United States Department of Agriculture 558-CPS-1

NRCS, NHCP

July 2021

NRCS reviews and periodically updates conservation practice standards. To obtain the current 

version of this standard, contact your Natural Resources Conservation Service State office or 

visit the Field Office Technical Guide online by going to the NRCS website at 

https://www.nrcs.usda.gov/ and type FOTG in the search field. 

USDA is an equal opportunity provider, employer, and lender.

Natural Resources Conservation Service 

CONSERVATION PRACTICE STANDARD 

ROOF RUNOFF STRUCTURE 

CODE 558 

(no)

DEFINITION 

A structure or system of structures to collect, control, and convey precipitation runoff from a roof. 

PURPOSE 

This practice may be applied to achieve one or more of the following purposes: 

• Protect surface water quality by excluding roof runoff from contaminated areas 

• Prevent erosion from roof runoff 

• Increase infiltration of roof runoff 

• Capture roof runoff for onfarm use 

CONDITIONS WHERE PRACTICE APPLIES 

Where roof runoff from precipitation needs to be— 

• Diverted away from a contaminated area. 

• Collected and conveyed to a stable outlet or infiltration area. 

• Collected and captured for other uses such as evaporative cooling systems, livestock water, or

irrigation. 

CRITERIA 

General Criteria Applicable to All Purposes 

Plan, design, and construct the roof runoff structure to comply with all Federal, State, and local laws and

regulations. Notify landowner and/or contractor of their responsibility to locate all buried utilities in the

project area, including drainage tile and other structural measures. The landowner is also required to

obtain all necessary permits for project installation prior to construction. 

Evaluate the condition of the existing roof structure prior to installation of a gutter. Install new fascia

boards as needed to support gutters and downspouts for the practice life span. Mount gutters on plumb

fascia boards. If the building does not have a fascia board, mount to the rafter ends, adding any

necessary appurtenances to ensure the gutters are securely attached and positioned to collect runoff from

the roof. 

Ensure that the gutter support system will withstand the anticipated loading from precipitation including

loads from snow and ice, where applicable. Where snow and ice are expected, install roof gutters below

the projection of the roof line. If this is not possible with the existing roof, install rigid supports or

wraparound straps. If structural supports are missing or insufficient, design the required supports for the

gutter. 
Protect the downspouts, laterals, and cross-pipe pipelines from damage by livestock or equipment with

heavy-walled pipe, fencing, or other protective measures to exclude livestock and equipment. 

Gutter design capacity 

To exclude roof runoff from manure contamination, design roof runoff structures to convey the peak flow

from a 25-year, 5-minute rainfall event. (Refer to NRCS National Engineering Handbook (Title 210), Part

651, Agricultural Waste Management Field Handbook, Chapter 10, Appendix 10B.) 

For other applications, design roof runoff structures to convey the peak flow from a 10-year, 5-minute

rainfall event. 

Downspout 

Design downspouts, collector pipes, lateral downspouts, or cross-pipes with a capacity that equals or

exceeds the designed roof gutter flow rate. If downspouts drain directly onto the ground, use an elbow and

energy dissipation device at the outlet to provide erosion protection and direct water away from the

foundation of the structure by ensuring that ground slopes away from the building. 

Ground gutter 

If roof gutters are not feasible, ground gutters may be used in some instances. Ground gutters may not be

practical for livestock housing where the purpose is to exclude roof runoff from contaminated areas. 

Use ground gutters only on buildings with eaves that extend 12 inches or more horizontally from the

building. Where runoff from the roof eave drops onto the ground surface, provide a gutter with the same

capacity as required for roof gutters. Ground gutters must convey runoff away from the building to a stable

outlet without erosion. 

Ground gutters can be rock-lined channels, rock-filled trenches with subsurface drains, or concrete

channels. 

Outlet 

Roof runoff can empty into a subsurface drain, underground outlet, a storage tank, a dry well, or onto an

energy dissipation device as described above in section “Downspout.” 

Size outlets to handle the design flow from the gutter system. Provide accessible cleanouts for subsurface

drains, underground outlets, and storage tanks that are used as outlets. 

Use NRCS Conservation Practice Standard (CPS) Subsurface Drain (Code 606) to design subsurface

drains if necessary to dewater ground gutters or infiltration ditches. 

Use NRCS CPS Underground Outlet (Code 620) to design underground outlets to convey roof runoff to a

stable outlet. In cold climates, ensure underground outlets are deep enough to avoid freezing or include a

method to bypass the outlet without damage to the downspout. 

Materials 

Roof gutters and downspouts may be made of aluminum, galvanized steel, plastic, or wood. Aluminum

gutters must have a minimum nominal thickness of 0.027 inches. Aluminum downspouts of 3 inches by 4

inches must have a minimum nominal thickness of 0.019 inches. Aluminum downspouts larger than 3

inches by 4 inches must have a minimum nominal thickness of 0.024 inches. Galvanized steel gutters and

downspouts must be a minimum of 28 gauge. Plastics must contain ultraviolet stabilizers. Wood gutters

must be made of rot-resistant wood free of knots. 

To prevent corrosion, avoid contact between components of dissimilar metals. 

To improve infiltration for rock-filled trenches and dry wells, use poorly graded gravel. 

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Where traffic, climatic, or other conditions necessitate the use of reinforced concrete for channels, pads,

and slabs, refer to NRCS National Engineering Manual (NEM) (Title 210), Part 536, Section 536.20,

“Design Criteria for Reinforced Concrete,” for design and installation of reinforced concrete. 

