Improper design of roof systems can be attributed to a lack of substantial knowledge of the mechanics of system components. Currently, no major architectural program in the United States addresses roof design for more than a few of hours of study. This leaves practicing architects to expand their role as a “generalist” tying in building components. Architects are often forced to rely on manufacturers to assist in roof system design. Problems in this relationship can arise because representatives of a given manufacturer can have limited analytical expertise in other manufacturers’ materials and systems, and their vested interests in selling their own products often preclude them from being impartial.
In some cases, roofing contractors act as specifiers, often recommending the systems that they are the most familiar with and those that can help them make the biggest profit. Contractors might lack the knowledge, expertise and resources to objectively evaluate potential problems and the suitability of materials that will perform best under field conditions.
A disturbing practice often utilized by roof designers is the insertion of manufacturers’ technical requirements as specifications. These documents - which are often represented as specifications to designers by overzealous manufacturers’ representatives - are in fact only guidelines established by manufacturers to set tolerances for the application of their materials.
Manufacturers state in these documents that they are not intended for use as design documents. The fine print attempts to absolve them of all liability in design issues. The guidelines and details provided by manufacturers often change, particularly with single-ply systems. In fact, at least one manufacturer is considering discontinuing the distribution of annual technical publications due to the rapid detail changes employed throughout the year. Reliance on manufacturers’ warranties as design criteria can bring another set of problems. As I speak to architectural groups and designers throughout the country, I have found that misconceptions about the value of these warranties abound. Warranties in the roofing industry were initiated as a marketing tool for manufacturers who were placing new and relatively unproven products on the market. Warranties limit the manufacturer’s liabilities, and most warranties provide no more retribution than the coverage of material costs for roof leaks that are reported within 24 hours of occurrence. The limitations often exceed the coverage. The roof designer should read the fine print of all warranties and decide what type of warranty is best suited for the project. Remember, no warranties include coverage for design issues.
Prior to the roof design process, the designer must perform four required calculations:
1. A wind uplift calculation must be made to determine design pressure for proper materials and substrate attachment requirements.
2. The required insulation and system R-value must be determined based on facility requirements and local codes.
3. A drainage calculation must be made to determine the required number of drains and their placement.
4. A perimeter edge calculation must be made to determine the material and attachment requirements at the roof’s most vulnerable points.
Once these calculations are completed, the determinations must be presented in the specifications. The specifications must thoroughly define procedures for material attachment to the substrate, the type and thickness of insulation(s) required, drainage construction and drain placement, and perimeter edge materials and attachment requirements.
On roof applications, the specification should be divided into three sections that address general requirements, products and execution. MASTERSPEC or CSI format specifications address these procedures in the following divisions:
• Division 1 - General Requirements.
• Division 2 - Products.
• Division 3 - Execution.
Section 1of the roofing specification should include standard language pertaining to general requirements. These requirements would include submittals, manufacturer requirements, installation methods and weather conditions required for application. This section should also provide material delivery and storage requirements and the type of warranty required.
Section 2should list the products required for the roofing systems. A brief description should be provided for all products listing information applicable to the application (i.e., size, type, thickness, etc.). Reference the ASTM number for all products.
Section 3of the specifications should include preparation procedures and application requirements for each roof component: substrate, insulation, membrane, flashings and penetrations.
Roof design begins at the roof deck. The roof deck is the base on which the roof system is assembled. As well as being the structural foundation, the roof deck also determines the design of the system applied, based on the type of building and its intended use. It is important that the stability of the structural deck can accommodate live loads, such as snow and ice, and prevent excessive deflection. Consideration must be given to loads, positive drainage, expansion and contraction, as well as its acceptability as a substrate for the roof membrane system.
On new construction projects, the roof designer can determine the type of roof deck that will best suit the application and ensure optimal performance of the roofing system. However, the roof designer on a reroofing project must take into account the type of roof deck installed and design a system that will perform well in conjunction with it. There are a number of different roof deck types available, and each requires separate roofing attachment procedures. It is essential that the roof designer study the attachment procedures of the system as they pertain to the specific deck type. If a roof system is applied incorrectly over the deck, premature roof failure will result.
Deck materials can be divided into three basic categories: wood, concrete and structural metal.
Wood is the oldest roof deck material and is the primary material used in residential roof construction. Although its use has declined in new commercial construction in recent years, it is still a major material to consider because of the reroofing sector. Wood decks are comprised of:
1. Plywood - must be a minimum of ½ - inch CDX.
2. Wood plank - must be a minimum of 1 inch by 8 inches, either tongue-and-groove or splinted together.
Concrete has been a viable roof deck material since the early 1900s. The standard poured-in-place concrete and the lightweight insulating fill concrete have provided significant advantages as a roof deck material through the years. The lightweight insulating fill concrete can be applied over fiberboard systems, making it a very economical design consisting of concrete and metal deck. Because the concrete decks are cast in place, the decks can be sloped to shape during application to provide adequate drainage.
