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After the roof system is selected and the specifics of the system (such as deck type, insulation type(s) and thickness, fastener patterns, and warranty requirements) are developed, it is necessary for the roof designer to determine what details are needed and to design the details so that they are suitable for the project conditions.
Roof Plan: A roof plan should be drawn to scale and be sufficiently large to adequately convey information. It should show all penetrations and all expansion, seismic and area divider joints. The slope directions and approximate amount of slope should also be shown. The different wind uplift areas (field, perimeter and corners) should also be shown and dimensioned. References to all penetrations, roof edges and roof-to-wall details should also be indicated on the plan. (Note: For some standard details such as plumbing vents and roof drains, rather than include the detail on the drawings, referencing the manufacturer’s detail in the specification is typically sufficient unless project conditions require enhancement to the standard detail). An example roof plan is shown in the Typical Roof Plan Layout detail below. If a manufacturer’s standard detail is referenced, the exact detail needs to be appropriate for the condition.
Details: Details should be drawn to scale and should be sufficiently large to adequately convey the information. Illustrating details in section typically suffices. However, with complicated details, an isometric drawing may be needed.
At parapet and roof-to-wall details, the low-point of the roof should be drawn and a dashed line placed at the high point if the roof elevation varies. See Parapet Wall Schematic detail below.
After details have been drawn, it is recommended that they be reviewed by the specified manufacturer(s) prior to bidding. Care should be taken to avoid drawing manufacturer-specific details that can only be met by a single manufacturer, unless a proprietary specification is intended.
Manufacturer’s Standard Details: Nearly all roof membrane manufacturers publish standard details to guide the proper installation of their products. These details should be considered as only a starting point in creating project specific details. The standard details produced by manufacturers represent the lowest acceptable level of performance. Due to competitive market pressures manufactures must keep the base price of their systems low and therefore they keep the standard details as simple as possible. The manufacturer can take an informed risk about the chances of a particular detail performing versus the cost of warranty repairs. Of course, it is the building owner who experiences the leak and suffers the bulk of the damages from this risk evaluation.
Some manufacturers publish enhanced details for high-wind warranties or for premium products. It may be advantageous to include these enhanced details into typical projects for improved performance.
It is important for design professionals to recognize which aspects of a manufacturer’s details can be altered or enhanced and which aspects must be left to the manufacturer. Typically, issues such as seaming, edge attachment, base flashing and drains must comply with the manufacturer’s details. On the other hand, the conditions on the standard details related to the interface with adjacent construction can be modified to include performance enhancements such as tighter air sealing, elevating base flashing on tapered edge strips, extending base flashing higher than minimums, and requiring redundant seals at penetrations.
In conclusion, do not rely exclusively on manufacturers’ standard details and do not allow custom details to be substituted for standard details during construction, unless they are an improvement. The evaluation of the details requires judgment, experience, and expertise in the systems being used.
Reference Details: There are a variety of sources to draw upon for design of details. Various details for low- and steep-slope systems are included in The NRCA Roofing Manual. The NRCA details are available on a CD-ROM and should be utilized as described herein. These details can be modified with CADD or incorporated directly into contract drawings. The NRCA details are widely accepted in the roofing industry and are generally recognized as being suitable for standard conditions.
The NRCA Roofing Manual: Roofing-system-specific details are available from NRCA.
This is one of the most widely recognized technical publications in the U.S. roofing industry and provides extensive information about the design, materials and installation techniques applicable to almost all types of roof systems. It contains the following four volumes:
One volume of the manual is updated each year.
The details included in the 4 volumes of The NRCA Roofing Manual are available on CD. This CD can be used with AutoCAD® software to customize the construction details to fit your specific project needs.
Two noteworthy NRCA details are in the Chapter 10 details in The NRCA Roofing Manual: Membrane Roof Systems. They are the details for equipment support stands and the guide for clearances between pipes/walls/curbs.
1. Detail for Equipment Support Stands: This detail provides recommendations for column height as a function of the width of the equipment. By following this guidance, stand-mounted equipment will be mounted high enough to allow roof mechanics sufficient room to work underneath the stand to properly install the new roof and future reroofing.
2. Guide for Clearances Between Pipes/Walls/Curbs: This provides minimum clearances between adjacent penetrations and between penetrations and roof edges. It is recommended that the guidance should be included in the contract drawings or referenced in the specifications).
Also note that the some NRCA details show a surface mounted termination at rising walls (base flashing condition) while other details that connect the roof membrane through the wall to the weather-resistive barrier on the backup wall (through-wall flashing). The through-wall flashing-style details are generally considered more durable and should be used wherever possible.
The Sheet Metal and Air Conditioning Contractors’ National Association, Inc. (SMACNA) Architectural Sheet Metal Manual also has details that are widely accepted and generally recognized as suitable for standard conditions.
AIA’s structural Graphic Standards also includes a variety of low- and steep-slope details. However, unlike the NRCA and SMACNA details, the details in Architectural Graphic Standards have not undergone extensive industry review. While several of the details are suitable for standard conditions, some may be inadequate. Hence, it is recommended that the NRCA details be utilized instead.
Manufacturers of roofing products also promulgate standard details. These may also be suitable for standard conditions. Many of these details are available in CADD. However, manufacturer’s details typically include propriety names for various products used in the assembly. Hence, modification of the details to delete propriety names will typically be necessary.
Modifying Reference Details: Whenever reference details are considered for inclusion in the contract drawings (or via reference in the technical specifications), the roof designer should determine whether or not the standard detail needs to be modified to account for unusual weather or building conditions. Standard details typically provide suitable performance when properly executed, provided the weather at the site and the building itself is “standard.” If unusual weather conditions are expected during the life of the roof (such as very high wind loads, frequent wind-driven rain, accumulation of slush under snow), standard details may need to be modified to accommodate the non-standard conditions in which they will be required to perform in. For example, in areas where deep accumulation of slush under snow is anticipated, the height of base flashings should be increased above the typical 8″ (200 mm).