Comparing roofing options

Roofs protect the plant, occupants, and contents from the elements. However, performing this seemingly simple task is made quite complicated by changing cycles of rain, heat, cold, wind, and UV radiation that continually attack the roof membrane.

05/01/2000


Roofs protect the plant, occupants, and contents from the elements. However, performing this seemingly simple task is made quite complicated by changing cycles of rain, heat, cold, wind, and UV radiation that continually attack the roof membrane. In addition, expansion and contraction, environmental pollutants, and foot traffic relentlessly deteriorate the surface. Wear, weathering, and aging created by these many factors evolve into cracks and fissures, and eventually cause the roof to fail. Consequently, creating a long-lasting roofing system requires a combination of sophisticated materials, detailed engineering, quality installation, and continual inspection and maintenance.

Roofs are categorized by the type of membrane, which is the fist line of defense against the elements and the only waterproof component in the system. This reference file looks at three of the choices for new or reroofing projects: built up, modified bitumen, and single ply.

Selection factors

The three roofing options have different installation procedures and maintenance requirements. Area of the country, code considerations, fire ratings, environmental conditions, aesthetics, and personal preferences of plant personnel also affect the type of roofing system selected. A life-cycle cost comparison is always a wise selection consideration.

No single system universally outperforms all other types, but there is usually a "best" choice for each plant situation. Regardless of the type selected, installation is the most critical factor in a successful roofing system. Choosing the best system and using the highest quality materials still doom the roof to eventual failure if they are not properly applied.

Built up

The built-up roof (BUR) includes asphalt or coal tar bitumen, which acts as the adhesive binding the membrane together and holds it to the substrate; and reinforcing material, which gives the roof its toughness.

Asphalt is the most common bitumen and is available in different types with various softening points and flow characteristics for applications on roofs with slopes from 1/2-6 in./ft. Coal tar is produced in pitch and bitumen versions, and is suited for dead-level and low-slope roofs.

Asphalt-impregnated fiber glass felts, available in several different types and strengths, are the most common reinforcement material. Surfacing material, such as aggregate, is placed on top of the membrane for protection and resistance to UV radiation.

A roofing crew usually hot-applies BUR in the field. (A cold-applied technique is also available.) Workers lay bitumen on the substrate with a mop or cart. Felts are rolled into the hot bitumen shingle style in order to obtain the desired number of plies.

Modified bitumen

Modified bitumen roofing systems closely resemble BUR in materials, equipment, and installation techniques. However, the bitumen includes elastomers or polymers mixed with basic asphalt to create a product with an altered softening point and improved elasticity and toughness.

Several modifiers are available, but two are most commonly used: styrene butadiene styrene (SBS) and atactic polypropylene (APP). SBS is a synthetic rubber membrane laid into hot asphalt and possesses greater elasticity than APP-modified products, remains more flexible at lower temperatures, and is usually mopped or applied with adhesives. APP members are installed with a propane torch, hot air, or cold adhesives; and provide good tensile strength, elongation, and flexibility.

Reinforcement-usually polyester or fiber glass mats-is added to keep the product in place and add tensile strength. Modified bitumen roofing requires a base sheet as part of the system.

Single ply

Single-ply roofing features lower labor costs, along with speed and ease of installation, since only one layer is used. There are three basic types of membranes: cured or vulcanized elastomers, noncured elastomers, and thermoplastics.

Cured elastomers, or thermosets, are commonly called rubber systems and possess great elasticity and return to their original dimensions after stretching. Uncured elastomers become rubberlike over time, weather well, and resist a broad range of contaminants. Thermoplastic elastomers are less resilient than the other two types. Polyvinyl chloride is the most common thermoplastic elastomer.

Several techniques are used to attach the material to the substrate and seal the seams between the sheets. Loose-laid-and-ballasted membranes are attached to the substrate only at the perimeters and around roof penetrations, with the material held down by smooth stone or paver blocks. Fully adhered membranes are attached with adhesives. Partially adhered installations are a compromise between the first two, with adhesive applied in strips or spots. Mechanically fastened systems use screws and reinforcement plates or battens to penetrate the membrane insulation and other roof components to hold the product to the deck. Subsequent sheets are lapped over the fasteners to avoid leaks at the penetration points.

-Ron Holzhauer, Managing Editor, 630-320-7139, rholzhauer@cahners.com

Plant Engineering magazine would like to thank Duro-Last Roofing, Inc., Firestone Building Products Co., and The Garland Co., Inc., for their special contributions to the development of this article.

For more information on types of roofing, visit the "Construction and buildings" channel on Plant Engineering Online at www.plantengineering.com.

