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On a low slope roof, the uppermost layer is the roof membrane. It is this layer that functions as the waterproofing layer and is most directly subjected to the weathering elements. The most commonly used roof membranes are as follows:

1. Built-up roof membrane
2. Modified bitumen roof membrane
3. Single-ply roof membrane

A built-up roof membrane consists of several individual layers laminated into one membrane. A single ply membrane consists of a single layer. A modified bitumen membrane is between a built-up and a single ply membrane and usually consists of 2 to 3 layers. A built-up roof is generally the most labor-intensive roof, followed by the modified bitumen roof and the single ply roof.

Anatomy of a built-up roof membrane

A built-up roof membrane consists of several layers of roofing felt adhered together by bitumen (asphalt or coal tar). In a typical built-up roof, they felt is laid over a mopping up pitching, followed by a second mopping of bitumen, and then the second felt, and so on. Thus, a number of felt layers (called plies), separated by inter-ply moppings of bitumen, are necessary to build a built-up roof membrane-hence, the name “build up” roof.

A built-up roof normally consists of 3 to 5 plies. The greater the number of plies, the thicker and, hence, stronger and more durable the membrane. The last felt is typically covered with a surfacing material to protect the membrane from the effects of weather and external fire.

The most common surfacing material is gravel laid over a floor coat of bitumen. Because the quantity of bitumen for the flood code is much greater than that required for inter-ply mopping, it is poured over the roof, while the inter-ply bitumen is simply mopped on. The following images show some of the tasks involved in the process.

Bitumen asphalt application – http://www.roofingannex.com/

Bitumen mopping – source: http://www.amerroofing.com/

Built up roof diagram – source: http://www.waynesroofing.com/

Bitumen dispenser

In the alternate layers of felt and bitumen in a built-up roof, the bitumen is the waterproofing material. However, bitumen alone cannot be used, because it is a thermoplastic material. It becomes soft at high temperatures and begins to flow. At low temperatures, it becomes hard and brittle, and cracks. Thus, bitumen does not have the requisite tensile strength to withstand stresses imposed by the changes in temperature, deck movement, foot traffic, hail storm, etc.

The felts work as reinforcing material, giving the required tensile strength to a built-up roof membrane. Thus, the function of felts in a built-up roof membrane is similar to that of steel bars in a reinforced concrete member.

The felt also stabilizes the bitumen against flow, since the interwoven felt fibers form mini-receptacles within which the bitumen is held, preventing its flow. A heavy mopping of bitumen without the felt simply cracks in cold weather due to the lack of tensile strength and flows like a thick paste during hot weather, due to the lack of containment. In other words, the felt allow for a more significant buildup of bitumen, which increases the weatherproofing and waterproofing of the membrane.

A felt consists of fibers (or strands) pressed into a sheet. It may be either an organic felt or an inorganic felt, depending on whether the fibers are of organic or inorganic type. The organic felts are made from paper or wood fibers, or a combination of both. In organic felts are made from glass fibers. Asbestos fibers were once used but have been discontinued due to health concerns.

The manufacturing process for both organic and fiberglass felts is similar to papermaking, in that a mass of fibers are pressed under rollers to give a thin, flat sheet. In fact, an untreated organic felt (without bitumen treatment) is virtually indistinguishable from a thick handmade paper. An untreated fiberglass felt looks like a woven mat. The weave in a fiberglass felt is so sparse that light and air can easily pass through it. By comparison, the organic felt is static, nonporous, and opaque to light.

After the rolling process, the felts are treated with bitumen. Since both coal tar and asphalt are black in color, a treated felt is black in color. Treated felt is used in a built-up roof. Therefore, the term “felt” in our discussion generally implies a bitumen-treated felt. The treatment consists of simply covering the felt with bitumen.

There is a fine distinction between how the bitumen is held in a treated organic or fiberglass felt. An organic felt soaks the bitumen like a blotting paper. Therefore, an organic felt is considered to be saturated with bitumen. A fiberglass felt, on the other hand, is impregnated with bitumen.

The impregnation of bitumen in a fiberglass felt makes it more opaque, but despite the treatment, a fiberglass felt is highly porous. The treated organic felt is nonporous and, hence, water resistant. Only an organic felt is specified as an underlayment for roof shingles and tiles or as a water resistant layer over wall sheathing. Because of its porosity, a fiberglass felt cannot be used in such applications. However, as we shall see later, the porosity of fiberglass felts is an advantage in built-up roof applications.

Technically, heating and air conditioning companies are known as HVAC, or Heating, Ventilation, and Air Conditioning companies. These companies handle, as the HVAC acronym suggests, anything to do with heating, ventilation, and air conditioning. Whether that be installing a new AC or heating unit, simply repairing one (although repairs of HVAC systems are generally never simple ), or setting up a system for a new home.

New homes

Whenever a new home is built, one of the design features is a complete HVAC system (it’s rare to find a home nowadays without such a system). This means air from all over the house needs to be ventilated out of the roof – that’s what those smaller pipes you see on the roofs of homes are for. If there’s a heating system, it’s exhaust needs to be vented out of the house and the hot air needs to have a way to circulate around the home. Circulation is achieved using ductwork, which you can usually see in your basement; the vents you have in each of your rooms are the ends of the duct work and deliver the hot or cold air to each room.

These systems have to be planned by the architect when the original plans for the home are drawn up.

Old homes

Many older homes may not have central air conditioning, although most of them have heating systems installed already. The good news with older homes is that because of the central heating systems that often already exist, there’s no need to install all new duct work. All an HVAC company has to do to add central AC is to install the AC unit and a small amount of duct work to connect the new AC unit to the existing duct work that was installed originally for heating. This saves considerable cost…such a scenario might cost the homeowner only $3,000 or so to add central AC to the home.

If the home does not have central heat, which is pretty rare, it likely doesn’t have duct work either. in cases like these, installing central AC & heat can be extremely expensive because duct work has to be installed all over the entire house. With the cost of heating and AC units, duct work, and labor, costs for something like this can easily get up to $15,000 for even a moderately-sized house.

The bottom line is that you want to choose an HVAC company who knows what they’re doing, as well as being reasonably priced. Look for companies that have been around for a while and by all means check out their reviews on the internet. I can personally recommend this Cincinnati heating and air conditioning company if you’re in that area. For other areas, check out Angie’s list, Yelp, the Better Business Bureau, or other review sites to help you choose a reputable and reasonably-priced company.