Back to basics

With the availability of new materials and technology, architects and designers are becoming aware of and using complex double-curvature geometry in the buildings they create more than ever. From a roof system designer's and roofing contractor's point of view, constructing a project the size of, for example, the Dubai International Airport terminal is a major undertaking. For a project of this magnitude and complexity, a considerable number of professionals are involved from disciplines such as structural engineering, hydraulics, thermal and condensation control, and acoustics.

Also, roof system designers have to consider the quality of labor and likely expertise of construction personnel in remote locations. As a result of the involvement of so many trades, it is of utmost importance all involved understand roof system design concepts.

A checklist

All personnel involved with a roof system's design no doubt will have checklists that cover a wide range of considerations from which recommendations are made and details developed. A checklist takes a considerable amount of time and experience to develop and generally is based on an individual's experience.

In my opinion, there are 10 major areas to consider that, in many instances, are influenced by an architect's requirements regarding materials, aesthetics and performance. But roofing professionals primarily are trying to develop systems that will provide satisfactory weatherproofing and consider the ultimate required performance from the structure.

Following is my checklist developed from my 50-plus years in the industry.

Architectural requirement

A building's shape and its roof system obviously will influence further considerations regarding material and construction. This typically has a major bearing on a building's aesthetics, and most architects have definite thoughts regarding a roof system's appearance.

Experience indicates an architect typically would like to see several options of the external appearance without necessarily being involved in the system's construction provided the system meets the physical performance requirements.

Options regarding external appearance could include color, texture, and size and frequency of ribs. Quite often, a roof's geometry will dictate the final appearance, which, in many instances, will be a compromise.

Weatherproofness

In my opinion, a roof system designer's first consideration should be whether a roof system is weatherproof and should involve a detailed study of rainfall intensity and other items, such as the probability of hail, snow and even sand buildup on a roof's surface. A hydraulic review also is of extreme importance, and the following questions should be asked:

• Will internal box gutters be used (if it is a metal roof system)?
• Is a standard drainage system adequate, or will it be necessary to employ special water evacuation techniques such as a syphonic drainage system?
• Considering the rainfall intensity, are the proposed roof ribs deep enough to cope with the volume of water?

Other considerations require preliminary decisions, such as:

• Structural aspects for additional live loads
• Membrane waterproofing versus sheet metal waterproofing (depending on geographic location, the options might include shingles, cement or terra cotta tiles, or slate)
• Other major considerations such as foot or vehicular traffic

Materials

Architects and roof system designers have a wealth of materials available to them, and a review of manufacturers' data and application details may result in more confusion than appropriate answers.

For material review (apart from architectural requirements), roofing professionals should consider the environment (industrial, coastal, rural, etc.), and availability can, in some cases, direct what will be the most appropriate roof covering. The selection, quite often, also will depend on cost and, in remote locations, available transportation methods.

In addition, in areas either part of or close to airports, a roof covering must be nonreflective.

Structural

Structural considerations require the input of an architect, structural engineer and roof consultant. If a structural metal deck is to be used, the deck selection obviously will dictate spans of subframing and overall structural layout.

Another important consideration regarding the structural aspect for most roof systems types is acceptable deflections. In numerous cases, acceptable deflections may not necessarily produce an acceptable appearance. Also, a detailed review is required to consider the likely live loads a roof may be subjected to and extraordinary conditions that may relate to access, building maintenance units or post-construction installation of mechanical equipment.

Acoustics

I have spent a considerable amount of time and money testing a variety of systems, profiles and rainfall intensities, and noise level is a complex subject on its own.

With the current trend toward lightweight building construction and possible use of a building as, for example, an auditorium, lecture hall or classroom, sound transmission loss through a roof system should be a major consideration.

Satisfactory solutions have been developed and are able to provide acceptable transmission loss values to satisfy a project acoustician's requirements for lightweight construction.

This design aspect also may influence materials. As with a sheet-metal roof system, heavy rain or hail can generate high sound levels that do not necessarily occur with membrane-type construction (particularly with gravel or trafficable surfacing).

Items to be considered when reviewing sound transmission loss through a roof system include panel resonance, acoustic decoupling from the main structure and flanking paths.

