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General Design Issues
This section provides a summary of major issues that should be considered by the designer. Much of the information is based on successful historic practice. The majority of the issues presented are related to the climatic conditions of the site and the configuration of the building.
- High Winds
In areas where high winds occur, the design of roofing systems must be carefully evaluated. High winds can place extreme positive or negative pressures on roof edges, such as, ridges, ridge vents, eaves, rakes, copings, gutters, and fascias. Under these conditions it is particularly important to ensure that the edges of roofing and flashing are securely fastened.
In areas of high winds and rain special care must be exercised in designing transverse seams and flashing. The headlap may be increased in these conditions, to reduce the likelihood that moisture will be driven past the roofing membrane.
- Heavy Rain
Where heavy rain is expected, special attention should be given to roof slopes, headlap, seam details, valleys, gutters and downspouts. Unless soldered flat seam roofing is used, low slopes, which do not promote positive drainage, should be avoided. The headlap of transverse seams or overlapping roofing components (at a valley, for example) can be increased, for improved protection. Increased seam heights can help prevent water infiltration. Seams may also be filled with sealant or fully soldered, to provide a watertight barrier. Valleys, gutters, and downspouts should be sized adequately, and their design should account for the volume, speed, and direction of the expected water flow.
- Ice and Snow
In areas where ice and snow may be severe, the designer should carefully consider their effect on all components. Many of the problems associated with ice and snow are caused by damming. This often occurs when snow on a roof, over heated space, melts and flows to the eaves. The roof temperature at the eaves can be significantly lower, especially if the roof has an overhang. The flowing water can freeze and form a dam which prevents proper roof drainage. These conditions can be intensified by roof shapes, such as valleys.
Eave snow flashing may be used to make the roof perimeter watertight. The headlap of roofing material over valley flashing, roof edge strips, gutter aprons, and other elements may be increased by extending the underlying copper material higher up the roof. The amount of increase is related to the slope of the roof or valley and the likelihood of a dam condition. Other steps that increase protection under these circumstances include: increased seam height and the use of solder or sealant in seams. In areas of extreme ice and snow conditions, heating elements can be installed on eaves and in gutters to help prevent ice build-up.
Another potential problem with ice and snow build-up is its weight. If snow accumulates on a sloped roof, it can slide down its surface with tremendous force, damaging the roof, eaves, or gutters. In areas where ice can accumulate in gutters, the proper design of gutter support is very important. The use of snow guards is neither supported nor opposed, rather their use must be determined and evaluated for each specific application.
- Temperature Range
When designing copper roofing systems or accessories, the effect of temperature changes should be considered. The expansion and contraction of copper material, as well as that of any other adjacent material, is a function of variations in temperature. The temperature during installation can also play an important role, since future thermal expansion and contraction will occur as the temperature rises and falls relative to this initial value. This is particularly important when installing components with potential restrictions to movement in one direction. For more information on expansion and contraction, see Roofing Systems—Long Pan.
The effects of direct solar gain should be carefully considered. The surface temperature of exposed copper can be significantly higher, by 70 to 80 degrees or more, than the surrounding ambient temperature. The effects of this additional gain, commonly called "superheat", must be anticipated, as increased thermal movement will occur.
- Building Orientation
Consideration should be given to the relationships between roof components, such as the ridge, eave, parapet, and the direction of prevailing wind, rain, and sun. The intensity of many issues discussed above will depend on the exposure of each roof component. For example, ice build-up on a gutter is more likely to occur on the North side of a building or a partially shaded area of an eave. Wind can exert extreme positive pressure on a coping in one area and negative in another.
With good design, staining from run-off can be minimized or eliminated. In brief, staining occurs when moisture that has been in contact with copper, drips off onto adjacent porous materials. The primary methods of preventing staining include the use of gutters, architectural overhangs, drip edges, silicone sealers, and lead coated copper. In addition, the designer can try to divert water away from places where it may cause stains. Where parapet covers or copings are used, for example, the top surface should be sloped toward the roof side, to minimize the amount of water that drips off the outside face.
The natural weathering process that leads to the patination of exposed copper surfaces usually takes many years to complete. A number of processes have been developed to quickly produce a patina. The industry has continued its development efforts in this area, and occasionally new methods are introduced. Please contact CDA Project Managers & Architectural Applications Specialists for current information.
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