DRAFT VA DCR STORMWATER DESIGN SPECIFICATION No. 1: ROOFTOP DISCONNECTION
DRAFT VA DCR STORMWATER DESIGN SPECIFICATION No. 1
ROOFTOP DISCONNECTION
VERSION 1.5
Note to Reviewers of the Stormwater Design Standards and Specifications
The Virginia Department of Conservation and Recreation (DCR) has developed an updated set of nonproprietary BMP standards and specifications for use in complying with the Virginia Stormwater
Management Law and Regulations. These standards and specifications were developed with assistance
from the Chesapeake Stormwater Network (CSN), Center for Watershed Protection (CWP), Northern
Virginia Regional Commission (NVRC), and the Engineers and Surveyors Institute (ESI) of Northern
Virginia. These standards and specifications are based on both the traditional BMPs and Low Impact
Development (LID) practices. The advancements in these standards and specifications are a result of
extensive reviews of BMP research studies incorporated into the CWP's National Pollution Removal
Performance Database (NPRPD). In addition, we have borrowed from BMP standards and specifications
from other states and research universities in the region. Table 1 describes the overall organization and
status of the proposed design specifications under development by DCR.
Table 1: Organization and Status of Proposed DCR Stormwater Design Specifications:
Status as of 9/24/2008
Practice
Notes
#
Status 1
Rooftop Disconnection
Includes front-yard bioretention
1
2
Filter Strips
Includes grass and conservation filter strips
2
2
Grass
Channels
3
2
Soil Compost
4
3
Amendments
Green Roofs
5
1
Rain Tanks
Includes cisterns
6
2
Permeable
Pavement
7
1
Infiltration
Includes micro- small scale and conventional
8
2
infiltration techniques
Bioretention
Includes urban bioretention
9
3
Dry
Swales
10
2
11 OPEN
12 Filtering Practices
2
Includes wet swales
13 Constructed Wetlands
2
Wet
Ponds
14
2
15 ED Ponds
2
1
Status: 1= Partial draft of design spec; 2 = Complete draft of design spec;
3 = Design specification has undergone one round of external peer review as of 9/24/08
Reviewers should be aware that these draft standards and specifications are just the beginning of the
process. Over the coming months, they will be extensively peer-reviewed to develop standards and
specifications that can boost performance, increase longevity, reduce the maintenance burden, create
attractive amenities, and drive down the unit cost of the treatment provided.
Rooftop Disconnection
1 of 10
9/24/08
DRAFT VA DCR STORMWATER DESIGN SPECIFICATION No. 1: ROOFTOP DISCONNECTION
Timeline for review and adoption of specifications and Role of the Virginia’s Stormwater BMP
Clearinghouse Committee:
The CSN will be soliciting input and comment on each standard and specification until the end of 2008
from the research, design and plan review community. This feedback will ensure that these design
standards strike the right balance between prescription and flexibility, and that they work effectively in
each physiographic region. The collective feedback will be presented to the BMP Clearinghouse
Committee to help complement their review efforts. The Virginia Stormwater BMP Clearinghouse
Committee will consider the feedback and recommend final versions of these BMP standards and
specifications for approval by DCR.
The revisions to the Virginia Stormwater Management Regulations are not expected to become finalized
until late 2009. The DCR intends that these stormwater BMP standards and specifications will be
finalized by the time the regulations become final.
The Virginia Stormwater BMP Clearinghouse Committee will consider the feedback and recommend
final versions of these BMP standards and specifications for approval by DCR, which is vested by the
Code of Virginia with the authority to determine what practices are acceptable for use in Virginia to
manage stormwater runoff.
As with any draft, there are several key caveats, as outlined below:

Many of the proposed design standards and specifications lack graphics. Graphics will be produced
in the coming months, and some of graphics will be imported from the DCR 1999 Virginia
Stormwater Management (SWM) Handbook. The design graphics shown in this current version are
meant to be illustrative. Where there are differences between the schematic and the text, the text
should be considered the recommended approach.

There are some inconsistencies in the material specifications for stone, pea gravel and filter fabric
between ASTM, VDOT and the DCR 1999 SWM Handbook. These inconsistencies will be rectified
in subsequent versions.

While the DCR 1999 SWM Handbook was used as the initial foundation for these draft standards and
specifications, additional side-by-side comparison will be conducted to ensure continuity.