Where nonreinforced concrete is acceptable, refer to NRCS 210-NEM-536, Section 536.22, “Design

Criteria for Concrete Slabs-on-Ground,” for design and installation of nonreinforced concrete. 

Additional Criteria to Increase Infiltration 

Increase runoff infiltration by directing flow to existing vegetation or infiltration features (e.g., lawns, mass

planting areas, existing natural areas, infiltration trenches, dry wells, rain gardens, or natural areas).

Ensure these areas have the capacity to infiltrate the runoff without flowing directly to surface or ground

water, causing excessive erosion, or adversely affecting the desired plant species. 

Additional Criteria to Capture Water for Other Uses 

Roof runoff can be contaminated with environmental pollutants that have settled on the roof between

runoff events. This may make the captured runoff unsuitable for uses such as drinking water for livestock

without treatment. The operator is responsible for ensuring that the quality of the runoff is suitable for the

intended purpose. 

If runoff water is to be stored, determine the tank size based on the planned use of the captured water.

Select tank materials that have adequate strength and durability to hold water for the intended purpose

and length of time required. Use materials that will not degrade the quality of the stored water for its

intended use. Include a drain to allow maintenance of the tank and to protect from damage from freezing. 

Install the storage tank on a firm, level foundation that will not settle differentially. Examples of suitable

foundation materials are bedrock, concrete, compacted gravel, and stable well-compacted soils. Where

necessary, prepare the foundation by removal and disposal of materials that are not adequate to support

the design loads. Anchor or brace aboveground tanks as needed to prevent overturning or sliding by wind

and animals. 

Use NRCS design procedures or manufacturer’s guidelines to ensure that buried tanks will withstand all

earth and vehicle loads anticipated for the site. 

Include provisions for access to the tank for maintenance and repairs. However, ensure that access points

will limit unintended or unauthorized access. 

Design tanks connected directly to gutters and downspouts to bypass runoff events that exceed the

design capacity of the tank. Include provisions to convey overflows to a stable outlet without excessive

erosion. 

CONSIDERATIONS 

Gutter size can be decreased by increasing the number of downspouts. When designing a gutter and

downspout system consider the balance between gutter size and the spacing and sizing of downspouts to

optimize the design. 

If roof runoff will be used for livestock drinking water, the runoff should be treated before being consumed

by livestock. This might include bypassing the first flush of runoff which often contains the majority of

pollutants. Depending on the use of the runoff, additional actions may be necessary, such as settling and

filtration to remove suspended particles and treatment of pathogens with ultraviolet light or chlorination.

See International Code Council, CSA/ICC 805-2018, “Rainwater Harvesting Systems,” for information on

water quality treatment options for different end uses of the collected water. 

Discharging roof runoff outlets near wells and sinkholes or directly into drainage ditches, streams, or

ponds can be a point source of pollution. Consider the use of vegetative filter areas, such as raingardens,

at outlets to minimize the pollution potential from roof runoff. 

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Consider the use of wraparound straps in lieu of rigid supports on steep roofs where the outer edge of the

gutter cannot be placed below the projected roof line. 

On roofs subject to snow and ice slides, consider additional supports even if the gutter is installed below

the projected roof line. 

PLANS AND SPECIFICATIONS 

Provide plans and specifications that describe the requirements for applying this practice to achieve its

intended purpose. As a minimum, include— 

• A plan view showing the layout of gutters, downspouts, and outlets. 

• Details of gutter installation, including necessary cross sections and slope of gutters. 

• Details on the protection of downspouts from damage. 

• Details on outlets, storage tanks, or infiltration areas as appropriate. 

• Any other site-specific detail drawings necessary for the installation of the practice. 

• Requirements for stabilization of any areas disturbed by the installation of the practice. 

• Construction specifications describing the installation of the practice, materials, and quantities. 

OPERATION AND MAINTENANCE 

Prepare an operation and maintenance plan and review it with the operator. Describe the actions that

must be taken to ensure that the facility functions properly for its design life. As a minimum, include— 

• A schedule for regularly checking the gutters and downspouts for removal of any accumulated

debris, damage from weather events, livestock, and equipment. Additional checks should be made

after significant weather events (rain, snow, ice, extended cold periods, or high winds). 

• Ensuring that the outlets are freely operating and not causing erosion. 

• Ensuring roof runoff structures are clean, operating properly, and overflows are not causing erosion. 

• Promptly repairing or replacing any damaged components. 

REFERENCES 

International Code Council. 2018. CSA/ICC 805-2018 Rainwater Harvesting Systems. ICC, Washington,

D.C. https://codes.iccsafe.org/content/CSAB8052018P1

Rose, W. 1990. Gutters and Downspouts for the Home. Small Homes Council-Building Research Council.

University of Illinois, Urbana-Champaign, IL. http://hdl.handle.net/2142/54649

USDA NRCS. 2021. National Engineering Handbook (Title 210), Part 650, Chapter 2, Estimating Runoff

Volume and Peak Discharge. Washington, D.C. https://directives.sc.egov.usda.gov

USDA NRCS. 2009. National Engineering Handbook (Title 210), Part 651, Agricultural Waste

Management Field Handbook, Chapter 10, Agricultural Waste Management System Component Design.

Washington, D.C. https://directives.sc.egov.usda.gov

USDA NRCS. 2014. National Engineering Handbook (Title 210), Part 642, Construction Specification 32,

Structure Concrete. Washington, D.C. https://directives.sc.egov.usda.gov

USDA NRCS. 2017. National Engineering Manual (Title 210), Part 536, Section 536.20, Design Criteria for

Reinforced Concrete Structures. Washington, D.C. https://directives.sc.egov.usda.gov

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July 2021