By C. Merrill Moter
YOUR ROOFING SYSTEM is vitally important to the comfort and maintenance quality of your facility. When the system is in good shape, with no leaks or maintenance problems, building occupants don't spend much time thinking about the roof over their heads. But, give them that first unsightly ceiling or wall stain -- or worse yet, a rain-soaked office --and they realize the importance of an effective roofing system.
You already know its importance, so you probably have scheduled a check of the roof twice a year, in the spring and fall:
Savvy managers, however, will have a plan in place for the time when roof replacement must be considered.
Built-up roofing (BUR) is this country's oldest and most prevalent low-slope roofing system. The membrane consists of several layers of felts alternating with layers of bitumen and topped with a surface material that protects the membrane from ultraviolet rays, weather exposure and general impact. Most felts applied today consist of glass fibers; the rest are organic. The two basic functions of the felts are:
Two types of bitumen can be applied: asphalt and coal tar pitch. Both have been studied for potential health hazards, and OSHA requires special handling of coal tar pitch, as it contains a known carcinogen.
Because bitumen can deteriorate when exposed to ultraviolet radiation, the final coat is protected in one of several ways. Gravel can be embedded into the last layer of bitumen while it is still fluid. Or a mineral-surfaced cap sheet -- a thicker-ply felt that has already been set with an aggregate at the factory -- can be laid on top. Reflective coatings that minimize heat gain are also possible. With up to four layers of felts, plus the alternating bitumen, a backup layer is always present in case one fails or was incorrectly installed.
BUR systems are inappropriate for relatively steep slopes and do not perform as well when deck deflection is excessive.
Modified bitumen can be considered a cross between BUR and a single-ply system. Instead of alternating bitumen with felt in the field, as is done with BUR, this system applies modified bitumen to reinforcing sheets of glass or polyester in the factory for a thick, waterproof membrane.
The finished roof is constructed by applying 3-foot-wide strips of this membrane over a base sheet. The top, or cap, sheet often has a factory-applied granular or metal surface to block UV rays, reflect heat, improve fire resistance and add visual interest.
Some manufacturers offer several BUR layers of fiberglass-based felt topped with a modified bitumen cap sheet. Although more expensive than either BUR or modified bitumen alone, such a hybrid system is very durable. Other manufacturers are starting their systems with a traditional felt at the base, adding a modified ply, and then topping it with a modified cap sheet.
Single-ply roofing systems, as the name suggests, have a single layer of waterproof membrane rolled down in one application. The sheets can be weighted down with ballast, partially attached by mechanical means, fully adhered with adhesive, or -- for a protected membrane -- covered with insulation board and then ballast.
Many types of single-ply systems are available; most fall under two categories: elastomerics and thermoplastics. One of the elastomerics, ethylene propylene diene monomer (EPDM) is used most often. The most commonly used thermoplastic sheets are polyvinyl chloride (PVC) and PVC copolymers. The systems vary in their compositions, installation methods, costs and performances.
In general, single-ply systems offer some decided advantages. They are easier to install, requiring fewer people and providing cleaner application techniques than the bitumen-based systems. They can be economical on large roofs with few penetrations, since the large-size sheets require fewer laps. Also, because a kettle of hot asphalt is not required to install the single plies, their cold-roof applications can extend what is otherwise a short roofing season in cooler climates.
A word of caution: Single-ply should be of excellent quality and applied by an experienced crew for adequate durability.
Sprayed-on systems usually consist of a foamed-on insulation with a silicone or vinyl coating applied over the insulation. These systems can be applied over unusual or irregular shaped surfaces.
While we don't recommend applying any roofing material over wet subsurfaces or deteriorated exposed surfaces, these materials can provide useful temporary solutions to unusual problems.
Metal roofing has increased in popularity in recent years as a reroofing material. Metal roofing has been used above existing roofing surfaces.
Supporting the additional weight of new materials and substructure is an important consideration.
Connections at the roof's edge and at penetrations must be carefully detailed to provide a watertight job. The life of metal roofing can be an attractive consideration in selection.
Our experience with roofing systems of all types, on all types of buildings over many years, leads us to caution against relying too heavily on any warranty. A warranty is not a satisfactory substitute for good roofing design and a good roofing contractor. More important points to consider are a roofing system's performance and a manufacturer's track record.
C. Merrill Moter is the owner and principal architect of Joseph & Joseph Architects, Louisville, Ky. Established in 1908, the firm serves commercial and institutional facilities nationwide.
If your roof has a history of problems or has reached the end of its designed life, it is time to determine if it should be repaired or replaced. For help in that decision, check with an architect, roofing contractor, roofing consultant or roof testing service. Any of these experts should be able to advise you on these replacement considerations:
We recommend two investigative tests: infrared inspection and asbestos testing. The infrared inspection shows if moisture has accumulated in the insulation or penetrated the system through the vapor barrier or the top membrane. This is important when considering the life of the present roofing, and the possibility of roofing over the existing membrane. The asbestos test indicates if asbestos-containing materials have been used and, if so, the project will require additional care and expense.
By C. Merrill Moter