The Importance of Properly Designed Tapered Insulation Systems

One of the first things to consider when exploring the use of tapered insulation systems is building code requirements. International Building Code requirements for low-slope roof coverings in new construction state that all membrane roof covering systems except coal-tar built-up roofs must have a design slope minimum ¼:12.

When it involves a roof replacement, deviations from that requirement can be acceptable as long as positive drainage is present. The key to this exception is positive drainage. Because positive drainage is difficult to achieve on roof slopes less than ¼:12, it is highly recommended to adhere to building code requirements to avoid potential ponding water issues.

Generally, tapered insulation is the most cost-effective method to achieving positive drainage on low-slope commercial roof replacements. However, tapered lightweight insulating concrete should be evaluated as an alternative to tapered insulation as it may be more efficient due to the roof configuration, details and location. In new construction, it may be less expensive to slope the structure to achieve the required drainage.

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 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,  eliminates any ponding water issues. 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 four-foot by four-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:

• Complexity of existing drain locations – Extended sumps or valleys can be used to simplify design while eliminating complex cutbacks.
• Multiple drains at a low point – With the absence of an overflow drain, valleys between drains provide a helpful water path to drain, should one of the drains fail.
• Cost – Tapered insulation is an expensive material element of the roofing system and; therefore, is often the primary target for value engineering. Although value engineering can reduce the cost, it also compromises the drainage effectiveness of the tapered system.
• High point restrictions – When a perimeter condition limits the thickness of a material.
• Rooftop penetration locations – Extended low points, crickets and/or sumps may be needed to move valleys past or around rooftop units or skylight penetrations. In order to provide the most efficient tapered insulation drainage system, one should prioritize design intent and field parameters for each project.

Insulation is often one of the most expensive components of a roof system, making it a target for value engineering. Any compromise in product or design also compromises the efficiency of the system. Noted author and roofing expert, Wayne Tobiasson, may have summed it up best when he said, “Money spent on roof slope is the best bargain in the construction industry.”

Oftentimes, building owners will agree to have a tapered insulation system installed only to have it value-engineered at some point during the process, reducing the roof system’s efficiency and increasing the potential of creating problems down the road.

In addition to potential drainage problems, the decision to value engineer a tapered insulation system could violate building code and void the roofing manufacturer’s warranty, which typically requires positive drainage and specifically excludes areas with ponding water. Pay close attention to the width of a cricket or saddle. A sign of value engineering is when those crickets or saddles are slimmed down.

The specified R-value also needs to be carefully evaluated. Sometimes, a minimum R-value is bid, while other times, a specification requires an average R-value be achieved. The design of a system requiring a minimum R-value of 20 versus one that requires an average R-value of 20 will vary greatly, as will the cost.