The types of concrete decks include:
1. Cast-in-place concrete.
2. Vermiculite concrete fill.
3. Gypsum concrete fill.
4. Cellular concrete fill.
5. Cementitous wood fiber.
The most common type of roof deck is the structural steel deck. Metal decks comprise over 60 percent of all roof decks currently installed. There are three types of steel decking: narrow rib deck, intermediate rib deck, and wide rib deck. The narrow rib deck, which has rib openings of 1 inch or less, requires that ½ -inch thick insulation be used over it. The intermediate rib deck has maximum rib openings of 1¾ inches and requires 1-inch thick insulation. The wide rib deck has a maximum rib opening of 2½ inches and requires that a layer of 1-inch insulation plus a layer of ½ -inch insulation be applied over it.
Early roof systems were often constructed without the use of insulation. The use of insulation in roofing systems became prominent in the early 1970s. It was not until the energy crisis that insulation became a common component of roofing systems. With the country’s focus on energy conservation, the thermal and heat resistance values that insulation provides became necessary in all buildings.
The thermal resistance that insulation provides is an economic asset to the build- ing owner. The proper amount of insulation will lower energy costs for the building owner. Savings can be attributed to the insulation’s ability to reduce cooling and heating loads, which can save on utility bills or even allow for the installation of smaller heating and cooling equipment. Insulation’s ability to restrict the flow of heat is measured by its thermal resistance, which is known as R-value. The higher the R-value of the insulation, the greater the material’s ability to restrict the flow of heat. The roof designer must specify insulation with adequate thickness to provide the thermal resistance required for the facility and to meet local building codes. There are seven conventional types of roof insulation that are used in the United States. These types of insulation are factory-formed into rigid sheet types, and they do not include formed-on-the-job types of insulation. They are as follows:
1. Wood fiber.
2. Glass fiber.
3. Expanded perlite-fiber combination.
4. Expanded polystyrene foam.
5. Extruded polystyrene foam.
6. Polystyrene foam.
7. Isocyanurate foam.
Regardless of the type of insulation applied, the following specification guidelines should be followed:
• Stagger the joints of the insulation so that no joints are aligned.
• Secure the insulation to the substrate with proper attachment procedures and materials.
• Ensure that insulation board joints do not occur over deck ribs. Once insulation board direction has been established, do not change that direction.
• The insulation board joints shall be butted and aligned in both directions with the end joints staggered by the maximum dimensions possible. Ensure that the board ends and sides touch along their entire length.
• No more insulation shall be laid at any one time than can be protected by roofing in case of sudden weather changes.
• The roof insulation must be kept dry at all times. No insulation, once wet, shall be allowed to be used in the roofing system.
• When two layers of insulation are applied, they should be applied at right angles to each other. The insulation board joints shall be butted and aligned in both directions with the end joints staggered by the maximum dimensions possible in relation to joints on the bottom layer of insulation. Ensure that the board ends and sides touch along their entire length. Minor gaps in boards shall be filled with roof adhesive.
The waterproofing component of the roof system is the membrane. There are currently five conventional membrane choices for commercial low-slope applications in the United States. The types of systems and application methods are as follows:
1. Built-up roofs
• Asphalt bitumen
• Coal tar bitumen
2. Modified bitumen
• Torch applied.
• Set in SEBS asphalt.
• Set in cold adhesive.
3. Thermosets (EPDM)
• Loose-laid ballasted.
• Mechanically attached.
• Fully adhered.
• Loose-laid ballasted.
• Mechanically attached.
• Fully adhered.
5. Sprayed polyurethane foam
Specification guidelines for proper application should include the following items:
• Comply with the membrane manufacturer’s requirements for required materials and application methods.
• Begin application at the low point (preferably the drain) of the roof area.
• Provide the required coverage rates for bitumen and adhesives at the proper application temperatures.
• Bitumen and adhesives should be applied in an even application with total coverage over the required area.
• For felt and sheet applications, roll all felts/sheets in a straight line so that no kinks or fishmouths result and the felts/sheets are laid completely flat. Once felts/sheets are rolled in one direction, do not change that direction over the entire roof area.
• Specify surfacing materials and application requirements.
The most critical points of a roof system are at the flashings and penetrations. Industry statistics indicate that over 75 percent of all reported roof leaks occur at these areas. The majority of these leaks could be avoided with the specification of proper materials and application procedures. The roof designer should provide comprehensive text and details that indicate application requirements at all of these locations. The text should match the details. Compliance with manufacturers’ requirements is important, particularly due to the fact that the manufacturer will be required to provide a warranty for the project. However, it is the specifier’s responsibility to determine the design that best meets the requirements of the building. With an existing building, the specifier should investigate the site and be aware of the as-built conditions that may not be covered in manufacturers’ literature or generic details.
It is the responsibility of the specifier to determine the exact standards that are required for each building and to properly define the requirements of the standards so that the contractors can comply with them during installation. Manufacturers’ literature, NRCA details and information from other industry organizations can serve as guidelines for design, and the specifier should be aware of the requirements established by these organizations. However, the design should be completed to best meet the requirements of the building - even if they are more stringent than industry requirements.