Building Loading – Rain Load

(Revised: 10/07/2024)

The Rain Load tab defines design-related information regarding the building’s Rain loads.

Rain loading only applies to the 2023 Florida code and 2024 IBC projects (i.e., codes requiring ASCE 7-22).

Rain loads need not be considered when the Rain load is less than the roof Snow load or the roof Live load.

Downspout Quantity, Rainfall Intensity:

The input Rainfall Intensity value is used for checking the external gutter size and the downspout spacings for primary drainage.

In this edit box, enter the Rainfall intensity measured in inches per hour. This is based on 5-minute duration, with a 5-year recurrence (MBMA I1 Rainfall Intensity). The Rainfall Intensity is used to determine how many downspouts are required for the building configuration. The check is done in accordance with MBMA Appendix A4, with a standard 4”x5” downspout and a 7”x7” eave gutter. For other configurations of gutter and downspouts, manual calculations are necessary to determine the proper number of downspouts required. A recommended website to find the Rainfall value is: PF Data Server-PFDS/HDSC/OWP (noaa.gov)

„ Note: Canada specifies rain fall in terms of mm/15-minutes with a 10 year MRI. This needs to be converted to inches/hour for software input.

Is ASCE 7-22 Section 8.2 Rain Loading Applicable?

· Yes = Rain Design Loading will be required if any primary or secondary drainage system could be blocked from allowing rainwater to drain from the roof

· No = If the Building Condition does not have an “Eave With Parapet” or a “Valley condition”

Building Condition:

There are two building conditions that may result in Rain Load needing to be applied to a roof surface:

· Eave With Parapet, or a

· Valley

Primary Drainage:

The primary drainage options are:

· Parapet Gutter, or

· Valley Gutter

Secondary Drainage:

The secondary drainage options are:

· Rectangular scupper

· Channel scupper

· Round standpipe

· Round scupper

15-min Duration, Rainfall Intensity:

The second Rainfall Intensity value is needed for roof Rain Design Loading to be calculated and applied:

In the same Rain tab, when Rain Design Loads are required to determine the size of roof drainage using scuppers, drainpipes, and secondary drains, this second Rainfall Intensity value is needed:

This second Rainfall Intensity value is from a 15-minute duration over the following Mean Recurrence Intervals:

· 100-year MRI for Risk Category I or II

· 200-year MRI for Risk Category III

· 500-year MRI for Risk Category IV

Conductor Tributary:

In the building length direction, the Conductor Tributary “length” is the spacing of the secondary drains.

Static Head:

Static Head is the elevation difference between the secondary drain and the bottom of the primary drain.

· When the secondary drain is a scupper, this would be to the bottom of the scupper

· When the secondary drain is a standpipe, this would be to the top of the standpipe

· A typical gutter system is the primary drain

Standard Controls:

§ OK, Cancel, Apply, Help

See also:

§ Building Loading - Building Codes

§ Building Loading - Live Load

§ Building Loading - Wind Load

§ Building Loading - Snow Load

§ Building Loading - Seismic Load

§ Building Loading - Tornado Load

§ Building Loading - Deflection Conditions