Rain Loads¶

The Rain Load module calculates the design rain load of ASCE 7 Chapter 8 for each drainage zone of a roof. You enter the rainfall intensity for the site, describe the secondary drainage system of each roof region, and list the drainage areas. Prose finds the flow rate, the hydraulic head at the drain, and the rain load R for every zone, and writes the calculation out with its code references.

Key Features¶

• Rainfall intensity entered once for the structure, for the 15-minute design storm
• Any number of roof regions, each with its own drainage system
• Four secondary drainage types with the inlet and outlet sizes tabulated in the ASCE 7 commentary
• Hydraulic head interpolated from the ASCE 7 commentary flow tables for each zone's flow rate
• A summary and report with the equation and code reference for every value

Adding the Module¶

1. Click the three-dot menu (⋮) next to the structure name in the sidebar
2. Select Rain Load

The module opens with a placeholder intensity of 1 in./h and one roof region that already holds a drainage system and one 1000 sq ft zone. Set every value for your project.

Rainfall Intensity¶

The ASCE Hazard Tool button opens ascehazardtool.org in a second window, where the intensity can be read for the project's coordinates. Check that the return period of the value you read matches the structure's risk category. For the 200-year and 500-year storms, the ASCE 7-22 commentary points to the NOAA Precipitation Frequency Data Server. The small button beside the field opens the rainfall maps.

Roof Regions¶

A roof region is a part of the roof served by one secondary drainage system. The module starts with one region. Add Roof Region adds another below it, and the three-dot menu (⋮) on a region's header offers Duplicate Region and Delete Region.

Drainage Systems¶

Each region describes its secondary drainage, the overflow path that carries water when the primary drains are blocked. ASCE 7-22 Section 8.2 also treats a secondary system as blocked if its inlet is less than 2 in. above the primary inlet, if it shares drain lines with the primary system, or if it uses controlled-flow drains. Describe the drainage system that remains when those are blocked, the secondary drainage system for structural loading (SDSL).

The figure beside the inputs shows the drainage system with ds, dh, and dp marked. Under the inputs, the module prints the two equations it uses: R = 5.2 (ds + dh + dp) from Equation 8.2-1, and the flow rate Q = 0.0104 A i from Commentary Equation C8.2-1. ASCE 7-10 and 7-16 use R = 5.2 (ds + dh), Equation 8.3-1, with no ponding head.

Drainage Zones¶

The Rain Design Loads table lists the drainage zones of the region, one row per drain or scupper. Enter the Drainage Collection Area of each zone in square feet; the rest of the row is calculated.

Add Zone adds a row. The trash button at the end of a row removes it.

Summary View¶

The summary on the right shows the general parameters, then for each region the drainage system, the interpolation source for the hydraulic head, and the design loads table with every zone. The outline button at the top jumps to a region.

PDF Export¶

1. Click the Export PDF tab on the module page
2. Review the report in the viewer
3. Save the PDF from the viewer toolbar. In a narrow pane the Save button sits in the >> menu at the right end of the toolbar

The module report has the same content as the summary, with the project header and your company information. The same pages appear in the project report.

Deleting the Module¶

1. Hover the Rain Load entry under the structure in the sidebar and click its three-dot menu (⋮)
2. Select Delete Module
3. Confirm with Delete Rain Load

Best Practices¶

• Take the rainfall intensity for the project coordinates at the design storm return period of the structure's risk category
• Model each secondary drain or scupper as its own zone with the area that actually drains to it
• Enter the static head as the height of the secondary inlet above the roof surface at the primary drain, and check it against the architectural roof drain details
• Coordinate the ponding head with the roof framing stiffness; ASCE 7-22 Section 8.2 defines it from the deflection of the roof under the unfactored rain and dead loads
• Compare the rain load with the roof snow load within the load combinations. ASCE 7-22 factors snow by 1.0 (0.7 in allowable stress design) and rain by 1.6 (1.0), so the unfactored loads do not compare directly

Code Reference¶

All three editions cover rain loads in ASCE 7 Chapter 8.