Other inconsistencies may exist regarding the specified setbacks from buildings, roads, septic
systems, water supply wells and public infrastructure. These setbacks can be extremely important,
and local plan reviewers should provide input to ensure that they strike the appropriate balance
between risk aversion and practice feasibility.
These practice specifications will be posted in Wikipedia fashion for comment on the Chesapeake
Stormwater Network’s web site at http://www.chesapeakestormwater.net, with instructions regarding how
to submit comments, answers to remaining questions about the practice, useful graphics, etc. DCR
requests that you provide an email copy of your comments, etc., to Scott Crafton
(scott.crafton@dcr.virginia.gov). The final version will provide appropriate credit and attribution on the
sources from which photos, schematics, figures, and text were derived.
Thank you for your help in producing the best stormwater design specifications for the Commonwealth.
Rooftop Disconnection
2 of 10
9/24/08
DRAFT VA DCR STORMWATER DESIGN SPECIFICATION No. 1: ROOFTOP DISCONNECTION
DRAFT VA DCR STORMWATER DESIGN SPECIFICATION No. 1
ROOFTOP DISCONNECTION
VERSION 1.5
SECTION 1: DESCRIPTION OF PRACTICE
This strategy involves treating runoff close to its source by intercepting rooftop runoff and
infiltrating, filtering, treating, or reusing it before it moves from the roof into the storm drain
system. Two kinds of practices are allowed. The first is for simple rooftop disconnection,
whereas the second involves disconnection combined with supplementary runoff treatment,
including the following:





Compost amended soils in the filter path
Installation of dry wells or french drains
Installation of rain gardens or front yard bioretention
Storage and reuse in a rain tank or cistern
Storage and release in a foundation planter
With proper design and maintenance, each of the disconnection options can provide relatively
high runoff reduction rates, as shown in Table 1. With the exception of dry wells and rain
gardens, most disconnection options have little or no capability to remove nutrients (Table 2).
Any runoff reduction achieved by rooftop disconnections can be subtracted from the overall
Rooftop Disconnection
3 of 10
9/24/08
DRAFT VA DCR STORMWATER DESIGN SPECIFICATION No. 1: ROOFTOP DISCONNECTION
channel protection volume for the site, and may also increase the time of concentration used to
model larger design storms to control flooding.
Table 1: Runoff Reduction Capabilities For Rooftop Disconnection
Annual Runoff Reduction Rate
HSG Soils A and B
HSG Soils C and D
Simple Disconnection
50
25
Compost-Amended Filter Path 1
75
50
Dry Wells or French Drains
75
50
Rain Gardens/Bioretention
75
50
Rain Tank or Cistern
actual storage volume x 0.75
Foundation Planter
40
1
CA= Compost Amended Soils, see Design Specification No. 4
Sources: CWP and CSN (2008), CWP, 2007
Rooftop Disconnection
4 of 10
9/24/08
DRAFT VA DCR STORMWATER DESIGN SPECIFICATION No. 1: ROOFTOP DISCONNECTION
SECTION 2: PERFORMANCE CRITERIA
Table 2: Pollutant Removal Capabilities For Rooftop Disconnection
EMC Reduction Rate
Total Phosphorus
Total Nitrogen
Simple Disconnection
Compost-amended Filter Path
Dry Wells or French Drains
Rain Gardens/Bioretention
Rain Tank or Cistern
Foundation Planter
Sources: CWP and CSN (2008), CWP (2007)
0
0
25
25-50
0
0
0
0
15
40-60
0
0
SECTION 3: DESIGN APPLICATIONS AND VARIATIONS
The flow chart in Figure 1 below provides guidance for choosing the best disconnection option
for an individual rooftop:
Figure 1: This simple flow chart helps homeowners decide whether simple disconnection, rain
barrels, french drains or rain gardens are most appropriate for their lot.
Rooftop Disconnection
5 of 10
9/24/08
DRAFT VA DCR STORMWATER DESIGN SPECIFICATION No. 1: ROOFTOP DISCONNECTION
SECTION 4: DESIGN CRITERIA
3.1: Simple Rooftop Disconnection

Simple disconnection is only allowed for residential lots greater than 6000 sq. ft. For smaller
sites, disconnection with supplementary runoff treatment may be considered.

The contributing flow path from impervious areas should not exceed 75 feet.

The disconnection length must exceed the contributing flow path.

A compensatory mechanism is needed if the disconnection length is less than 40 feet and/or
the post construction Hydrologic Soil Group is in the C or D Category.

Pervious areas used for disconnection should be graded to have a slope in the 1 to 2% range,
and should never exceed 5%.

The total impervious area contributing to any single discharge point shall not exceed 1000
square feet and shall drain continuously through a pervious filter until reaching a property
line or drainage swale.

The disconnection shall not cause basement seepage. Normally, this involves extending
downspouts at least 10 feet from the building if the ground does not slope away from the
building.

Maintenance of disconnected downspouts is essentially maintaining the lawn or landscaped
areas in the path of the water from the downspout.
3.2: Compost-amended filter path
The design should conform to Stormwater Design Specification No. 4 (Soil Compost
Amendments), and include the following elements:

Flow from the downspout should be spread over a 10-foot wide strip extending downgradient from the building to the street or conveyance system.

