Roofing SourceFile

Dec. 1, 1998
Roofing tips for education institutions.

Determining if you Need a new Roof

When a roof leaks, thoughts naturally turn to thinking a new roof may be needed. Before that expensive and often disruptive action is undertaken, make sure that a new roof really is required. It may be possible that with some relatively minor repairs, the life of the roof can be extended. On the other hand, if the roof is old or leaks continue even after repairs have been made, then it probably is time for a new roof. When it has been determined that a new roof is needed, there are a number of questions to consider, including:

-Is a low-slope (less than 2 inches in 12 inches) or a steep-slope (more than 3 inches in 12 inches) roof needed?

-Can the roof be recovered, or is a tear-off needed? Investigation of existing roofing systems will determine if recover is an option. For example, there may be a limit on adding another roof to an existing roof because of code or building-load capacity. In addition, if the existing roof contains more than 40 percent entrapped water or moisture, complete tear-off may be more cost effective than partial tear-off and replacement.Other issues will determine whether recover or tear-off is the best option, including:

-Is the existing insulation and structure sound? If so, recover may be the best option.

-Is the new system under consideration compatible with the existing system, and does the existing system contain materials that make it expensive to remove or dispose of the old roof? If so, recover may provide the best benefits.

-Will the cost of a recover exceed the cost of a tear-off? (i.e., there is not enough parapet wall height to accommodate the flashing and extra insulation.) In this case, tear-off should be considered.

-What is the deck type? The type of deck dictates the methods of attachment of the membrane system and/or the insulation. Wood decks are easy to attach the roof membrane to, but may not have enough fastener-holding strength for some screw-attached systems. Steel is easy to fasten to, but not all insulation can be applied directly to the deck. Concrete is easy to adhere to, but not fasten to. Also, size, shape and load limits need to be considered.

-Does the roof have a large number of penetrations? Large sheets work well on open roofs; systems with narrow sheets work well on roofs that are broken up by penetrations or shape.

-What will the building occupancy and use be? If chemicals and/or animal fats (from cooking) will be deposited on the roof, then systems that are not damaged by these contaminants should be used. If there will be high humidity in the building, then a roof system that includes an effective vapor retarder may be needed.

-Are there any building-code or insurance requirements? If so, a class-A fire-rated roof assembly may be needed. Building code or insurance carriers may require a certain level of wind resistance, or lower insurance rates may be available if the roof is designed for wind resistance. Many states have adopted the National Energy Code, which has minimum insulation requirements. Lastly, many codes require a minimum of a 1/4-inch-per-foot slope for new or re-roofed buildings, so slope may have to be added.

-Will the roof surface regularly be subjected to excessive activity? If so, a plaza deck-like system or a membrane system that has a more protected surface should be considered. If the roof is not expected to get any activity, but be difficult to access so that even maintenance will be difficult, a low-maintenance system should be investigated.

-What is the location and climate? Hot climates may benefit from light-colored reflective roof systems. Gravel- and ballast-surfaced roofs do well in hail areas. Very cold climates may benefit from a protected membrane roof or a membrane system with good low-temperature flexibility.

-Are there any aesthetic requirements? Colored membrane sheets may be desired, favoring modified bitumen and single-ply systems, or standing-seam metal roofs.

-Is there a system preference? The preference may relate to the use of one type of system to make maintenance easier, or just a comfort level with a type of roof system based on past performance or other reasons.

Understanding Low-Slope Roofing Systems 

Very few people are experts on every kind of roofing system. However, the various types of low-slope roofing systems frequently considered for schools basically can be divided into two categories: multi-ply or single-ply. Multi-ply systems generally are bitumen-based (asphalt or coal tar), while single-ply includes polymer and metal systems. Several factors should be considered when selecting a roofing system. The installation design, its application and potential problems are important when determining the optimum system for a particular building. Another consideration is the environment. Although there are no nationwide programs to recycle roofing waste, some materials can be recycled or put to use in other ways.


Description: Historically, built-up roofing (BUR) systems are the most common low-sloped roofing type. These systems are built-up from alternating layers of roofing felt and asphalt or coal-tar. It is these alternating layers that provide redundancy in these roofing systems. The reinforcing felts of yesterday were made from recycled rags and paper or asbestos. Today, the vast majority of these roofs use fiberglass as the reinforcing felt. These roofs function by using felt to reinforce thin, uniform layers of bitumen, which make the system waterproof. BUR systems commonly are referred to by the number of layers of roofing felt that are overlapped, the most common being 3-ply or 4-ply roof systems. Thus, in a 4-ply roof, there are a minimum of three films of waterproofing bitumen, depending on the method of membrane attachment to the substrate and the top coating used. In most cases there will be upwards of five films of bitumen in a 4-ply roof. The felt provides the strength to resist thermal and other rooftop stresses. The system is adhered to the top of an insulation layer, or to a base sheet, that is attached to the roof deck.

Ideal Application: Owner-maintained facilities commonly use built-up roofing because these roofs can be maintained by trained in-house staff. Repair materials are readily available, and the techniques are not difficult to master. Built-up roofs are reasonably abuse-resistant and difficult to vandalize.

Potential Problems: The odors from the hot bitumen during installation can cause concerns with building occupants and neighbors. Today, there are fume-control kettles available that greatly reduce fumes and odor. As with all roofing systems, inspection is recommended during the roof installation process, rather than the alternative of taking completed roof samples to check that correct amounts of materials were used.


