Types of Roof Systems

There are five generic classifications of low slope membranes or systems, with most having three principle components: Weatherproofing Layers, Reinforcement and Surfacing.  

Built-Up Roof (BUR) Membranes
Commonly referred as "tar and gravel" roofs, BUR roofs are composed of alternating layers of bitumen and reinforcing fabrics to create a finished membrane. The number of plies in a cross section is the number of plies on a roof, i.e. a four ply room membrane construction is four plies. 

Metal Panel Roof Systems
There is only one category of metal roof systems used in low-slop applications: structural metal panel. These can be used because of their hydrostatic (water barrier) characteristics and are designed to resist the passage of water at laps and other joints with sealant or anti capillary methods. 

Polymer-Modified Bitumen Sheet Membranes
Polymer-modified bitumen or modified bitument (MB) are composed of reinforcing fabrics that serve as carriers for the hot polymer-modified bitumen as it is manufactured into a rool material. They are composed of multiple layers, much like BUR membranes, and are typically installed as a two-ply system and almost always are fully adhered. 

Single-Ply Membranes
These are factory-manufactured sheet membranes generally categorized as either thermopolastic or thermoset. Thermoplastic materials, such as polyvinyl chloride (PVC) and thermoplastic olefin (TPO) can be repeatedly softened when heated and hardened when cooled. Thermoset materials, such as ethylene propylene diene monomer (EPDM) solidify, or "set," irreversibly after heating. Single ply membranes are referred to by their chemical acronyms, like TPO or EPDM.  

Spray Polyurethane Foam-Based (SPF) Roof Systems

After photo of a hospital roof with the
Spray Polyurethane Foam System.

SPF-based roof systems are constructed by mixing and spraying a two-component liquid that forms the base of an adhered roof system. It can installed in various thicknesses to provide slope to drain or meet a specified thermal resistance (R-value). A protective surfacing is then applied to the foam to provide protection from the elements. The first component is rigid, closed cell, spray polyurethane foam insulation. The second component is the protective surfacing and is typically a spray applied elastomeric coating. This provides weatherproofing, protect the foam from UV exposure, provide protection from mechanical damage and assist with the fire-resistant characteristic of the roof system.

From the National Roofing Contractors Association website