Tables: Blue Roofs (Non-GI)
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1. Table 1
BMP 2026, Ch 11.1, p. 1
| N.J.A.C. 7:8 Stormwater Management Rules – Applicable Design and Performance Standards |
|
|
|
Green Infrastructure |
Not Allowed |
|
Stormwater Runoff Quantity |
Only with a waiver or variance from N.J.A.C. 7:8-5.3 |
|
Groundwater Recharge |
Not Allowed |
| % |
Stormwater Runoff Quality |
Not Allowed |
| Stormwater Runoff Quantity Mechanism and Corresponding Criteria |
|
|
| --- |
--- |
|
| Peak Flow Reduction |
|
|
| Maximum ponding depth |
Based on required structural calculations |
|
| Partition requirement |
Required when roof slope > 2% |
|
Source context and notes
Source revision: January 2026.
# 11.1 BLUE ROOFS (NON-GI)
[Figure: img-0.jpeg (original PDF, p. 1)](SOURCE.pdf#page=1)
Blue roofs are systems that are designed to provide stormwater detention. Precipitation that falls onto the roof is managed using orifices, weirs or other outlet devices that control the discharge rate of rooftop runoff. By reducing flow rates from rooftops, blue roofs are effective in reducing the size of down-gradient stormwater management systems. However, blue roofs can only be used to address the quantity control aspects of development when a waiver or variance from N.J.A.C. 7:8-5.3 is granted as they are not considered to be green infrastructure.
| N.J.A.C. 7:8 Stormwater Management Rules – Applicable Design and Performance Standards | | |
| --- | --- | --- |
| | Green Infrastructure | Not Allowed |
| | Stormwater Runoff Quantity | Only with a waiver or variance from N.J.A.C. 7:8-5.3 |
| | Groundwater Recharge | Not Allowed |
| % | Stormwater Runoff Quality | Not Allowed |
| Stormwater Runoff Quantity Mechanism and Corresponding Criteria | |
| --- | --- |
| Peak Flow Reduction | |
| Maximum ponding depth | Based on required structural calculations |
| Partition requirement | Required when roof slope > 2% |
2. Table 2
BMP 2026, Ch 11.1, p. 7
| Roof area: |
40,000 sf |
| Roof width, W: |
125 feet |
| Roof length, L: |
320 feet |
| 2-year storm rainfall, P: |
3.31 inches |
| 10-year storm rainfall, P: |
5.06 inches |
| 100-year storm rainfall, P: |
8.51 inches |
| Maximum ponding depth: |
6 inches |
| Roof slope: |
2% |
| Existing land cover |
Lawn, Good Condition |
| Hydrologic soil group |
C |
| Existing curve number |
74 |
| Existing time of concentration |
20 minutes |
Source context and notes
# Designing a Blue Roof
Blue roofs may be designed in many different ways. Below are two examples of how a blue roof can be designed using partitions or outlets. The examples below show possible configurations and flow paths and are not intended to limit the design.
## Example 1: Blue Roof Designed with Partitions
A 40,000 sf blue roof is being designed based on the following parameters:
| Roof area: | 40,000 sf |
| --- | --- |
| Roof width, W: | 125 feet |
| Roof length, L: | 320 feet |
| 2-year storm rainfall, P: | 3.31 inches |
| 10-year storm rainfall, P: | 5.06 inches |
| 100-year storm rainfall, P: | 8.51 inches |
| Maximum ponding depth: | 6 inches |
| Roof slope: | 2% |
| Existing land cover | Lawn, Good Condition |
| Hydrologic soil group | C |
| Existing curve number | 74 |
| Existing time of concentration | 20 minutes |
## Step 1: Calculating the Distance Between Partitions
This roof is being designed with a series of partitions; therefore, the ponding area will be split into sections, each with a different maximum elevation but with the same maximum depth. The distance between the partitions may be calculated as follows:
$$
\begin{array}{l}
\text{Distance between Partitions} = \frac{\text{Maximum Ponding Depth}}{\text{Roof slope}} \\
= \frac{6 \text{ in}}{0.02 \times 12 \text{ in/ft}} = 25 \text{ ft}
\end{array}
$$
3. Table 3
BMP 2026, Ch 11.1, p. 8
| Storm Event |
Pre-development Peak Flow (cfs) |
Traditional Roof Peak Flow (cfs) |
Blue Roof Peak Flow (cfs) |
Peak Flow Reduction Rate |
| 2-year |
0.76 |
2.95 |
0.33 |
56.6% |
| 10-year |
1.74 |
4.55 |
0.43 |
75.3% |
| 100-year |
3.90 |
7.67 |
2.63 |
32.6% |
Source context and notes
# Blue Roofs (Non-GI)
Source revision: January 2026.
