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Flat roof types

Modified Bitumen: When selecting a product for a new roof membrane system, there are many products from which to choose. One of the best products, for flat roofs, is a modified bitumen membrane.

Originally developed and installed in Europe, modified bitumen has enjoyed widespread success. Modifieds are now one of the fastest growing segments of the commercial roofing market.

The modified membranes are reinforced with one or more plies of fiberglas, polyester, or fiberglass/polyester composite. These sheets are encapsulated in polymer asphalt. The sheets are then either surfaced with ceramic granules (just like asphalt shingles) or remain unsurfaced. The result is a roof system that combines the benefits of multi-layer built-up roofs (hot asphalt and fiberglass systems) with the installation advantage of a single-ply membrane.

The two most proven and common types of modified bitumen membranes are SBS and APP.

SBS (Styrene-butadiene-styrene) formulations perform well in cold weather applications, possess superior elongation and recovery properties, and are quite versatile in their application. They can be installed using hot asphalt, torched, cold adhesives, or self-adhered. Mineral granules are generally used in surfacing this type of membrane, as resistance to ultraviolet light is somewhat limited. The newest type is the self-adhered product. This product looks to be the better application as it takes away the danger of the roof being set on fire due to the torch. Many insurance companies no longer will cover roofing contractors who use a torch on the roof.

APP (Atactic polypropylene) products are more suited to application in hot weather, are resistant to weathering and aging, and are more resistant to ultraviolet radiation than are the SBS membranes. The method of application is generally limited to torching. These membranes are generally manufactured without a surfacing, although granule surfaced products are available. We recommend that protective coatings be installed and maintained on the unsurfaced sheets. We do not recommend the use of this product anymore as the use of a torch on a roof is a risk that is not worth taking for most homeowners.

Neither APP or SBS membranes are well suited to application in ponded conditions. Furthermore, the effectiveness of these membranes is limited in recover situations where moisture in the original roof system exists. For this reason, we recommend that roofs be torn off before a new roof is installed.

Metal roofs

The two main systems are named after the method of joining panels together: "standing seam roofing" has a raised seam and "batten roofing" employs a wider cover cap. The standing seam we use is either “double locked” or we use a “snap lock” system. The “double lock” is what is typically seen on a lower sloped roof, such as those on flat roofs in the District. The “snap lock” is installed on roofs that have a steeper slope. The difference between the two is that the “double locked” seam is much more water tight than the snap locked seam. (see figure)

The forming of the standing seam can be a tedious process and the quality can be vary. For this reason, we have purchased state of the art computerized equipment that allows us to pre-form our panels in our fabrication facilities. This equipment allows us to provide a consistent quality and saves on the labor costs, which we can pass on to the homeowner.

Several types of metal are used for roofing: steel, stainless steel, aluminum, copper and zinc alloys.
Steel, which is heavier than aluminum, is also much more difficult to form than aluminum. This can result in an increase in labor costs, depending upon the roof application. The steel sheets come with a fluorocarbon coating called Kynar. These coatings come in a myriad of colors.

Stainless steel is a very expensive roofing material, however, it won't rust or corrode.

Terne coated steel is very common in the District. This coating is easy to solder and has excellent paint adhesion. This finish - a mixture of 80% lead and 20% tin--must be painted after installation.

Aluminum, extremely lightweight, is the material used most commonly in residential metal roofing. It won't rust, but must be painted or coated for appearance. Coatings are similar to those used on steel. Since they are factory applied, maintenance is minimal. The lightweight makes it easy to work with and form in common applications.

Copper, rooted in centuries of use, will not rust, has no "finish" to scratch or peel, is soft enough to easily tool, and weathers naturally to a beautiful verdigris patina. Unfortunately, it's very expensive. However, when you factor the cost of painting over the years, it may just be less expensive than terne coated steel.

Properly installed, a metal roof should last as long as the house, completely sealing out water, surviving high winds and easily shedding snow. Metal is resistant to fire, mildew, insects and rot. We are so convinced of the superiority of our metal roofing systems, we back up our products with a lifetime limited warranty and will even transfer the warranty to one future owner.

EPDM – a synthetic rubber ( comprised of ethylene, propylene and a diene) material, is installed in overlapping sheets to form a membrane on top of a roof. The sheets are available in 45 mil (thickness of a dime) or 60 mil (thickness of a quarter). The sheets are sold in solid sheets up to 50’ wide and 100’ long. This makes this product good for very large roofs, but no so good for small roofs (such as in Washington DC) because of all of the penetrations. The sheets are bonded together with solvent-based adhesives formulated with volatile organic compounds. There are now solvent-free, pre-formed tapes that are available. The current trend is to use these tapes as there have been concerns over the effect of VOCs on the environment. One of the major problems associated with EPDM is a defect known as “shrinkage”. This is not to be confused with problems resulting from expansion and contraction of the membrane. The material can actually shrink, most notably around wall flashings. This is a major problem especially on homes in Washington DC, where most leaks occur along the wall flashings.