Description: These roofing systems also use multiple layers of bitumen and reinforcement. However, the products come from the factory with a heavy coating of a polymer-modified bitumen on each side of the reinforcement. The polymers are added to modify the properties of bitumen so that it will be stable and flexible at a wide range of rooftop temperatures. Asphalt commonly is modified with either styrene butadiene styrene (SBS) or amorphous polypropylene (APP). These materials can be assembled into a completed roof by mopping (SBS), torching with a propane burner (APP or SBS), or using special cold adhesives designed for the specific system (SBS or APP). There also are some self-adhering systems available (SBS). The modified bitumen sheets are then assembled into the final roofing product, typically consisting of two or more plies. Some of the plies may be traditional BUR felts. Modified bitumen sheets are the most common type of flashing material in use today on all bituminous membrane systems.

Ideal Application: Modified roofs are very versatile. They can go onto most buildings that have adequate drainage. They are tough and, when damaged, not too difficult to repair using cold repair methods and products similar to BUR, or by torching a patch made of the same type of modified bitumen sheet.

Potential Problems: Open torches on a rooftop create fire risk. As with BUR, the odor of the hot bitumen can be a concern. Care and correct application is necessary, as with other roofing systems. Thus, the installation crews need to be knowledgeable about the type of system being installed.


Description: These roofing systems are made from large sheets of synthetic rubber that are placed on the roof and seamed in place using adhesives or special tapes. The roof is complete in a single layer; thereby the term single-ply.

The rubber used in these systems is ethylene propylene diene terpolymer (EPDM)-a synthetic polymer that is blended with carbon black, oils and other additives to make a weather-resistant and thermally stable membrane sheet. EPDM and other single-ply membranes are attached to the building using one of three techniques:

-Ballasted is the most common attachment method for EPDM, and uses stone (nominally 11/2-inch diameter) or concrete pavers (typically at 10 to 12 pounds per square foot) to resist wind-uplift forces. The membrane is laid over loose-laid or attached insulation boards, or other substrates, and then seamed together. It is then covered with the ballasting material.

-Mechanically attached systems use rows of fasteners to hold the membrane to the roof deck. Wind and thermal forces are transferred from the membrane to the fastener and, ultimately, to the deck. These systems typically use a reinforced membrane and are designed with row and fastener spacing set to meet wind-uplift-requirement needs; thus varying greatly between manufacturers and individual jobs.

-Fully adhered is the last anchoring technique and involves the use of either a contact adhesive, sprayed foam, or asphalt to attach the rubber to the top of an insulation layer or other substrate. The type of adhesive used will depend on the type of membrane. Fleece-backed EPDM is needed with spray foam or asphalt. The contact adhesives contain flammable solvent; thus there should not be any open flames on the rooftop during installation, and steps should be taken to keep solvent fumes out of the building.

Ideal Application: EPDM is best suited for roofs with vast, open areas where the large panel size allows fast installation. The roofs should also not have much traffic.

Potential Problems: The major concerns with rubber roofs are cuts, tears and seam strength. Other issues with ballasted roofs, due to the ballast, are the weight of the ballast and the difficulty in inspection and finding leaks. It is recommended that walkways are provided to allow easier movement across the roof.


Description: Thermoplastic sheets are seamed using hot air to heat-weld the two sides together. The resulting fusion of the sheets creates a seam as strong as the sheet itself. These sheets are manufactured from thermoplastic resins, the most common being PVC (polyvinyl chloride) and TPO (thermoplastic olefin). These sheets are typically reinforced with either a fiberglass or polyester scrim. Thermoplastic systems typically are mechanically attached to the deck through the lap area. This allows them to take advantage of their light color, scrim reinforcement and strong seams. Weldable systems also can be installed as a fully adhered or ballasted system.

Ideal Application: Thermoplastics are best suited for buildings where a highly reflective roof is desired, or the need for chemical resistance to animal fats is needed. They also are good for roofs where concerns about seams or puncture-resistance are an issue. For mechanically attached systems, it is necessary to have decks that will take and hold a fastener.

Potential Problems: These materials are very slippery when wet. On sloped roofs, walking can be dangerous if the roof is wet. The earlier versions of some of these products suffered premature failures that today's products overcome.


Description: Metal roofing systems are classified into two basic types: architectural and structural. Architectural systems need the metal to be supported on a continuous deck, and are designed primarily to be aesthetically pleasing. Structural systems are designed to span structural supports and, therefore, do not require an additional decking material. Structural systems are generally watertight systems and can be used at low slopes, down to 1/4 inches in 12 inches. Architectural systems typically are water-shedding systems and, therefore, require slopes greater than 2 inches in 12 inches. Galvalume coated steel is the most popular metal for structural applications. For architectural applications, painted steel (in a wide variety of colors) is common. Other metals commonly used in architectural metal roof applications are copper, aluminum and zinc.

Ideal Application: For architectural metal roof systems, application is best on buildings where the roof is part of the visual appearance and that are not too wide, making it difficult to accommodate the slope. Structural metal roof systems work well for buildings in need of additional slope, or in cases where a low-slope (1/4 inches per foot) metal roof is desired.

Potential Problems: Metal systems generally have a higher initial cost than other roof systems. The amount is dependent on the choice of materials and system. Maintenance and repair can be difficult, and may require special knowledge and skills.


Description: Foam roofs are a combination of a sprayed-in-place polyurethane insulating foam, typically coated with liquid-applied waterproofing membrane. The waterproofing membrane also protects the foam from ultraviolet attack and is commonly either silicone-, urethane- or acrylic-based.

Ideal Application: Foam is best suited in recover applications where low weight is needed, or additional slope is desired. It also is good on new construction where there are unusual roof configurations or domes.

Potential Problems: Weather during application is critical, as the foam cannot be applied in winds over 25 mph, on very hot or cold days, or when water (rain, standing water, very high humidity) is present. These systems are not recommended in areas of high roof traffic. Bird damage to the foam and coating has been reported.

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