BY RONALD R. SPADAFORA
The use of the saw-tooth design to exploit daylight more effectively is not new. It has been around for centuries. During the American Industrial Revolution, saw-tooth roofs were first built to illuminate factories. This was especially true of textile manufacturing mills in New England. Although daylighting techniques (see “Sustainable Design”) are centuries-old, they have been “rediscovered” and have regained popularity with 21st century architects. Benefits from daylighting can be readily reviewed and analyzed by design professionals using computers and industry software tools. Natural lighting in industrial, commercial, and institutional modern buildings is commonly provided by roofs with transparent surfaces (photo 2). Saw-tooth roof design uses opaque building materials combined with inclined glazing. The dimensions of saw-tooth roofs are critical in terms of vision quality and energy savings.
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| (1) The author is standing at the apex of a saw-tooth roof section. Firefighters should not place themselves in this position during hoseline operations. (Photos by author.) |
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| (2) The Cooper Union (Landmark Foundation Building) for the Advancement of Science and Art, one of the nation’s oldest institutions of higher learning, located in the East Village, New York City, reveals its saw-tooth roof silhouette when observed from the Northwest. |
DESIGN FEATURES
The saw-tooth roof gets its name from its peculiar profile, which is similar to the teeth in an upward-facing saw blade. Saw-tooth roof design uses a number of parallel roof sections of triangular shape. They can be built on site of wood, steel, or reinforced concrete. Some saw-tooth roofs are preengineered off site where they are manufactured using separately molded structural members. Major components of the saw-tooth are the following.
Sloping Roof
Sloping, opaque roof surfaces in a row constructed of combustible or noncombustible material are the trademark characteristic of the saw-tooth roof. Angles vary depending on the span they cover. Steep-angled roofing material will seriously affect the pace at which firefighters can perform the work required at fire and emergency operations.
Glazing
The major advantage of installing a saw-tooth roof on a structure is the uniform diffusion of light throughout the space below it. This allows the entire interior area of the building to be usable to the owner. It is the ideal solution to the problem of lighting large-dimensional buildings (lofts, machine shops, warehouses, and factories). Saw-tooth roofs prevent the influx of direct sunlight while providing northern light. In saw-tooth design, every bay has an angled skylight. Less glare and unwanted heat also offer better working conditions, increased production, and a reduction in sick leave hours. Today, however, many saw-tooth roofs have had their glazing removed for security reasons and are covered over with opaque roofing material.
A sloped glazing is typically installed within a metal or a wooden sash, facing away from the equator side of the building. Glass used for saw-tooth roofs may be plain or reinforced with wire mesh. The mesh is inserted between two plates of glass, making it much more durable. Double-glazing with an airspace between the panes may also be found. Glass will be single strength (1⁄16 inch, approximately 3 feet × 5 feet) or double strength (1⁄8 inch, approximately 4 feet × 8 feet). The glass is inclined to the vertical to take advantage of the brighter light in the upper sky. Firefighters venting on the roof must be in communication with interior forces. The area inside the building directly below the vertical glass panes being broken must be confirmed to be clear of all operating personnel.
The saw-tooth’s glass side may contain operable louvers at the upper section of the sash. These louvers should be opened manually to facilitate vertical ventilation when warranted. From inside the building, adjustable louvers may have hand cranks or small motors, which, when activated, open them over large sections of the roof.
Valley Gutters
Valley gutters are rain trough areas separating one saw-tooth section from another. They are on both sides of the apex of the saw-tooth. The valley of the saw-tooth is commonly used by firefighters to transverse the roof. Ice and snow naturally falling off the glazing and roof surface of the saw-tooth accumulate in this area. Typical widths of the valley gutters range from one to two feet, although much larger widths are common in new construction. Narrow gutters and clogged drains may cause large amounts of ice and snow to build up in this section of the roof during winter months in northern climates. This will lead to dangerous operating conditions on the roof. Members must move slowly, preferably in a crouched position, to ensure secure footing and a low center of gravity in the event of slippage.
