Tar and Gravel

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Building Material

A finished Tar and Gravel roof showing various penetrations for venting and equipment


Contents

Background

Tar and gravel flat roof systems are installed by laying out three to four layers of asphalt-saturated felt roof paper (tar-paper) and mopping on hot asphalt. The surface is given a flood coat of hot asphalt and pea sized gravel is evenly distributed on top. Some of the gravel is permanently embedded into the hot asphalt and some gravel remains loose affording some protection against puncture when walked upon. This system of flat roof protection is known as Conventional Built-up Roofing, BUR, or simply Tar and Gravel roofing.

Care and attention must be paid when new equipment is installed on a tar and gravel roof such as an electrical access or an air conditioning unit. Areas where the membrane has been penetrated must be thoroughly resealed to maintain the integrity of the entire roof system.

With regular maintenance and repair, a tar and gravel roof can last as long as 50 years. Some tar and gravel roofs have been known to last a century.

History

Tar and gravel roof being installed in 1940

Tar and gravel roofs exist all over the world and each area has its own tradition or preference for materials used. In warmer climates where rainfall is less and freezing is unlikely to occur, many flat roofs are simply built of masonry or concrete and this is good at keeping out the heat of the sun and is cheap and easy to build where timber is not readily available.

In areas where the roof could become saturated by rain and leak, or where water soaked into the brickwork could freeze to ice and thus lead to 'blowing' (breaking up of the mortar/brickwork/concrete by the expansion of ice as it forms) these roofs are not suitable.

An important consideration in tarred flat roof quality is differentiating what many call simply 'tar' into rather different products: tar or pitch (which is derived from wood resins), coal tar, asphalt and bitumen. Some of these products appear to have been interchanged in their use and are sometimes used inappropriately, as each has different characteristics, for example their ability or not to soak into wood/boards, their anti-fungal properties and their reaction to exposure to sun and weather as well as varying temperatures.

Use In Building Construction

Characteristics

Diagram of a tar and gravel roof




  • Like individual homes that commonly have sloped roofs meant to aid in drainage, flat tar and gravel roofs rely on gravity to drain water.
  • It is imperative that tar and gravel roofs are tightly sealed to insure that there is no leakage into the inner membrane of the structure.
  • Tar and gravel roofs do have a slight slope that enables water to escape through provided drainage areas.






Common Problems with Tar and Gravel Roofs

Pooling and vegetation are common problems on tar and gravel roofs
  • Blistering

Blisters result from air or water becoming trapped between layers of the roof. As the sun heats the roof, trapped gasses expand creating bubbles and blisters in the surface of the roof. If gone unnoticed, water can leak into the roof through these blisters, potentially weakening the roof and leading to future leaks.

  • Water Pooling

If water is sitting for more than 48 hours it is referred to as pooling. This excess of water places increased weight on the roof and can change the roof structure. Depressions may also result, affecting the drainage slope and causing the pooling to continue.

  • Vegetation

Continued pooling may result in vegetation growing on the surface of the roof. Vegetation will grow roots, hold moisture and deteriorate the surface of the roof.

  • Patches

It is common to see patches on tar and gravel roofs. Patches indicate past leaks or weak areas. If the patching has not been done correctly, these areas are likely to leak again. As a general rule, if 25% of the roof is covered with patches, then the roof needs to be replaced.

  • Old or Damaged roofs

With older roofs the best way to determine how worn out they are, besides the obvious signs, is to walk on them and see how they feel under your feet. Look for loose areas, depressions, deteriorated surfaces, cracking, and discoloration. [1]

Building Materials

Tarand gravelroofmaterial.jpg

There are four main materials that compose a tar and gravel roof:

  • Asphalt
  • Tar paper
  • Tar
  • Gravel [2]

Examples of (architectural style) Condominiums

Encore 235 West 4th Street, North Vancouver, BC

Originally built in 1981 by Intrawest, the first building was built on a concrete perimeter as a wood-frame three-story low-rise, with stucco exterior walls and and a tar and gravel roof. Beneath the building is a level of parking and at its core is the elevator shaft containing the building's one hydraulic elevator. The building was designed to include purely cosmetic awnings and sloped roofs which are accented in asphalt shingle.

In 2006, local investors finally purchased all of the suites from Intrawest, which by now required extensive renovations to update them to current market trends. The property was substantially rebuilt by general contractor PCM Pomeroy in 2007. Since the re-build, it has been re-branded as "Encore".

518 Beatty 518 Beatty Street, Vancouver, BC

518 Beatty is located in a row of buildings along the 500 block of Beatty Street, near the crossroad of Beatty Street and West Pender Street. Historically, this area of Beatty Street was a wholesale warehouse and commercial district, due to the fact that a railroad spur ran next to the buildings.

Today, the area is a neighbourhood filled with small shops, restaurants,and cafes--as well as residential buildings. Well known places nearby include BC Place Stadium, Rogers Arena, the Vancouver Film School, Pigeon Park, Steamworks Vancouver, and the Old Spaghetti Factory.

Chatham Green 165 Park Row, New York City, NY

Completed in 1960, Chatham Green at 165 Park Row in New York City has an award winning design that was conceptualized by Kelly & Gruzen Architects. This modern 1960s architecture building has the undulating Italian-influenced was once at the forefront of American architecture.

  • Gangs of New York is a 2002 historical film set in the mid-19th century in the Five Points district of New York City where Chatham Green is now located. It was directed by Martin Scorsese and written by Jay Cocks, Steven Zaillian, and Kenneth Lonergan.[3]

Mirabella 10430 Wilshire Boulevard, Los Angeles, CA

The steel framed, modernist style building with a brick facade was built in 1982.[4] Mirabella was designed by noted Los Angeles based architect Maxwell Starkman, who began designing tract homes for the post-World War II Southern California housing boom and capped his career with the Museum of Tolerance and Sony Pictures Plaza. Starkman studied architecture at the University of Manitoba after having served with Royal Canadian Engineers in the Second World War. He died of natural causes at the age of 82 in December of 2003.[5]

Wilshire Boulevard and Westwood is the home to some of the first skyscrapers built in Los Angeles after the lifting of earthquake related height restrictions in the early 1960s. The area was was home for many years to the ABC Entertainment Center, which housed network operations for the ABC Television Network.



References

  1. Sean Moss Home Inspection Services
  2. Eureka Modern Roofs
  3. Gangs of New York on Wikipedia
  4. Emporis
  5. LA Times


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