Residential Rainwater Gutter Systems
By Ron Mueller
June 11, 2014
Rainwater gutter systems, including downspouts and leaders, are common features installed on many single-family homes, townhomes, and multi-family residences with sloped roofs. These serve an important function in collecting, controlling, and directing rainwater away from the building, thus reducing the likelihood of water damage to building components. As prevalent and important as these systems are, many people are surprised to learn that gutters are not required by the model building codes, except for locations with rare soil conditions. Surprisingly, even where they are required, building codes do not prescribe the parameters for their design or the minimum requirements for installation and performance. Guidelines for the design and installation of gutter systems have been published by various industry organizations; however, there is a great deal of inconsistency and contradiction between them, and very little evidence of acknowledgement by manufacturers and installers. Most disturbing is the lack of attention to gutter systems when specified by design professionals. A single source of comprehensive design guidelines and installation standards should be developed and adopted by the model building codes.
Gutter Function and the IRC Building Code
When installed, gutters collect rainwater runoff from sloped roofs, which directs the water to downspouts that conduct the water vertically, from the roof edge to the ground. The importance of discharging water away from buildings to prevent water infiltration through basement walls and damage to building foundations is widely acknowledged in the construction industry. The International Residential Code (“IRC”) ; however, requires controlled collection and discharge of roof run-off only in locations where “expansive or collapsible soils” exist.
These soils types are susceptible to extreme volumetric changes when large quantities of water are introduced. Geological maps prepared by the Army Corp of Engineers and others indicate that the occurrence of these types of soils are extremely rare throughout much of the United States east of the Mississippi river. For the majority of American households, the IRC allows for the uncontrolled drainage of water directly off the edge of sloped roofs. Protection of the building from water deposited from roof runoff is only provided by the code through a requirement for grade surrounding the foundation to be sloped away from the building.
A major concern in building construction is the prevention of water infiltration through the building envelope. The ASTM E 241 Standard Guide for Limiting Water-Induced Damage to Buildings  suggests that an effective method to minimize the amount of water that contacts the exterior cladding and thereby reduces the opportunity for water infiltration includes the provision of roof overhangs or gutters. Where the IRC does not require gutters, it also does not include an alternate requirement for installation of a minimum sized roof overhang.
Advocacy for Gutter Systems
There is a growing consensus in the building industry to acknowledge the necessity of gutters in promoting durable construction and healthy habitats. At the Federal level, both The Environmental Protection Agency  and The U.S. Department of Housing and Urban Development  have recognized that improperly installed gutter systems can damage exterior walls, direct water into the structure, and lead to moisture problems including the formation of mold. Gutter systems are now commonly recommended in green building and energy efficiency standards including those published by National Association of Home Builders, U.S. Green Building Council, Energy Star, and the Energy & Environmental Building Alliance.
Available Gutter Standards
Gutter system manufacturers and installers do not appear to have a common source for design and installation standards. When reviewing the websites of gutter system manufacturers and installers, one is confronted with a great deal of conflicting information regarding ‘standard’ installation procedures such as hanger spacing, types of hangers, etc. Most admit to using “rule of thumb” shortcuts for sizing gutters and downspouts based on roof area.
Several trade organizations have published guidelines for the design and installation of gutters; however, these contain inconsistent and conflicting recommendations. Guidelines published by SMACNA , the Copper Development Association (“CDA”) , Architectural Graphic Standards , and HUD [8,9] provide directions for determining the necessary capacity and sizing of gutters and downspouts; however, these guidelines also have many variations among them. For example, some guidelines suggest that the capacities should be adjusted for roof slope while others do not. All offer a variety of rainfall rates on which to base the calculations; however, none state definitively which should be used, leaving that decision up to the “designer”.
Gutter System Design
In many cases for residential construction, the installing contractor is or becomes the “designer”. Unfortunately, this is true even for construction where gutter systems are called for by a design professional. Many times construction documents indicate that gutters and leaders are to be installed, yet no size is specified, or worse, the documents simply state use of one “standard” size, typically 5” gutters with 2” x 3” downspouts.
With “complicated” roofs being the trend in residential design, the lack of consideration for roof drainage has become evident. Intersecting gables, dormers, and valleys all concentrate drainage from large roof areas to shortened roof edges. Often the length of eave available for a gutter is insufficient for a “standard” gutter and downspout to handle the runoff capacity. The deficiency in the gutter system design is often not evident until the first big rain comes and overflowing gutters cause all of the landscaping at the base of the wall to wash away in the summer or icicles of significant proportions form in the winter.
The conditions where gutter systems are required should be greatly expanded in the building code to include the necessity of systems which minimize exposure of the exterior cladding and soils surrounding the structure to excessive water. Additionally the code should define the rainfall rates for which the gutter system capacity should be calculated and place this responsibility on the designer of the roof. SMACNA, CDA, Architectural Graphic Standards, and HUD have all produced viable guidelines; however, a consensus standard should be developed with unified recommendations for the design and installation of gutter systems based on the results of scientific testing. This standard should be adopted by reference in the building code.
Until industry standards and resources are consolidated, and the requirements are incorporated into the building code, contractors, installers, and designers should recognize the responsibility for the proper design and specification of gutter systems. Where gutters are utilized, but not required by the code, it is imperative that they function properly and not contribute to the deterioration of other building elements. Without specific code requirements, consumers, unless they self educate themselves are at a disadvantage and susceptible to the purchase of inferior or inadequate gutter systems. This may occur not only due to the activities of an unscrupulous contractor, but also from those who may be well intentioned such as contractors or designers that are misinformed or unaware of the standards and guidelines available to them.
List of References
 International Code Council, International Residential Code, 2009
 ASTM International, E241-04 Standard Guide for Limiting Water-Induced Damage to Buildings, 2004.
 U.S. Environment Protection Agency, Mold Remediation in Schools and Commercial Buildings, 2008
 U.S. Department of Housing and Urban Development, Residential Rehabilitation Inspection Guide, 2000
 Sheet Metal and Air Conditioning Contractors National Association, Inc., Architectural Sheet Metal Manual, 2003
 Copper Development Association, Inc., www.copper.org
 Architectural Graphic Standards, Ramsey/Sleeper, Eleventh Edition, 2007
 U.S. Department of Housing and Urban Development, Durability by Design, 2004
 U.S. Department of Housing and Urban Development, Moisture Resistant Homes, 2006
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