Calculators / Rain Loads

Free Rain Load & Ponding Calculator

Size one roof bay for ordinary roof loads, then find the ASCE 7-22 rain load it must carry, where the water held by the deflected roof counts. The ASCE 7-16 rain load on the same framing is shown for comparison.

ASCE Hazard Tool

Framing

Girder Span,

L_p

Joist Span,

L_s

Joist Spacing,

S

5.00 ft on center: 8 equal spaces across the girder span

Rain and Drainage 4.00 in./hr · 4.00 in. of water at the low point

Risk Category

Sets the design storm return period, ASCE 7-22 Table 8.2-1

15-Minute Intensity,

i

in./hr, 100-year storm, from NOAA Atlas 14 or the ASCE Hazard Tool

Roof Area per Drain,

A

sq ft draining to one overflow

Overflow Drain or Scupper

The secondary drainage system for structural loading

Overflow Drain Location

along the low girder, which deflects with the roof

Static Head,

d_s

in., low point of the roof up to the overflow inlet

Hydraulic Head,

d_h

in., water above the overflow inlet at the design flow

Roof Loads and Design Basis D 15 · Lr 20 · S 0 psf · LRFD

Dead Load,

D

psf, including the framing

Roof Live Load,

L_o

psf, ASCE 7: Table 4.3-1

Snow Load,

S

psf, for ASCE 7-22

Design Method

Members Sized To

Edition whose load combinations size the framing

Live Load Deflection Limit

Reduce the roof live load for tributary area (ASCE 7: Section 4.8.2) Use 0.8 of the W-shape stiffness in the ASCE 7-22 analysis (AISC Design Guide 40)

Roof Slope and Bay Position 0.250 in./ft · interior bay

Roof Slope

in. per ft; camber is not included

Low Edge Runs Along

Static head is measured at the low edge

Bay Position

Every other edge continues into an identical bay

Roof bay, plan view

Preliminary sizes: lightest members that carry the uniform roof loads under ASCE 7-22, LRFD.

Girders W21×44

Joists at 5 ft 22K4

Preliminary sizes. These members are sized only for the uniform dead, roof live and snow loads above, so the ponding analysis starts from realistic stiffness. They are not a member design. Snow drift, wind, seismic, mechanical units, other roof loads and diaphragm collector forces are not considered. Choose "I'll choose" and enter the members you have designed to check ponding on the actual framing.

Gray bays are for illustration only. The analysis takes each neighbor as a mirror image of this bay, with the same members and the same water. Not to scale.

ASCE 7-22 peak rain load, $R$

50.8 psf

$R = 5.2(d_s + d_h + d_p)$ , with ponding

ASCE 7-16, no ponding

20.8 psf

2.4 times the ASCE 7-16 load, on the same roof with the same members and the same storm.

Depth before the roof deflects, $d_s + d_h$

4.00 in.

Added depth as the roof deflects

+5.77 in.

Roof live load, for scale, $L_o$

20 psf

Joist J5 loading diagram (critical) · 22K4, 40 ft span

Factored loads along the joist, low end at left.

ASCE 7-22 rain with ponding ASCE 7-16 rain Dead, 90 plf Equivalent uniform

Max moment, 1.2D + 1.6R

73.0 k-ft

Max end reaction, 1.2D + 1.6R

8.38 k

Equivalent uniform load, 1.2D + 1.6R

419 plf

End shear governs

Standard joist load tables assume a uniform load. The equivalent uniform load is the larger of the uniform load that matches this moment everywhere along the span and the one that matches the larger end reaction. Check it against the LRFD table, or specify the joist for this load diagram.

Member schedule

ASCE 7-22 rain with ponding, 1.2D + 1.6R. Select a row to see its loading diagram.

Line	Trib., ft	Equiv. uniform, plf	R low end, k	R high end, k	M, k-ft
J1	5.00	207 (V)	4.13	2.51	32.5
J2	5.00	263 (V)	5.26	3.10	42.6
J3	5.00	343 (V)	6.85	4.09	57.9
J4	5.00	399 (V)	7.98	4.90	69.0
J5 critical	5.00	419 (V)	8.38	5.20	73.0
J6	5.00	399 (V)	7.98	4.90	69.0
J7	5.00	343 (V)	6.85	4.09	57.9
J8	5.00	263 (V)	5.26	3.10	42.6
J9	5.00	207 (V)	4.13	2.51	32.5

(V): end shear governs the equivalent uniform load.

Girder panel point loads, kips

Girder	P1	P2	P3	P4	P5	P6	P7	M, k-ft	V, k
Low girder	10.51	13.71	15.96	16.76	15.96	13.71	10.51	596.7	48.56
High girder	6.19	8.19	9.80	10.40	9.80	8.19	6.19	363.9	29.38

The ponding analysis follows the approach of the Steel Joist Institute's Roof Bay Analysis Tool and AISC Design Guide 40. It is an independent implementation, not reviewed or endorsed by SJI or AISC.

A computational aid, not a professional engineering opinion. Results must be independently verified against the governing building code before use in any design.

About the Free Rain Load & Ponding Calculator

What the calculator does

The Free Rain Load & Ponding Calculator sizes one roof bay for ordinary roof loads, then runs an iterative ponding analysis to find the water the deflected roof holds. It reports the ASCE 7-22 rain load with ponding head and the ASCE 7-16 rain load on the same framing.

Supported rain-load calculations

Hydraulic head from overflow drain and scupper tables
Member selection for W girders, W beams, joists, and joist girders
Iterative ponding analysis and ponding instability
Ponding head and rain load, R = 5.2(ds + dh + dp)
AISC 360-16 Appendix 2 simplified ponding check

Supported ASCE editions

Calculations support ASCE 7-16 and ASCE 7-22. ASCE 7-22 adds ponding head to the rain load, so the water held by the deflected roof is part of the design load.

Use this rain load calculator with a 15-minute rainfall intensity from NOAA Atlas 14 or the ASCE Hazard Tool, at the return period ASCE 7-22 Table 8.2-1 sets for the building's Risk Category, and confirm the result against the governing building code and local jurisdiction.

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Free Rain Load & Ponding Calculator

What the calculator does

Supported rain-load calculations

Supported ASCE editions