It cannot be overemphasized that rain on a metal roof can and has produced acoustic disasters. For example, in a television production building in Sydney, production could not take place during heavy rain simply because of the generated noise levels.

Condensation

As with curtain walls and any cladding system (roof or walls), a condensation review is important and, quite often, depending on the external and internal ambient conditions, double vapor retarders may be required.

Some excellent computer programs that can predict the probability of condensation and its possible location in a system (interstitial condensation) are available to roof system designers. This is important because many roof systems appear to be satisfactory but may have unobserved interstitial condensation damage.

Some buildings require an in-depth review for possible condensation problems. For example, heated indoor swimming pools in air-conditioned environments can produce a whole range of unexpected difficulties.

In some instances, it is necessary that a vapor retarder has the characteristics of a balloon for the total building area because minor leaks through the vapor retarder can render the roof system useless.

It is of particular importance to review with an HVAC consultant whether an air-conditioning system will run on positive or negative pressure. If it is positive, there is a possibility of high humidity-laden air being forced into all the cracks and cavities within the roof system construction, ultimately causing excessive condensation. This may reach the point where further consideration of the structural performance may be required because of the additional dead load of condensed water and saturated insulation.

Most of the work my company designs in the Middle East requires double vapor retarders (external and internal face) because temperature variations are considerable with possible ranges being 50 F to 122 F externally and 68 F to 77 F internally.

Buildability

I have had project involvement where on-site manufacturing is far more preferable than using the typical factory-produced products and shipping and/or trucking to a site.

As an example, the Bangkok International Airport's main terminal's roof system (total area about 1 million square feet) was manufactured in a factory built adjacent to the main terminal building. The roof structural steelwork (including industrial painting); skylights; and roofing, stretcher leveling, roll-forming, and many structural and sheet metal activities were produced in the factory.

The total roof was assembled into prefabricated modules, each 25 by 30 feet, which were lifted by crane into position as a complete unit (weighing about 5 tons per unit). It was not possible to factory fabricate components off-site because of the poor condition of the road system adjacent to the airport.

This same system of on-site manufacturing has been used for roofing projects on Kuala Lumpur International Air Terminal, Malaysia; Hong Kong International Air Terminal; Expo Railway Station, Singapore; Dubai International Air Terminal, United Arab Emirates; and Dubai Metro.

An equally important component of buildability is safety, which also has to be a major consideration. Scaffolding is an extremely costly system and, particularly in grandstands, railway stations and factory-type construction, the finished ceiling can be incorporated into the roof system and the whole system installed from above the structural steelwork, which allows for the development of a working platform and safe working practices for installation.

Service and maintenance

Another design consideration should be a roof system's life expectancy.

Building economists will suggest a building's economic life should have some influence on the products and materials specified. An architect requiring titanium cladding on a building with a maximum economic life of 40 years is an example of what not to do. Costs could be reduced substantially by using an alternative material with acceptable durability and appearance.

Insulation

Roof consultants obviously need to meet the expectations of mechanical engineering and air-conditioning consultants to be certain appropriate levels of insulation are achieved.

As with other roofing materials, the type of insulation chosen often will depend on the manufacturing plant's location in relation to the installation location.

The correct selection of insulation also can assist with condensation control and sound transmission loss. A balance has to be made regarding the cost and likely performance requirements for all these items.

Contractor's expertise

When selecting a roofing contractor to undertake major roofing work, price should not be the sole determinant.

We all have experienced widely differing levels of expertise among roofing contractors, and review of staff training, equipment capability, financial stability and, in particular, experience should be the prime considerations when selecting roofing contractors for major roofing projects.

Staffing appears to vary considerably among roofing contractors with some roofing contractors insisting on professional qualifications for their senior project personnel as opposed to long-term experienced (but unqualified) people.

In my experience, a combination of professional training and practical experience ultimately results in a good roofing contractor who understands all aspects of the work he or she is undertaking.

Additionally, a structural engineering degree appears to be the most appropriate initial qualification for senior design and construction personnel.

A place to start

The items I mention are by no means a complete list of what it takes to make a roofing project successful; however, they form a good starting point for complicated construction. And all successful construction needs a solid beginning.

Bill Mansell is managing director of Chadwick Technology Property Ltd., Forestville, New South Wales, Australia.