse is that water intrusion accounts for more than 70 percent of construction litigation.
Improper drainage on a low-slope roof system often leads to ponding water, accelerating deterioration of the roof system and then leading to damaging and potentially costly water intrusion. It’s a dangerous ripple effect and one that can be avoided with the proper design and installation of tapered insulation systems. The goal of tapered roof insulation systems is to eliminate ponding/standing water on a membrane when the roof deck does not provide adequate slope. A properly designed tapered roof insulation system not only provides the necessary R-value and the support for the roof system, but it helps extend the life of the roof system because it provides the required drainage.
As with any product or system, the proper design and installation of tapered insulation systems is directly tied to its performance. Without it, the overall effectiveness of the system can be compromised.
When designing a tapered insulation system, polyisocyanurate insulation is likely to be the type of insulation used. It’s a rigid foam insulation used in the majority of low slope commercial roof construction. Polyisocyanurate insulation is compatible with virtually every roof membrane as well as a variety of composite products and provides superior R-values among other advantages.
Tapered polyiso panels are manufactured in standard 4-foot by 4-foot sizes with a minimum thickness of ½-inch at its low edge and maximum thickness of four inches for a single board. A flat polyiso underlayment board, often referred to as flat fill, is used beneath continuing, repeating tapered panels.
As stated previously, a ¼-inch per foot slope is required by International Building Code. However, a variety of standard (⅛-inch and ½-inch per foot) and special order slopes (1/16- inch and ⅜-inch per foot) are available and acceptable based on field conditions and building parameters. Various degrees of slope may be needed to address or overcome deflected areas in the existing roof deck or reverse slopes in the existing roof deck.
In addition to the panel sizes and required slope, existing components on the roof such as curbs, edges, through-wall flashings, as well as internal and external drain locations, expansion joints and heights of parapet walls need to be taken into consideration during the initial design phase. For example, a roof area having a 50-foot distance from the roof edge to the roof drain with a tapered insulation system sloped at ¼-inch per foot and 2-inch minimum at the roof drain will be 14½ inches thick at the roof edge. That thickness has to be accounted for in the wood nailers when developing the roof edge detail as well as in wall flashing heights and curbs. Two Way and Four Way Systems
There are numerous ways to design a tapered insulation system, but the most commonly used designs are the two-way and four-way tapered systems. Industry professionals recommend the use of a four-way system (Figure 1) as the most effective way of moving water off the roof.
Assuming you have a square and the drain is in the center of that square, a four-way system is designed to drain down from the perimeter edge on all four sides at a 45-degree angle.
A two-way tapered system (Figure 2) is more rectangular, with the two longest sides sloping to the drain. In theory, the surface is flat to begin with. Therefore, the tapered insulation starts on a dead level surface with a low point that is in a drain line. The slope goes from that point to a high point either at the roof perimeter or at a point between two drain lines. Once the primary slope is created, crickets are installed both between drains and between the end of the building and the drain to direct water to the outlets.
Ideally, the tapered system, either two-way or four-way, in combination with the drains would eliminate any ponding water issues. In order to keep water from ponding at the drains, it is important to ensure the drains are properly sumped so that there is enough momentum for the water to get down the drain.
According to NRCA, drains should be located in square-shaped, gradually tapered sumps formed in the insulation to facilitate localized drainage at drains. A typical interior roof drain assembly occupies about a 4-foot by 4-foot square area with a ½-inch per foot-tapered slope. As a general rule, if the insulation is two inches thick or less, a 4-foot by 4-foot sump is sufficient. If the insulation reaches three inches or thicker, an 8-foot by 8-foot creates a more gradual slope while still providing the necessary momentum.
Although it is highly recommended to use a four-way tapered system, there are valid reasons to consider installing a two-way tapered system when certain conditions exist. According to the Polyisocyanurate Insulation Manufacturers Association (PIMA), some of those exceptions include:
Crickets are a secondary application of tapered insulation used to divert water from rooftop curbs, valleys and low points to drains and scuppers. Crickets (Figure 2) are most often used in conjunction with two-way tapered systems in the low points of the tapered system. They are typically diamond or triangle in shape, and slope upwards from the drainage location, scupper or drain to keep water from remaining in the low points and also help direct water around rooftop mechanical units.
When designing a cricket, two items need to be designed – slope and configuration of the cricket. A general rule for designing sufficient slope in crickets is that its slope shall be twice the slope of the adjacent roof field. Typically, that will keep water from remaining on the cricket surface. Some residual water could be expected but it should dissipate within 48 hours.
When designing the configuration of the cricket, the NRCA recommends designers recognize the importance of cricket geometry and valley slope and provide the following guidelines:
| Roof Slope | Material Slope | L:W Ratio |
|---|---|---|
| 1/8 | 1/4 | 3:1 |
| 1/4 | 1/2 | 3:1 |
| 1/2 | 1/2 | 4:1 |
When it comes to R-value, there are various ways design professionals can specify the required thermal resistance of the roof. For example, one specification may require a minimum R-value while another may require an average R-value across the entire roof. Code appears to require minimum thickness based on the demands of energy efficiency. However, average R-value is usually used due to the physical constraints of slope, fastener length, and flashing height, as well as the general concern for budgets.
One important change worth noting is that PIMA recently updated its QualityMark-certified R- value program to incorporate a new test method for determining long-term thermal resistance (LTTR) of polyisocyanurate insulation. As a result, new LTTR values will decrease about 7 percent from current values.
The minimum R-values are as follows:
| Thickness | LTTR Value/Inch | LTTR Value |
|---|---|---|
| 1.0 | 5.6 | 5.6 |
| 2.0 | 5.7 | 11.4 |
| 2.5 | 5.8 | 14.4 |
| 3.0 | 5.8 | 17.4 |
| 4.0 | 5.9 | 23.6 |
As recommended by NRCA Associate Executive Director Mark Graham, designers should specify thickness of insulation rather than R-value to ensure clarity in construction documents.
