Technical Library

Roofing Systems

EPDM is an extremely durable synthetic rubber roofing membrane (Ethylene Propylene Diene Terpolymer) widely used in low-slope buildings in the United States and worldwide. Its two primary ingredients, ethylene and propylene, are derived from oil and natural gas. EPDM is available in both black and white, and is sold a broad variety of widths, ranging from 7.5 feet to fifty feet, and in two thicknesses, 45 and 60 mils.

EPDM can be installed either fully adhered, mechanically attached or ballasted, with the seams of the roofing system sealed with liquid adhesives or specially formulated tape.

Built-Up (BUR) systems generally are composed of alternating layers of bitumen and reinforcing fabrics that create a finished membrane. The number of plies in a cross section is the number of plies on a roof: The term “four plies” denotes a four ply roof membrane construction. Sometimes, a base sheet, used as the bottommost ply, is mechanically fastened. Built up roofs generally are considered to be fully adhered if applied directly to roof decks or insulation.
Modified Bitumen (MB) is asphalt that has had modifiers added to it to give it plastic or rubber-like properties. The most common types of modifiers being used are APP (Atactic Polypropylene) and SBS (Styrene Butadiene Styrene). Modified Bitumen roof systems consist of one, two, or three ply systems. The type of substrate will often determine the type of system being installed. Modified membranes can also be installed in conjunction with built-up roof materials (such as multiple plies of fiberglass felt) to form a “hybrid” roof system. Modifieds have proven performance on residential, commercial, and industrial applications.

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 calendaring 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

Steep-slope roof systems typically are composed of individual pieces or components installed in shingle fashion. Steep-slope roof assemblies typically consist of three primary parts:

  • Roof deck — a roof deck is the structural substrate and usually is a wood-based material such as plywood or oriented strand board (OSB).
  • Underlayment — underlayment provides temporary protection until a roof covering is installed and provides a secondary weatherproofing barrier. Sometimes underlayment is referred to as “felt” or “paper.”
  • Roof covering — the roof covering is the external water-shedding material.