Low-slope roofing (Commercial)

There are five generic classifications of low slope roof membranes or systems. Click on a roof system for more information.

  • Built-up roof (BUR) membranes
  • Metal panel roof systems for low-slope applications
  • Polymer-modified bitumen sheet membranes
  • Single-ply membranes
  • Thermoplastic membranes (e.g., PVC, TPO)
  • Thermoset membranes (e.g., EPDM)
  • Spray polyurethane foam-based (SPF) roof systems

Most low-slope roof membranes have three principal components:

  1. Weatherproofing layer or layers — the weatherproofing component is the most important element because it keeps water from entering a roof assembly.
  2. Reinforcement — reinforcement adds strength, puncture resistance and dimensional stability to a membrane.
  3. Surfacing — surfacing is the component that protects the weatherproofing and reinforcement from sunlight and weather. Some surfacings provide other benefits such as increased fire resistance, improved traffic and hail resistance, and increased solar reflectivity.

With some roof membranes, a component may perform more than one function.

The most common thermoplastic roof membranes are PVC and TPO. The following provides general descriptions of these two systems.

  • Polyvinyl Chloride (PVC)

PVC sheets are produced by calendaring, spread coating or extruding, and typically are reinforced with polyester or glass-fiber mats or scrim. PVC sheets contain plasticizers and stabilizers, as well as other additives to impart flexibility and achieve other desired physical properties. Some membranes are available with nonwoven fleece backing adhered to the underside of a sheet.

  • Sheet widths range from 6 feet to 12 feet wide.
  • Sheets are typically 45 mils to 90 mils thick.
  • Seams are sealed by heat or chemical welding.
  • PVC membranes are produced in numerous colors, though gray and white are the most common.

Thermoplastic Olefin (TPO)

TPO membranes are produced by calendering with lamination, extrusion with lamination, or extrusion-coating techniques. TPO sheets are a blend of polypropylene and ethylene propylene polymers and usually are reinforced with polyester. TPO sheets contain colorant, flame retardants, UV absorbers and other proprietary substances to achieve desired physical properties.

  • Sheet widths range from 6 feet to 12 feet wide
  • Sheets are typically 40 mils to 100 mils thick
  • Seams are sealed by heat welded with hot air
  • TPO membranes commonly are white

PVC and TPO roof membranes can be installed fully adhered, mechanically attached or ballasted. Most PVC and TPO membranes do not receive surfacings.

CTA_commercial_32Example of a TPO roof system

Commercial Metal Roofing System Steep Slope

There are three general categories of metal roof systems used for steep-slope roofing applications: architectural metal panel, structural metal panel and metal shingle/shingle panels.

Generally, architectural metal panel roof systems are watershedding and are intended for use on steep slope roofs. Structural metal panel roof systems are used on low and steep slope roofs. Structural metal panel roof systems can be used on low slope roofs because of their hydrostatic, or water barrier, characteristics.

Because architectural metal panel roof systems typically are designed to be used on steep slopes that will shed water rapidly over the metal panels’ surface, the seams typically are not watertight. Many architectural metal roof systems are well suited for use on roof slopes of 3 inches per foot (14 degrees) or greater. One exception to the general slope guidelines for architectural metal panel roof systems is the traditional flat seamed, soldered or welded metal roof system, such as copper. It may be specified on slopes less than 3 inches per foot (14 degrees). Solid roof sheathing, or decking, is required for architectural metal panel roof systems, and NRCA recommends using underlayment.

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Most structural metal panel roof systems are designed to resist the passage of water at laps and other joints, as sealant or anti capillary designs can be used in the seams. Structural metal panel roof systems possess strength characteristics that allow them to span supporting members.

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Metal shingles and shingle panels are available in numerous varieties for use as steep-slope roof coverings. Most of the metal shingles are press-formed during the manufacturing process to provide a variety of shapes. These products can take the shape of individual or multiple asphalt, tile, slate or wood shingle configurations.

Seam Types

There are many categories of metal panels. The term standing seam often is used as a generic description for a class of metal roof seams. The name standing seam is derived from the fact that the seams are joined together above the panel flats. The term also is used to refer to a panel profile that includes a standing seam: the vertical leg/flat pan and the trapezoidal seam. The trapezoidal standing seam is more commonly associated with structural metal panels. Other panel types are batten, flat, bermuda and shingled.

The original batten seam consisted of vertical leg panels placed between wood batten strips and covered with a cap. Today, many batten seam panels are constructed entirely of metal. Because they are designed to shed water, batten seam panels primarily are used in architectural applications.

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