Common roof problems

A number of problems attack the roof. Proper materials and installation minimize the possibility of their development.

Alligatoring is found on smooth surface asphalt roofs. Constant exposure to the sun forms deep grooves similar to an alligator's skin pattern. Asphalt emulsions fill the grooves and return the roof to it original look.

Blistering is usually caused by the presence of moisture beneath the surface. Water develops vapor pressure between the felt plies on either gravel or smooth-surfaced roofs and forms a bubble. Proper installation usually eliminates this problem.

Dry felts and bare spots result from oxidation of a gravel roof's bitumen surface. Felts become exposed to UV rays and deteriorate. Roof resaturants help alleviate this condition.

Splits are caused by excessive movement in the roof system. Constant expansion and contraction accompanying freeze-thaw cycles bring about the movement. Proper adhesion and reinforcement control this situation.

Wet insulation loses its insulating value and prevents the roof from performing one of its primary duties. All moisture trapped in the roof system must be removed. Replacing the insulation is the best approach.

Worn flashings lead to total roof failure. Emulsions, cements, and coatings help bring flashings back to their optimum condition.

Inspecting the roof

The roof begins to deteriorate immediately after it's installed as weather and environmental factors begin their attack. A regular program of preventive and corrective maintenance enables most roofs to last 20 yr or more. It costs just a few cents per square foot per year for maintenance, while replacing a neglected roof might cost up to $10/sq ft.

The roof should be inspected shortly after it is installed, and at least twice a year thereafter. In the spring, check for possible damage that occurred during the winter and to prepare for the rainy season. In the fall, get ready for the problems of winter. Also look over the surface after any severe weather or construction activity on the roof, even if a semiannual inspection was recently completed.

Before heading to the roof, gather all past records and note any recent repairs or modifications Original drawings, specifications, work or purchase orders, warranties, inspection forms, photographs, and test results should be examined.

Check the plant interior for warped deck materials, discolored ceiling areas, peeling paint, or rust spots. Check all masonry above the roofline for cracks or water intrusion. Monitor the plant perimeter for potentially damaging signs, such as loose mortar or open joints.

Take a note pad, pencil, camera, and marking paint for the inspection on the roof. The perimeter, membrane, flashings, parapet walls, seams, and roof penetrations should be examined for any type of cracking, splitting, puncturing, or erosion problems. Most manufacturers offer an inspection checklist or form to assist in the specific features of the examinations.

Looking for leaks

Finding the exact location of a roof leak is often a difficult task, since the problem may not be right above where the moisture first shows up inside the plant. To begin the leak location search, remember that about 80% occur around the perimeter or at roof penetrations.

Determining when the leak starts helps pinpoint its location. If the problem occurs as soon as rain begins, the entry point is near where the water first shows up inside. A delay between the start of rain and its presence inside indicates the moisture has traveled a considerable distance. If water continues after the rain stops, the moisture is probably being absorbed into the insulation.

Flooding the roof or spraying water in an established pattern duplicates the effects of rain, and is useful when searching for a leak's origin in better weather conditions.

Confusing world of warranties

Roofing generally accounts for less than 5% of the plant's budget, yet is involved in over 50% of all construction litigation. Manufacturer warranties are provided for most roofing, sometimes at an additional cost, and often give the plant owner the feeling that he is well protected when problems develop. However, in reality, this piece of paper is not a security blanket for the owner, but issued to protect the manufacturer.

Roof warranties ensure manufacturer quality materials and installation, require an approved or certified roofing contractor, and provide specifiers with a third party reference when dealing with the effects of contractor proposed changes and the specified system's warrantability. Warranties typically run 10-20 yr.

The warranty states what is covered, but even more important are the exclusions and conditions that make the protection ineffective or null and void. The National Roofing Contractors Association (847-299-9070; www.nrca.net) publishes lists covering these two conditions.

Specific inclusions cover failure of material and/or metalwork not supplied by the manufacturer issuing the guarantee or warranty, and moisture entering the plant through any part of the structure or adjacent building. There are over 20 other specific inclusions commonly listed.

Conditions voiding the warranty include repairs, alterations, and additions without manufacturer approval; changes in building use; failure to submit accurate inspection reports or notify manufacturers of leaks within a prescribed time period; and a dozen others.

For maximum protection, read every line of the warranty, understand exactly what it says and does, and comply with and document all actions dealing with inclusions/exclusions. Employ only financially sound contractors and manufacturers. Use regular preventive maintenance and do not let small problems grow into catastrophes. Have the legal department study the warranty. In many cases, the law protects the plant owner better than a warranty.





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