Low slope refers to a roof that has a minor slope.

Modified Bitumen is a rolled roofing material that was developed in Europe in the 1970s and has been manufactured in this country since the 1980s. Often referred to as a torch-down roof, it is applied over nailed base sheets and heat-welded in place with a hand-held torch fueled by propane. Modified bitumen consists of asphalt-based tar products applied to polyester or fiberglass mats, and bonded with modifiers added to provide flexibility and improve aging and curing after heat application. The ease of installation and relatively low-cost of this material has enabled it to supplant built-up (tar & gravel) as the most ubiquitous low-slope roofing membrane in residential applications in this country. Various forms are available; typically a mineral surface is presented to the weather, with several choices of color available.

LIFE: 20-30 years
COST: $350-$600
Built-up roofing is the type of roof associated with the smelly kettle we occasionally see (and smell) on the street and do our best to avoid. With the development of modified bitumen and the increase in the cost of oil-based products, built-up roofing has become less common. The hot asphalt is melted in the kettle and pumped up onto the roof, where it is used to laminate various sheets or felts one layer upon another. In this manner we build up layers or plies of roofing: hence built-up roofing. Granular surfaced cap-sheets are often set in the hot asphalt as the finished surface-this is most often seen on larger warehouses, etc. Or a layer of crushed granite (gravel) is applied to protect the membrane from the damaging sunlight. Unlike modified bitumen, built-up roofing does not have seams in the conventional sense, and therefor has more redundancy of membrane, which can make it a reasonable choice on roofs that have standing water or sluggish drainage. Additionally the crushed granite dressing serves to displace standing water, which may sound counter-intuitive, but can be quite effective.

LIFE: 15-30 years
COST: $400-$500
Thermoplastic Polyolefin is another rolled roof product with seams. It can be nailed to the roof deck over base sheets, or simply glued down. This product was introduced in the mid-1990s and over the past several years has become very popular. Its worth noting that no fumes or off-gassing occurs while this system is being applied and is therefore well liked by both installers and environmentalists. Noted for its seam strength, resistance to puncture and tear, and the UV-reflective qualities of the white membrane, TPO is increasingly specified by designers and architects. It is expected to soon become the most common flat roof membrane in the country. It is also available in various shades of grey.

LIFE: 25-30 years
COST: $450-$600
Vinyl roofs have been around for over 40 years. It is another seemed rolled roof product that can be fully adhered to the base sheets or nailed down with the seam of the second sheet covering the nails of the first. The seams are heat-welded and are reputed to be several times stronger than the field of the product. Like TPO, the associated flashings (edge-flashings, pipe and vent-flashings etc.) are proprietary. In this case, they have to be vinyl-clad to bond into the membrane. PVC roofing is usually bright white in color and is considered energy efficient. Due to its long life and durability, this system is often used as the base for a Green Roof or growing roof assembly with the addition of various drain-mats and growing medium. Interestingly, some Green builders object to the use of PVC due to the chemicals used in the manufacturing process and the potential for toxic release in a fire. PVC products are also available as deck coverings. These walkable membranes are typically applied over smooth-surfaced plywood on a deck or patio area, making pavers, tiles or decking unnecessary. They tend to have a slightly textured (non-skid) surface reminiscent of linoleum.

LIFE: 25-35 years
COST: $700-950
Liquid applied membranes-most notably polyurethane based walkable deck surfaces such as Gaco-Flex provide a seamless walking surface, usually with an applied granular component to deliver a non-slip surface. These membranes are installed over solid smooth surfaced substrate and produce a pleasant homogenous walking surface, rather like smooth sandpaper. Several coats of the product are applied with curing (often 24 hours) between coats. This necessitates several site visits that tend to add to the cost of these systems. Various other liquid applied membranes are used to coat existing roofs-often to provide a Cool-Roof reflective surface. Other applications include waterproofing under tile decks or concrete slabs.

LIFE: 15 years, 35 years with re-coating
COST: $850-$1500
Galvanized (corrugated) steel, aluminum and steel with (or without) powder-coated paint finish, zinc, and copper are all available as roofing materials, either as shingles or as roof panels. The conductive properties of metal usually require that a heat resistive base sheet be installed. In hotter areas a gypsum heat barrier may be worth considering. Standing seam roofs have become increasingly popular over the last few years. They provide a clean modern look and are relatively simple to install. As they have become more widespread, they are one of the very few assemblies that have actually come down in price!

LIFE: 40-90 years
COST: $500-$1,100 Standing Seam: $700-$1,000
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