A pea gravel or riverstone diaphragm should be installed at the downspout outlet to distribute
flows across the filter path.

Existing soils in the strip will be scarified or tilled to a depth of 12 to 18 inches and amended
with well-aged compost to achieve an organic matter content in the range of 8 to 13%.
The depth of compost amendment is based on the relationship of the contributing rooftop
area (RA) to the area of the soil amendment strip (SA), using the following general guidance:
o RA/SA = 1, use 4 inches compost.

Rooftop Disconnection
6 of 10
9/24/08
DRAFT VA DCR STORMWATER DESIGN SPECIFICATION No. 1: ROOFTOP DISCONNECTION
o RA/SA = 2, use 8 inches compost.
o RA/SA = 3, use 12 inches of compost and till to a depth of 18-24 inches.
3.3: Dry Wells and French Drains
Depending on soil properties, roof runoff may be infiltrated into a shallow dry well or French
drain. The design for this option should meet the requirements of micro-infiltration, as described
in Design Specification No. 8 (Infiltration), and summarized in Table 3.
Table 3: Design Requirements for Micro-Infiltration
Design Factor
Micro Infiltration Design
Impervious Area Treated
250 to 2500 sq. ft.
Typical Practices
Dry Well and French Drain
Runoff Reduction Sizing
Minimum 0.1 inches over CDA (the RA)
Minimum Soil Infiltration Rate
0.5 inches/hour
Observation Well
No
Type of Pretreatment
External (leaf screens, etc)
Depth Dimensions
Max. 3 foot depth
UIC Permit Needed
No
Head Required
Nominal, 1 to 3 feet
Underdrain Requirements?
Only on marginal soils
Required Soil Test
One per practice
Building Setbacks
5 feet downgradient, 25 feet upgradient
In general, the size of Micro-Infiltration Facilities needs to be10- 15% of the contributing roof
area.
An on-site soil test is needed to determiner if soils are suitable for infiltration. The microinfiltration facility should be located in an expanded right of way or stormwater easement so that
it can be accessed for maintenance.
3.4: Rain Gardens and Front Yard Bioretention
Depending on soil properties, roof runoff may be filtered through a shallow bioretention area.
The design for this option should meet the requirements of micro-bioretention, as described in
Design Specification No. 7 (Bioretention), and summarized in Table 4.
Rooftop Disconnection
7 of 10
9/24/08
DRAFT VA DCR STORMWATER DESIGN SPECIFICATION No. 1: ROOFTOP DISCONNECTION
Table 4 Design Requirements for Micro-Bioretention
Design Factor
Micro-Bioretention aka Rain Garden
Impervious Area Treated
250 to 2500 sq. ft.
Type of Inflow
Sheetflow or roof leader
Runoff Reduction Sizing
Minimum 0.1 inches over CDA
Minimum Soil Infiltration Rate
0.5 inches/hour (or use underdrain)
Observation Well/ Cleanout Pipes No
Type of Pretreatment
External (leaf screens, etc)
Minimum Filter Media Depth
24 inches
Media Source
Mixed on site
Head Required
Nominal, 1 to 3 feet
Required Soil Borings
One, only when an underdrain is not used
Building Setbacks
5 feet downgradient, 25 feet upgradient
For high density sites, front yard bioretention may be an attractive option. This form of
bioretention captures roof overflow and lawn and driveway runoff from low to medium density
residential lots in a slight depressed area between the home and the street. The bottom of the
bioretention area then connects by an underdrain to the main storm drain pipe located underneath
the street The concept is to take advantage of the drop from the roof leader to the street storm
drain pipe, by creating a 10 foot wide bioretention corridor from roof to the street. The
minimum effective length of the bioretention corridor is 20 feet long. The bioretention corridor
is subtly graded to create a shallow 6-12 inch deep ponding area between the roof leader and the
edge of a sidewalk or road. The ponding area may have a turf or landscape cover, depending on
homeowner preference.
The bioretention media is approximately 3 feet deep, and is located over a 12-24 inch deep stone
reservoir. A perforated underdrain is located above the stone reservoir, to promote storage and
recharge, even on poorly draining soils. In highly urban settings, the underdrain is directly
connected into the major storm drain pipe running underneath the street or in the street right of
way. A trench needs to be excavated during construction to connect the underdrain to the street
storm drain system. Construction of the remainder of the front yard bioretention system is
deferred until after the lot has been stabilized. The front yard design should reduce the risk of
homeowner conversion because it allows them to choose whether they want turf or landscaping.
Front yard bioretention requires regular mowing and/or landscape maintenance to perform
effectively and should be located in an expanded right of way of stormwater easement so that it
can be accessed in the event that it fails to drain properly.
3.5: Rain Tanks and Cisterns
This form of disconnection must conform to the design requirements outlined in Design
Specification No. 6 (Rain Tanks and Cisterns). The runoff reduction rates for rain tanks and
cisterns depend on their storage capacity and ability to drawdown water in between storms for
reuse as potable water, greywater, or irrigation use.
Rooftop Disconnection
8 of 10
9/24/08
DRAFT VA DCR STORMWATER DESIGN SPECIFICATION No. 1: ROOFTOP DISCONNECTION

Designers will need to estimate the water reuse volume, based on the method of distribution,
frequency of use, and seasonally adjusted indoor and/or outdoor water demands for the
building.

