According to the website for the National Roofing Contractors Association, roof systems and materials are divided into low-slope and steep-slope categories. Low-slope roofing includes weatherproof types of roofing systems installed on slopes less than or equal to 14 degrees. Steep-slope roofing includes water-shedding types of roof coverings installed on slopes exceeding that.
The five low-slope roof systems include: built-up roof membranes; low-slope metal panel roof systems; polymer-modified bitumen sheet membranes; single-ply membranes, including Thermoplastic or Thermoset membranes; and spray polyurethane foam-based roof systems.
Most low-slope roof membranes have three principal components.
The first is weatherproofing layer or layers, which is the most important element because it keeps water from entering a roof assembly.
Next is reinforcement, because it adds strength, puncture resistance and dimensional stability to a membrane.
And the final component is surfacing, because that protects the weatherproofing and reinforcement from sunlight and weather. Some surfacing provides 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.
There are six classifications of steep slope roof coverings: asphalt shingles, clay or concrete tiles, steep-slope metal roof systems (such as copper), slate, wood shakes or shingles, and synthetic.
Steep-slope roof systems typically consist of individual pieces or components installed in shingle fashion.
These types of roof assemblies typically have three primary parts:
1. A roof deck, which is the structural substrate and usually is a wood-based material such as plywood or oriented strand board.
2. An underlayment, which provides temporary protection until a roof covering is installed and provides a secondary weatherproofing barrier. Sometimes underlayment is referred to as “felt” or “paper.”
3. The roof covering, which is the external water-shedding material.