Flat and Low Slope Roof Systems
There are currently five basic classifications of commercial roofs that have flat or low slope configurations:
- Built-up roofing
- Metal roofing
- Modified bitumen roofing
- Single-ply roofing
- Spray polyurethane foam (SPF roofing)
In low slope roofs, there are generally three main parts:
- Reinforcement, which adds strength, stability, dimension, and some resistance against wear-and-tear to a roofing system.
- Weatherproofing layers, arguably the most critical part of a roof as it guards against rain or water entering into a roof system.
- Surfacing, which protects the other two layers from the outdoor elements and exposure to the sun’s UV rays.
Built-up roofing — Typically referred to as “BUR”, built-up roofs are generally known as “tar and gravel” roofs. In general, they are made up of alternating bitumen layers and reinforcing fabric layers that, together, form finished roofing membranes. These membranes are laid out in cross sections across a building top’s surface. For the most part, built-up roofing is fastened to roof decks and insulation for adherence. The membranes are generally known as plies, but the reinforcement layers have been known to be called “roof felts” or “ply sheets” as well.
The bitumen found in BUR is typically composed of asphalt, coal tar, or cold-applied adhesive. For surface layering, this roof type can include a layer of hot asphalt applied across a building top’s whole surface, aluminum coatings, elastomeric coatings, aggregate “mixtures” (such as gravel or slag), or glass-fiber-surfaced or mineral-surfaced cap sheets.
Metal roofing — Metal roofing is one of the most popular choices for commercial roofing today. There are several metal materials which can be used for metal roofing: corrugated galvanized steel, aggregates of zinc, aluminum, and silicon-coated steel, metal tile sheets, stainless steel, copper, aluminum, stone-coated steel, lead, and tin. Because certain kinds of metal roofs, especially steel roofs, can become rusted from prolonged exposure to the sun, these roof types have surfacing layers applied that protect against or resist damaging influences such as sun exposure.
These surfacing layers are typically coatings, which are applied for resistance against rust, waterproofing, or reflectivity of the sun’s energy. They are typically made of materials such as epoxy, ceramic, or acrylic. Modified bitumen roofing—These roofing systems are composed of reinforcing roof fabrics which function as “carriers” for bitumen, when it is being manufactured into rolls. Similar to BUR membranes, bitumen roofing membranes are installed in layers. They are usually fastened to building tops as two-ply systems that are fully adhered to the roofing deck.
Modified Bitumen — There are two types of bitumen roof membranes, generally speaking: SBS polymer-modified bitumen membranes and APP polymer-modified bitumen membranes. The SBS modifier is installed in hot asphalt or cold adhesives, while the APP modifier is applied by heat welding or torch application. Of the two, the SBS modified membrane is more common. These membranes’ surfacing layers include: aggregate layers, smooth liquid-applied layers, mineral layers, or metal foil-laminate layers.
Single-ply roofing — Single-ply roofing membranes are factory-made membranes. Generally speaking, there are two kinds of single-ply roofing membranes: thermoplastic membranes, which include PVC and TPO membranes, and thermoset membranes, which include EPDM membranes, a popular rubber roofing system.
Single-ply membranes are able to be fully installed, held down with ballast, or mechanically attached. They do not usually feature roof coatings for surfacing layers. The membranes do, however, often contain reinforcement layers such as polyester fabrics or scrims, glass fiber, or a felt or fleece backing.
SPF roofing — SPF roofing is foam-based and created by mixing and applying a two-part liquid that serves as this roofing system’s base layer. It can easily be applied with different amounts of thickness for greater R-value, or insulation value, or sloping for drainage.
The first part of the two-part liquid is a tight, close-celled spray polyurethane foam insulation, of which the foam is composed of isocyanate and polyol. These are mixed in a one-to-one ratio and then pumped through dual hoses, and then mixed again and applied to the roofing’s substrate. The second part of this liquid formation is the surfacing layer, which provides protection against inclement weather, the sun, and incidences such as fire. This layer can be membranous, but roof coatings are usually applied as this roofing type’s surfacing layer.
General Low Slope Assembly