Roofing Systems

Tapered Roofing Systems

Poor drainage on commercial roofs can result in damage far more destructive than either wind or other natural elements. Building owners often under estimate the advantages of tapered roofing when evaluating the life expectancy of a roof.

Positive drainage is necessary to insure a safe, long lasting roof. Ponded water and freeze-thaw cycles can result in costly repairs and even pre-mature roof failure, which is rarely covered under warranty or by insurance. Positive sloped roof systems result in reduced maintenance and less stress to the building structure.

According to the Uniform Building Code, roof systems must be sloped a minimum of ¼ inch in 12 inches for drainage. All three major code bodies accept this standard. However, some local codes accept retrofit roofs with a slope of only 1/8 of an inch per foot.

Tapered roofing is equally suited for new and replacement construction. In new construction, this can be achieved by designing a slope into the structure of the building. In this case, the roofing support columns are varied in height so the final roof deck has high and low points from which to place the roof drains. The easiest way to improve the slope of an existing roofing system is with the addition of insulation underneath the membrane. The thickness of the insulation is varied to create the necessary slope.

A Proven Track Record

Expanded polystyrene is a logical, cost effective choice in low-slope roofing systems. Tapered EPS insulation provides the required positive slope to drain while retaining the structural and economic advantages of a flat roof deck. It maintains long-term thermal performance, dimensional stability and consistent moisture-resistance. EPS effectively controls the transfer of heat within all roofing systems.

Moisture is extremely detrimental to any roofing system. The purpose of the membrane is to keep the water out. However, in the event of water penetration, EPS has been shown to effectively resist moisture and retain its mechanical properties. In a study conducted by Structural Research Incorporated, Madison, Wisconsin, in 1984, it was demonstrated that no wet insulation was found in the EPS or wood fiberboard overlayment. The moisture content of EPS insulation samples ranging in age from 6 to 15 years was found to be very low, i.e., a maximum of 0.04% by volume. Likewise, results for R-value performance indicated no deterioration in R-value over time, meaning EPS thermal resistance values may be used without adjustments for aging.

Expanded polystyrene can be molded into thicknesses up to 50 inches and then cut to obtain the desired pitch, ranging from 1/8" per foot or greater. EPS can be applied in a single layer creating a continual form. This design versatility translates into considerable savings in labor and framing costs. Usually the ridges and valleys necessary to provide correct drainage are supplied at 45° increments to the horizontal, but custom angles can be easily furnished.

Design Considerations

A designer must take into consideration the details of the roofing area as well as budgetary concerns. A roof might include skylights, internal and external drain locations, expansion joints and height limitations of parapet and other adjacent walls as well as house HVAC units. The layout of the insulation must work around these components and within a specified budget.

Solutions are found by utilizing various layout options to move unwanted water off the roof.

Shed Roof Design - This is the most basic layout design. The roof slopes from a high point to low point causing the water to travel to the edge of the building or into a gutter.

Two-Way Slope - The roof is split into two parts with a slope on each part. It can either slope inward toward the center of the building or to the outside. The two-way slope is more cost effective than the one-way slope because the total thickness of the insulation is less at the high point since it is split into two.

Three-Way Slope - This layout is used on roofs of smaller building attached to larger structures because it slopes to a point that lies on a side of the drained area. Instead of forming a valley as seen in the two-way slope, the water is forced to a small area, usually a roof drain.

Four-Way Slope -It is considered the most effective layout to insure proper drainage. The insulation is sloped to a single point located inside the area being drained. Or, the slopes are inverted to create a high roof center where the water is brought down to edges of the building. In some designs, the center of the slopes can create ponding. To eliminate this chance, a special set of tapered products, crickets and saddles, should be installed.

Once the layout is determined, the thickness and slope of the insulation must be calculated. The designer must take into account the cost of the insulation as well as the required R-value when deciding on the needed thickness to achieve an adequate slope. Steep slopes may do a great job in eliminating ponding but the problems could outweigh the benefits. Cost can be escalated and the roof drains may be overloaded. Extra long fasteners will also increase the cost of the tapered roof.

The Perfect Choice

Depending on the type of roofing system chosen, EPS insulation is available in both faced and unfaced rigid foam. It may be laminated at the point of manufacture or on the jobsite and helps to provide added strength and durability. To protect from roof traffic, hot asphalt, single ply adhesives and modified bitumen applications, the lamination is adhered to the top of the insulation. It is laminated on the bottom when a thermal barrier is required.

EPS manufacturers are available to provide complete specification and design assistance. Field assistance becomes even more valuable when unanticipated modifications become necessary. When problems occur on the jobsite and the slope of the insulation needs to be adjusted, contractors typically experience delays while requisitioning a reconfigured piece. Delays are avoided with EPS since the material can be cut to specification on site, achieving a precise fit and decreased labor costs.

Other considerations include interior and exterior temperature variations. When interior relative humidity is 80% or less at 70°F, with cold exterior conditions, no moisture build-up will occur in EPS insulation. However, the use of a vapor retarder is recommended with extreme indoor humidity (above 80% relative humidity at 70°F) and cold exterior conditions (average winter temperatures less than 40°F). A vapor retarder is recommended with high interior humidity (above 40% relative humidity at 70°F) and cold exterior conditions (average winter temperature less than 40°F) when a wood fiberboard overlayment for EPS is installed. Based on NRCA/MRCA sponsored studies, vapor retarder placement for EPS insulated roof systems is less critical than for other types of roof insulations.

Code Compliance

EPS roof insulations are compatible with most commercial roofing systems, including, but not limited to built up roofing and modified bitumen systems and single-ply membrane systems that are either ballasted, mechanically fastened or fully adhered. Expanded polystyrene insulation meets the requirements of ASTM C 578 Standard Specification for Rigid, Cellular Polystyrene Thermal Insulation. When specifying EPS insulation using ASTM C 578, the best possible product selection is ensured.

Roof assemblies are tested based on the type of fire exposure. The different roof construction assemblies are listed in the materials and equipment approval manuals published by each of the testing facilities.

Expanded polystyrene insulation meets extensive model building code requirements. More than 48 EPS manufacturers with hundreds of locations in the U.S. and Canada maintain numerous listings at Factory Mutual (FM), Underwriters Laboratory (UL), Underwriters Laboratory of Canada (ULC) and other testing facilities, representing various roof construction types. SBCCI and some state code authorities allow for direct-to-deck applications of EPS foam insulation.

For product information, installation guidelines and specific code approval information, contact your local EPS manufacturer.

Design Issues:

  1. Allow time for "Design Assistance" by taper manufacturers
  2. Determine minimum acceptable slope
  3. Determine if layout is suitable for 4-way or 2-way design.
  4. When designing crickets and saddles, insist on a minimum valley slope of 1/8" where possible
  5. Projects that taper and flat insulation areas should be noted on plans with hatching and clear notes.



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