Based on the prevailing climate for the region, a conservative runoff reduction estimate of
40% is recommended for initial design.

Pretreatment measures may need to be employed to keep leaves, bird droppings, and other
pollutants from entering the tank or cistern.

All devices should have a suitable overflow area to route extreme flows into the next
treatment practice or stormwater conveyance system.
3.6: Foundation Planter
This form of disconnection must conform to the design requirements for foundation planters as
outlined in Design Specification No. 9 (Bioretention).
Foundation planters are another option to disconnect and treat rooftop runoff. They consist of
confined planters that store and/or infiltrate runoff through a soil bed to reduce runoff volumes
and pollutant loads. Stormwater planters combine an aesthetic landscaping feature with a
functional form of stormwater treatment. Stormwater planters generally receive runoff from
adjacent rooftop downspouts and are landscaped with plants that are tolerant to both periods of
drought and inundation. The two basic design variations for stormwater planters are the
infiltration planter and the filter planter.
An infiltration planter filters rooftop runoff through soils in the planter followed by infiltration
into soils below the planter. The recommended minimum width is 30 inches; length and shape
can be decided by architectural considerations. The planter should be sized to temporarily store
at least 1/2-inch of runoff from the contributing rooftop area in a reservoir above the planter bed.
Infiltration planters should be placed at least 10 feet away from a building to prevent possible
flooding or basement seepage damage.
A filter planter has an impervious liner on the bottom. The minimum planter width is 18 inches
with the shape and length governed by architectural considerations. Runoff is temporarily stored
in a reservoir located above the planter bed. Overflow pipes are installed to discharge runoff
when maximum ponding depths are exceeded to avoid water spilling over the side of the planter.
Since a filter planter is self-contained and does not infiltrate into the ground, it can be installed
right next to a building.
All planters should be placed at grade level or above ground, and sized to allow captured runoff
to drain out within four hours after a storm event. Plant materials should be capable of
withstanding moist and seasonally dry conditions. Planting media should have an infiltration
rate of at least 2 inches per hour. The sand and gravel on the bottom of the planter should have a
minimum infiltration rate of 5 inches per hour. The planter can be constructed of stone,
Rooftop Disconnection
9 of 10
9/24/08
DRAFT VA DCR STORMWATER DESIGN SPECIFICATION No. 1: ROOFTOP DISCONNECTION
concrete, brick, wood, or other durable material. If treated wood is used, care should be taken so
that trace metals and creosote do not leach out of the planter.
SECTION 4: OPERATIONS AND MAINTENANCE
The rooftop disconnection and supplementary treatment device must be covered by a drainage
easement to allow inspection and maintenance. When the disconnection occurs on a private
residential lot, its existence and purpose shall be noted on the deed of record. Homeowners must
be provided a simple document that explains their purpose and routine maintenance needs. A
legally binding maintenance agreement must be in place to ensure that downspouts remain
disconnected, treatment units are maintained and filtering/infiltrating areas are not converted or
disturbed. The agreements should grant authority for local agencies to access the property for
inspection or corrective action.
SECTION 5: REFERENCES
City of Portland, Environmental Services. 2004. Portland Stormwater Management Manual.
Portland, OR. http://www.portlandonline.com/bes/index.cfm?c=dfbbh
CWP. 2007. National Pollutant Removal Performance Database Version 3.0. Center for
Watershed Protection, Ellicott City, MD.
Northern Virginia Regional Commission. 2007. Low Impact Development Supplement to the
Northern Virginia BMP Handbook. Fairfax, Virginia
Philadelphia Stormwater Management Guidance Manual
http://www.phillyriverinfo.org/Programs/SubprogramMain.aspx?Id=StormwaterManual
Schueler, T., D. Hirschman, M. Novotney and J. Zielinski. 2007. Urban stormwater retrofit
practices. Manual 3 in the Urban Subwatershed Restoration Manual Series. Center for
Watershed Protection, Ellicott City, MD
Schueler, T. 2008. Technical Support for the Baywide Runoff Reduction Method. Chesapeake
Stormwater Network. Baltimore, MD www.chesapeakestormwater.net
Rooftop Disconnection
10 of 10
9/24/08