Step 3: Designing the Partitions
Four partitions will be used at 25-foot intervals from the four required outflow drains. Each partition will be 6 inches high and will have a series of 25 weep holes, each approximately 12 feet apart to allow stormwater runoff to flow from one section of the roof to the next. Each weep hole will be 1 inch in diameter, and each invert must be at the roof level to ensure there is no standing water behind the partitions. The table below shows the reductions in peak flow rate due to the blue roof system.
| Storm Event | Pre-development Peak Flow (cfs) | Traditional Roof Peak Flow (cfs) | Blue Roof Peak Flow (cfs) | Peak Flow Reduction Rate |
| --- | --- | --- | --- | --- |
| 2-year | 0.76 | 2.95 | 0.33 | 56.6% |
| 10-year | 1.74 | 4.55 | 0.43 | 75.3% |
| 100-year | 3.90 | 7.67 | 2.63 | 32.6% |
The blue roof meets the peak flow rate reductions required by the Stormwater Management rules for the 2-, 10- and 100-year storm. If there are other disturbed areas on-site, such as parking lots and sidewalks, they will also require stormwater management measures and hydrologic analyses to demonstrate compliance with the Stormwater Management rules.
Example 2: Blue Roof Designed with Orifices
A 20,000 sf traditional roof is being retrofitted as a blue roof to reduce peak flow rates leaving the site. The blue roof has a single drainage area based on the following parameters:
4. Table 4
BMP 2026, Ch 11.1, p. 9
| Storm Event |
Traditional Roof Peak Flow (cfs) |
Blue Roof Peak Flow (cfs) |
Peak Flow Reduction Rate |
| 2-year |
1.48 |
0.67 |
55% |
| 10-year |
2.27 |
0.95 |
58% |
| 100-year |
3.84 |
2.70 |
30% |
Source context and notes
Source revision: January 2026.
Step 2: Designing Orifices and Overflows
The peak flow rate reductions will be determined by the size and number of orifices. Many commercial drains are available which have pre-determined flow rates. For this example, we will assume that each drain is designed as a vertical orifice which slowly releases stormwater runoff from the rooftop, and water depths in excess of the maximum ponding depth are freely discharged (modeled as a weir). It is also assumed that the rooftop is designed with crushed stone with a porosity of 0.4 to hold the waterproofing membrane in place.
Using 4 drains, two located at the bottom of the roof each with an orifice with a diameter of 5 inches and two elevated 6 inches above the bottom of the roof with an orifice diameter of 12 inches, the blue roof achieves the peak flow reductions provided in the table below:
| Storm Event | Traditional Roof Peak Flow (cfs) | Blue Roof Peak Flow (cfs) | Peak Flow Reduction Rate |
| --- | --- | --- | --- |
| 2-year | 1.48 | 0.67 | 55% |
| 10-year | 2.27 | 0.95 | 58% |
| 100-year | 3.84 | 2.70 | 30% |
These flow rate reductions apply to the entire roof area, though only the 15,000 sf area calculated above will have ponding. Retrofitting an existing roof to provide peak flow reductions could be used as part of a mitigation plan in instances in which a variance or exemption for a separate project must be granted for the stormwater runoff quantity standard, or a way to reduce impacts due to flooding or combined sewer overflows.
Considerations
A number of factors should be considered when using a blue roof to manage stormwater runoff.
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