Trusses
Saw-tooth roofs may be constructed of heavy timber1 or steel trusses (Figures 1, 2). These special type “inverted” trusses do not have a telltale peak to warn firefighters of their existence. In addition, heavy timber trusses may use metal rods for the bottom chord elements to carry tension loads to columns and bearing walls. With all truss roofs, sudden collapse without warning is always a threat should the fire compromise the strength of any one component. For this reason and for fear of cutting into the structural top chord of the truss, do not attempt to use power saws on a saw-tooth roof of truss construction to accomplish vertical ventilation.
| Figure 1. A Heavy Timber Truss Saw-Tooth Roof |
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| The bottom chord component of this truss consists of a metal tension rod.2 |
| Figure 2. A Steel Frame Truss Saw-Tooth Roof2 |
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Note: Type IV-Heavy Timber (also known as “mill” construction) structural, load-bearing elements have a minimum nominal dimension of six inches. Exterior masonry walls are two-hour-rated with one-hour-rated heavy timber floor/ceiling/roof assemblies.
Reinforced Concrete
In reinforced concrete constructed buildings, a typical arrangement for this type of saw-tooth roof has a reinforced concrete roof slab (the long side of the saw-tooth assembly) supported at the top by inclined steel posts. The glazing is installed between the roof slab and the posts. For large slabs, splicing of the steel reinforcing bars is required. The slab and posts are supported by steel beams that are supported by columns (Figure 3). Reinforced concrete saw-tooth will negate the possibility of cutting into the roof.
| Figure 3. A Reinforced Concrete Saw-Tooth Roof |
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SIZE-UP
The saw-tooth configuration on the roof may be completely invisible from the vantage point at the front of the building. High facades, ornamental cornices, parapets, and communications/data equipment may hide it (photo 3). The saw-tooth may be spotted, however, from the sides or rear of the building by members at the scene or by responding units en route to the incident (photo 4). The existence of a saw-tooth roof on a building must be communicated to the incident commander (IC), all units at the scene, and responding companies.
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| (3) The parapet of this storage warehouse conceals a saw-tooth roof. |
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| (4) Responding units en route to the incident may spot a saw-tooth roof. |
FIREFIGHTING OPERATIONS
The success of firefighting operations in buildings having a saw-tooth roof depends on responders’ having a keen knowledge of its important construction features. Ignited combustible roofing material will generate large volumes of smoke, which can hamper visibility. Preferably, hoseline stretches to the roof should use apparatus and portable ladders since interior access may not be readily discernable. Firefighters stretching a hoseline on the roof will be hard-pressed to maneuver it in and around the many peaks and valleys. Tight supervision of hoseline stretches and use is essential for an effective and safe operation. Evaluating the roof prior to stretching hoselines is critical to avoid positioning firefighters in precarious areas such as on steep peaks or roof edges. Moving deliberately and spreading out on the hoseline are two additional ways to enhance safety. Ensure all information concerning hoseline operational difficulties or delays in getting water on the fire are communicated to the IC.
Ventilation
Primary vertical arteries (skylights, scuttles, roof access doors, and staircase bulkhead doors), when approved by the IC, should be opened first during fire operations. Removing glazing from the saw-tooth roof will, in most cases, also effectively assist in relieving the structure of heat, smoke, and hot gases. Wind blowing from the north or in the direction of angled glazed openings, however, could inhibit the upward flow of the products of combustion out of the openings. In this instance, for nontruss saw-tooth construction that incorporates either a row of columns or supporting wall partitions that run under the low part of the saw-tooth roof, cutting a hole through the adjacent roofing material, where feasible, may aid in alleviating this operational problem. Roofing materials may include wood planking, metal, bituminous mixtures, rubber, asphalt, gravel, and insulation requiring saws, axes, prying tools, and pike poles. This work is labor intensive and involves the services of many firefighters under the supervision of a designated company or chief officer. Ensure all information concerning difficulties ventilating the roof and the removal of glazing is transmitted to the IC.
Saw cuts into the solid roof surface of the saw-tooth will necessitate using the saw at shoulder level and in positions other than a horizontal surface. In this situation, the saw blade guard may have to be adjusted prior to use to better protect the operator from roofing material being kicked up. The firefighter making the cuts should be backed up by a firefighter positioned directly to the rear. This will enhance the stability of the operator during the cutting procedure.
Make saw cuts in a triangular fashion (similar to cutting a roll-down gate) with the apex angle at the top of the individual saw-tooth bay. This ventilation hole can then be expanded laterally, should fire conditions dictate, along the length of the roofing material parallel to both the top and the base of the bay (photo 5). Roof ventilators may also be found installed through the angled roof surface. These devices can be removed and their openings expanded to help improve vertical ventilation.