Thermoplastic - The most common thermoplastic roof membranes are PVC and TPO. Vinyl is the shortened term for “polyvinyl chloride.” Vinyl, or PVC as it is more commonly referred, is a molecule comprised of carbon, hydrogen and chlorine (taken from salt). Vinyl membranes for roof applications were first developed in Germany and then arrived in North America. The performance history of TPOs in this country is not quite as long, although at least one TPO has been on the market for almost 10 years. However, it is still a fact that all TPOs are not created equal. Recent research by the National Roofing Contractors Association (NRCA) found that there are significant differences between TPO membranes currently being sold in the U.S. The effects of some of these differences are still unknown at this time. NRCA believes other differences will have notable effects on the usability and long-term field performance of TPO membranes. Since these types of roofs have either had a history of problems or not enough of a track record, we are closely watching the performance of these products before we install them.

Built-Up Roofs are roofs consisting of multiple plies of roof felts laminated together with bitumen. Built-up roof material usually consists of bitumen-saturated fiberglass felt. A surfacing is generally applied and can be asphalt, aggregate (gravel or slag), emulsion or a granule-surfaced cap sheet. While this is often regarded as one of the best roofing systems for a flat roof, the installation cost is high due to the danger of the material. While we still offer built-up roofs, we do not usually recommend them for residential properties in the District as the access to the roofs make it a difficult and expensive option.

Specialty Roofs

Turrets – A turret is a small tower that projects from the wall of a building such as a castle. A turret projects from the edge of the building, rather than continuing to the ground. In Washington, we usually see either round or square shaped turrets. The square shaped turrets are easier to roof and can be roofed with a wider option of materials. The round turrets usually only look good if they are roofed with natural slate.

Built-in gutters –These are usually found (on the homes in Washington) along the front of the mansard roof. They are usually located directly below the turrets as well. Built-in gutters are very popular among architects. Properly installed, built-in gutters can provide a lifetime of trouble-free service. However, built-in gutters expose a building to higher risk of water intrusion than any exposed-type gutter. When leaks occur in built-in gutters the water is likely to intrude directly into the building. Because out of sight usually translates to “out of mind”, we only install built-in gutters using materials that we can solder. We also try to use only materials that do not need maintenance such as copper or (preferably) lead-coated copper. Downspouts should be located in a manner that minimizes or eliminates the need for expansion joints. Continuous cleat and drip edge are recommended along the front, while water proof membranes should extend at least 24 inches behind wall lines in ice dam areas.

Slate – Slate is one of the most aesthetically pleasing and durable of all roofing materials. Installed properly, slate roofs require little maintenance and will last 60 to 125 years or longer. Some slates have been known to last over 200 years. Found on virtually every type of building, slate roofs are perhaps most often associated with institutional and government buildings, where longevity is an especially important consideration in material choices. Slate is available in a variety of colors. The most common are grey, bluegrey, black, various shades of green, deep purple, brick red, and mottled varieties. Generally, the slates of Maine, Virginia, and the Peach Bottom district of York County, Pennsylvania are deep blueblack in color. The slates of Lehigh and Northampton Counties, Pennsylvania, are grayish-black in color. Green, red, purple, and mottled slates come from the New York-Vermont district. The slate producing region of New York, which centers around Granville and Middle Granville, is particularly important because it contains one of the few commercial deposits of red slate in the world.

Slates are also classified as fading or unfading. Fading slates change to new shades or may streak within a short time after exposure to the atmosphere. For example, the "weathering green" or "sea green" slates of New York and Vermont are grayish green when freshly quarried. Upon exposure, from 20% to 60% of the slates typically weather to soft tones of orange-brown, buff, and gray while the others retain their original shade.

Slates designated as unfading maintain their original colors for many years.

The fact that the slates are fading or unfading provides no indication of the durability of slate. Rather, time has shown that the Vermont and New York slates will last about 125 years; Buckingham Virginia slates 175 years or more; and Pennsylvania SoftVein slates in excess of 60 years; Pennsylvania HardVein slates and Peach Bottom slates, neither of which is still quarried, had life spans of roughly 100 and at least 200 years respectively. The life spans provided should be used only as a general guide in determining whether or not an existing slate roof is nearing the end of its serviceable life.

In recent years, slates from China, Africa, Spain and other countries have begun to be imported into the United States. The use of imported slates should probably be limited to new construction since their colors and textures often do not match those of U.S. slate.

Clay tile – The clay tiles that are prevalent in this area are the flat shingle tile, the Spanish Tile, and Mission Tile.

The flat shingle tile replicates the materials used on medieval European and early American homes, ranging from rugged tree bark, rustic natural stone, to the highly sophisticated polished surfaces of quarry tile.

Spanish Tile, one of the most popular field tile, features a simple one-piece barrel design that is classic yet aesthetically versatile. The origin of this tile goes back to the pan and cover barrel tiles found on European buildings and old western missions.

Mission Tiles are a two-piece tile comprised of curved pans and covers installed at regular or random exposures for design creativity.


 

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roofing needs of clients in Maryland, Virginia and Washington DC.

Washington DC - 5505 Connecticut Ave., NW # 364 Washington, DC 20015
Maryland - 6476 Sligo Mill Road Takoma Park, MD 20912
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