Gutters · Key Biscayne

Gutters in Key Biscayne, FL

A 10-year downpour on Key Biscayne can fall at 10.5 inches an hour for five minutes. Gutters here are sized for bursts like that, not for steady light rain. A 25-year storm runs 12.4 inches an hour in that same five-minute window, and a 100-year storm reaches 15.4.

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Updated October 2026

Twisted tree on a beach leaning over calm water at dusk with distant buildings on the horizon. — Photo: Key Biscayne Miami Florida Travel Photography by Giuseppe Milo, CC BY 3.0, cropped

Drone view of a single-story home with a brown-gray shingle roof and box vents under large oak trees, a metal carport roof in the corner and a street behind. — Photo: Key Biscayne Miami Florida Travel Photography by Giuseppe Milo, CC BY 3.0, cropped

Key Biscayne figures this page uses

People (2020 Census): 14,809
Population estimate (ACS 5-year): 14,791
Housing units (ACS 5-year): 7,310
Median year homes were built: 1977

Sizing for local rain

Sizing Key Biscayne gutters for a short, heavy burst

Gutters are sized from how fast rain lands on the roof, not from the yearly total. The plumbing code ties gutter capacity to the roof surface flow rate. On this island, the five-minute burst is the number that matters most. Miami-Dade County's wet season brings 76.6 percent of the year's rain between May and October, so the burst can repeat many times each summer.

The same house in the same downpour with a white seamless gutter catching the runoff and a downspout discharging onto a splash block, flower bed intact (illustration)

Illustration: downpour with gutter

Moving from a 5-inch to a 6-inch half-round gutter raises capacity from 74 to 110 gallons per minute at the same slope, a jump of about 49 percent. Downspouts often fail before the gutter does. A 2 by 2 inch leader carries only 30 gallons per minute, against 92 for a 3 inch round pipe. Add half the area of any wall that sheds water onto the roof before you pick the size.

Discharge rules

Where downspout water must go on Key Biscayne lots

Gutters with downspouts are required on Key Biscayne buildings whose eaves project less than 6 inches, except at gable end rakes. Each leader must end at least 1 foot out from the wall, which matters on the narrow lots common near the causeway. Piping, a tail extension or a splash block can all meet that distance, so plan the outlet before the leader is cut.

Ask the Village of Key Biscayne building office whether a permit or inspection applies to your gutter or drainage work. The answer depends on the work involved.

Cleaning and debris

Leaves, trees and cleaning on Key Biscayne roofs

Southern live oaks can grow 60 to 80 feet tall and drop litter year round, so a gutter under one needs more cleaning. Pine needle fall peaks in autumn, from September through November, and the hurricane season peak of September 10 lands in that same window. For trees over a roof, a Florida insurer suggests cleaning at least twice a year, in spring and fall, and more often under heavy cover.

Fine-mesh guards kept more debris out in a 2010 Consumer Reports test, but the top screen still handled a severe downpour only fairly well. Guards reduce cleaning. They do not remove the need for it.

Rain on your roof

Roof area (square feet)Rain (inches)

Preset: 7.93 inches, the 10-year 24-hour rain (NOAA Atlas 14 at Key Biscayne).

9,881 gallons run off that roof, all of it toward the gutters and downspouts.

Math: square feet × inches × 0.623 gallons.

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Questions

Gutters in Key Biscayne: questions

How much rain falls in a storm on Key Biscayne?

A 10-year storm on Key Biscayne drops rain at 10.5 inches an hour over five minutes, according to NOAA Atlas 14 values for this town. A 25-year storm runs 12.4 inches an hour and a 100-year storm 15.4. A 24-hour 10-year storm totals 7.93 inches, and a 100-year storm 14 inches. Gutters and downspouts should be sized for the short burst, because that is where overflow starts. Confirm local design values with the Village building office before you buy.

Source: NOAA Atlas 14 Volume 9 version 2, partial-duration intensity estimates at the Key Biscayne 2020 Census internal point (25.6907, -80.1656), read Oct 5, 2026

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How big should gutters be on a Key Biscayne home?

The plumbing code sizes gutters by roof flow rate, and a 6-inch half-round gutter at 1/8 inch per foot slope moves 110 gallons per minute. Roofs that receive water from a taller wall need extra capacity. Downspouts must match the gutter, and a 4 inch round leader carries 192 gallons per minute. With 10.5 inches an hour falling in a 10-year burst here, have a qualified person measure your roof area before choosing.

Source: 2023 Florida Building Code, Plumbing, 8th ed., Chapter 11 (UpCodes viewer, unofficial transcription), read Oct 5, 2026

Related: Gutters

Where should downspout water go on Key Biscayne?

Keep it at least 1 foot from the sidewall of the building, using buried pipe, a tail extension or a splash block. This protects the foundation and walls during the heavy bursts this island gets. Check with the Village of Key Biscayne building office before you dig or add drainage piping, since the answer depends on the work.

Source: 2023 Florida Building Code, Residential, 8th ed., Chapter 8 (UpCodes viewer, unofficial transcription), read Oct 5, 2026

Related: Gutters

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