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| (5) This triangular ventilation hole has the apex angle at the top of the individual saw-tooth bay. It can be expanded laterally should fire conditions dictate. |
Safety
Saw-tooth roofs can prove difficult to ladder properly, which can affect safe access and egress from the roof. As stated previously, it may be extremely difficult to get to the roof through the interior of the building. Often, a straight ladder leading to a panel roof door will allow members to set foot on the valley gutter (photo 6). Slipping in the valley gutter can cause a firefighter to slide off the side of the roof.
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| (6) Interior access to the door off a vertical ladder reveals a narrow valley gutter of the saw-tooth roof. |
Once on the roof, the saw-tooth is a myriad of high points and valleys that will be difficult to transverse. Steep angles when climbing or descending roofing material are a fall hazard as well as an ankle/knee strain and sprain risk. High points along the apex of each saw-tooth may be wide enough to walk on. Don’t do it, however; one misstep could cause you to fall.
Do not step on glazing at any time to avoid being cut by broken glass or, in a worst-case scenario, falling through the glazing into the interior of the building. Some saw-tooth roofs have bays that abut on an exterior wall. These roofs are exceptionally dangerous. A firefighter climbing up roofing material could step over the apex and tumble forward on covered-over glazing to the ground below (photos 7, 8). This is a definite possibility under fire/smoke conditions or at any time during nighttime hours.
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| (7) This saw-tooth roof has a bay that abuts on an exterior wall. A firefighter climbing up roofing material could step over the apex and tumble forward on covered-over glazing to the ground below. |
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| (8) This is a view of the saw-tooth roof in photo 7 from across the street. A fall from this roof would most likely be fatal. |
Saw-tooth roofs can present unique construction features that may affect your standard operating procedures. Familiarization drills and preplanning are musts at all levels to enhance effectiveness of size-up and safety during extinguishment and ventilation operations. Information regarding the framing and construction of the roof, ventilation options, and unique hazards associated with the saw-tooth roof should be discussed. In the age of sustainable design and “green building,” many of these roofs are being restored to their former function and new ones are being installed.
REFERENCES
1. International Code Council (ICC), International Building Code (IBC) ICC: Washington, DC, 2012.
2. Kidder, Frank E., CE, Ph.D., (compiled by a staff of specialists, Thomas Nolan, editor-in-chief). The architects’ and builders’ pocket-book: a handbook for architects, structural engineers, builders and draughtsmen, 16th Edition. New York: John Wiley & Sons, Inc. 1916.
3. Monash, John, “Engineering enterprise prior to WW1,” Retrieved 10/22/13 at: home.vicnet.net.au/…/jm/bldgtext/bldgs08.html.
Sustainable Design
Sustainable design seeks to enhance building performance, thereby improving the health and comfort of building occupants while reducing the negative impact on the environment. Basic objectives of sustainability include optimizing construction site potential, lessoning the consumption of nonrenewable resources, minimizing waste, decreasing nonrenewable energy consumption, protecting and conserving potable water supplies, enriching the indoor environment, utilizing environmentally friendly products, and optimizing operational and maintenance practices.
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| The purpose of a daylighting system is to provide natural light to the interior of a building to enhance the visual performance of its occupants. |
DAYLIGHTING
Daylighting is using windows, openings, or reflective surfaces during daytime hours to provide supplemental, effective internal lighting. It uses the sun as a source of light but avoids letting direct sunlight into a building. It allows the designer of the building to maximize visual comfort while reducing energy use. Natural light is economical, making the structure less reliable on artificial lighting fixtures. Energy savings are achieved through the diminished need for electric lighting as well as fossil fuels in HVAC applications.
RONALD R. SPADAFORA is a 35-year veteran of the Fire Department of New York (FDNY), where he is an assistant chief in the Bureau of Operations. He writes extensively for and is editorial advisor of WNYF, the official training publication of the FDNY. His most recent book Sustainable Green Design & Firefighting: A Fire Chief’s Perspective was published in 2013.
Ronald R. Spadafora will present “Firefighting Strategy and Tactics for Green Building Construction” on Wednesday, April 9, 10:30 a.m.-12:15 p.m., at FDIC 2014 in Indianapolis.
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