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Responding to Roof Failure Re-cover has Economic and Environmental Advantages
October, 2007
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METAL FINISHES FOR BUILDING ENVELOPE
By Blake Merrick
The flexibility of metal finishes, combined with up-to-date rain screen principles, creates many possibilities for designers and builders in today's marketplace. These two technologies provide proven longevity and low maintenance durability of the architectural metal finishes, with the latest in weather protection and membranes associated with building envelope technology.
Craftsmen have been forming and installing metal roofs on buildings for centuries. In Europe and Asia, metal finishes have been very popular for their durable, long lasting and low maintenance qualities, with replacement occurring approximately every 100 years or upon building modification.
Beginning in the early 1800s, tin and cornice makers were the first to implement metal roofs in North America. Through design progression, coppersmiths became popular in the 1860s as copper roofing was the finish of choice.
Then, during the early 1900s, corrugated roll-formed metal sheeting became the leading option for roof finishes. Light weight and transportable, these sheets covered larger areas in faster application times.
Today, there are many forms of metal products and finishes available along with a multitude of colours and textures. The malleability of metal has allowed for innovative design shapes and techniques to prevent weather and water infiltration into buildings.
Whether the project ranges from metal roofing, as seen on Richmond's River Rock Casino, to diamond-shaped panels transitioning from the roof onto the walls of Vancouver's Lift Restaurant or the Verde green patina copper and zinc wall panels of Omaha, Nebraska's Holland Performing Arts Center, the design concepts include bold architectural statements and a large part of the overall building envelope.
Installation of these building envelope components must complement other associated trade components in order to operate properly and efficiently. In British Columbia, the Industry Training Authority has recognized the need for specific training in architectural sheet metal applications and has organized a steering committee towards developing this required training program. The process is ongoing and should conclude in a full architectural sheet metal apprenticeship training program by the end of 2008.
Blake Merrick is the metals production manager for Flynn Canada Ltd., Vancouver branch. The preceding article is reprinted from the May/June 2007 issue of Construction Business magazine.
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By Peter Kalinger
It is important to understand how roofs fail in service and identify those factors that contribute to their failure. Roofs begin deteriorating shortly after they are applied and continue deteriorating until they are replaced. The rate, severity and extent of deterioration are affected by a wide variety of factors, including:
* Their exposure to the environment.
* The quality and properties of the materials comprising the roof.
* How they are assembled.
* The quality of installation.
* The level of maintenance and care applied during its life.
* The age of the roof.
A roof acts as an environmental separator between the building interior and the outside and is exposed to the ambient environmental conditions (sunlight, rain and snow) of its particular geographic location, orientation and geometry.
Deterioration and damage are not always the result of environmental exposure. Quite often, roofs have to fulfill several functions they were not intentionally designed for. Roofs are routinely used as staging areas, temporary storage areas and even recreational spaces for building occupants. They are often replete with mechanical equipment that requires periodic servicing, exposing the roof to damage from roof top traffic.
From time to time, roofing products are introduced into the market, which fail to provide satisfactory performance. Where inferior products form part of the roof, the building owner is left with little choice but to remove and replace them, often requiring the removal of the entire roof.
The quality of application will affect the performance of any roof. Even the most durable and robust materials will fail if incorrectly installed.
The degree of maintenance will also affect service life. There is ample evidence that a well-maintained roof will perform better and longer than one that is infrequently serviced.
When failure of the roof occurs, whatever the cause, the owner is faced with the difficult task of selecting the appropriate course of action. At one time, the current materials and designs limited available options. If the roof failed, the alternatives were to attempt repairs or tearing-off and replace all, or part of, the roof. Increasingly, another option is becoming a viable alternative to complete tear-off and replacement - re-cover.
REPAIR
Repairing the roof is almost always the most desirable option for building owners providing the roofing contractor can effectively restore the roof to an acceptable level of service and it is economically viable. The three general levels of repair that can be undertaken are:
1. Spot patching of small areas where isolated defects require attention.
2. General repairs, normally undertaken when there are numerous deficiencies of varied severity throughout the roof.
3. Comprehensive repairs as part of a planned maintenance program. They may include resurfacing or recoating of the membrane and the limited removal and replacement of parts of the roof.
RE-ROOF
Re-roofing generally involves the tear-off and replacement of the existing roof. A tear-off consists of the removal of all the components down to the deck. It may include the entire roof or selected portions of the roof.
A partial tear-off may include the complete removal of all the existing roofing down to the deck in selected areas or may be limited to the removal of the membrane cover only, leaving the underlying insulation and other above deck elements in place.
Complete tear-off and replacement is preferred as it provides the building owner the opportunity to make improvements, such as upgrading the insulation, air and vapour barriers, improve the drainage, inspect the deck and affect any repairs that may be necessary.
RE-COVER
Gaining popularity, an alternative to complete re-roofing is to re-cover. FM Global, a world-wide commercial and industrial property insurance and risk management organization, defines re-cover as a system where, "the existing roof system is left in place and new components are installed over it."
The increasing viability of re-cover is due to a number of factors, including the use of lightweight membrane systems that can be mechanically fastened, fully adhered or loosely laid over the existing roof.
Re-cover has a number of economic and practical advantages, including:
* Lower overall costs.
* The potential for reusing the existing roof insulation, provided it is dry and in sound condition.
* Avoiding financial and environmental costs associated with roof tear-off.
* Diverting materials from the waste stream and landfills.
* Reduced labour costs and shorter work schedules.
* Minimizing disruptions to building operations.
* Offering a higher degree of security and protection from the elements while the work is being carried out.
Many membranes and systems are being designed specifically for re-cover. Manufacturers are designing roofs that facilitate future re-covering by incorporating cover boards as separators between the membrane and underlying insulation, mechanical fastening components and use of lightweight flexible membranes. In addition, analytical tools, such as infrared thermography and electronic vector field mapping, have improved the reliability of roof condition assessments.
A thorough assessment of the wind resistance of the existing roof is required prior to any re-covering. Roofing systems resist wind uplift if mechanically fastened, secured with adhesive or weighed down with some ballast.
Re-cover will not correct problems arising from inadequate condensation control, defective air and vapour barriers, poor drainage, or leaks resulting from poorly weatherproofed roof top equipment. Neither will it address problems caused by defective materials or poor workmanship in the original roof. Instead, the performance of the re-cover depends on the quality and soundness of the base to which it is being applied.
The addition of insulation or additional layers of membrane or adding insulation to an existing roof may compromise the fire resistance of the entire assembly. In most instances, superimposing a new re-cover membrane onto an existing roof will add to the dead load of the system and care must be taken to ensure that the design load limit is not exceeded.
Peter Kalinger is the technical director of the Canadian Roofing Contractors' Association. The preceding article is reprinted from the May/June 2007 issue of Construction Business magazine.
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