1. Your roof & KC location

Roof footprint (square feet)

Just the area covered by roof - if your house is 2,000 sq ft on the ground, that's your number (one story). Two-story homes use the ground-floor footprint.

Roof pitch (rise over run)

Pitch multiplier per SMACNA - steeper roofs catch more wind-driven rain than their footprint suggests.

KC metro ZIP code

Used to look up NOAA Atlas 14 5-minute, 10-year rainfall intensity for your exact location. Default: Lee's Summit (64063).

Design storm return period

Higher return period = larger gutters that handle rarer, bigger storms. KC's 100-year 5-min intensity is ~9.5 in/hr.

2. Recommended spec

5″ K-style aluminum + 2 downspouts

Effective roof area (pitch-adjusted)2,400 ft²

NOAA Atlas 14 intensity (KC, 5-min)7.5 in/hr

Required flow capacity187 gpm

Gutter size (K-style)5 inch

Downspout count2

Downspout size2 × 3 in

Max recommended spacing40 ft

Read above as: "This is the minimum spec to handle a [design storm] without overflow. Going to the next size up adds ~15% material cost and 30% capacity headroom."

The math, fully shown

This calculator is not a black box. Here's the methodology in plain text - the same sizing approach used in SMACNA's Architectural Sheet Metal Manual and Section 1106 of the International Plumbing Code.

# Step 1: Pitch-adjust the roof area effective_area = footprint × pitch_multiplier # Step 2: Look up rainfall intensity from NOAA Atlas 14 i = NOAA_Atlas_14[zip, return_period, 5-min duration] # in/hr # Step 3: Required flow rate (per IPC 1106.2) Q = effective_area × i × 0.0104 # gallons per minute # Step 4: Select gutter size from SMACNA capacity tables # 5" K-style @ 1/16" per ft slope = ~120 gpm # 6" K-style @ 1/16" per ft slope = ~190 gpm # 7" K-style @ 1/16" per ft slope = ~280 gpm gutter_size = smallest size where capacity ≥ Q # Step 5: Downspout sizing per IPC 1106.6 (1 ds per 100 ft gutter run typical) # 2×3 ds: ~600 sq ft of roof per downspout # 3×4 ds: ~1,200 sq ft of roof per downspout ds_count = ceil(effective_area / capacity_per_ds)

Primary sources:
· NOAA Atlas 14 Precipitation-Frequency Atlas (NWS) — the authoritative US rainfall intensity dataset, used by AASHTO, FHWA, and all state DOT drainage manuals.
· SMACNA Architectural Sheet Metal Manual, 7th Edition, Chapter 1 — gutter and downspout capacity tables.
· International Plumbing Code Section 1106.2-1106.6 — storm drainage sizing formulas.
· Manning's equation (open channel flow): Q = (1.486/n) × A × R^2/3 × S^1/2 — the underlying hydraulic basis for SMACNA's gutter capacity tables.

KC-specific values used by this tool

For a Lee's Summit, MO location (ZIP 64063), the NOAA Atlas 14 lookup returns these design intensities (5-minute duration):

Return period	5-min intensity (in/hr)
2-year	5.8
5-year	6.9
10-year (Premier standard)	7.5
25-year	8.4
100-year	9.5

Values rounded to nearest 0.1 in/hr. Per NOAA Atlas 14 Volume 8 Version 2.0. Slightly higher in north metro (Clay/Platte counties), slightly lower in southwest (rural Johnson Co).

Technical Gutter Sizer for KC Homes Engineer-grade