Sizing gutters and downspouts for a Southeast cloudburst

Step 1: measure the area that drains to each gutter run

Sizing is done run by run, for the part of the roof that sheds into a given stretch of gutter and its outlet. The plumbing code's drainage language refers to projected roof area, the footprint seen from above, so a steep roof and a shallow one with the same footprint count the same. One adjustment matters on many houses: under Section 1106.4, half the area of any vertical wall that diverts rainwater onto the roof is added to the projected roof area. A two-story wall above a porch roof is the classic case.

If you need the numbers, the National Roofing Directory's roof pitch calculator and roofing squares calculator help you work out the roof's dimensions.

Step 2: pick the rainfall intensity

Section 1106.1 of the International Plumbing Code bases the size of vertical conductors, leaders and storm drains on the 100-year hourly rainfall rate shown in the code's maps, or on other rainfall rates determined from approved local weather data. For a specific address, NOAA Atlas 14's Precipitation Frequency Data Server gives point estimates of intensity for durations from five minutes upward. Here are the rates at seven Southeast airport points:

The 60-minute column matches the kind of rate the code names. The five-minute columns show what the heaviest part of a storm can do.

Step 3: convert rain into gallons per minute

Section 1106.2.1 of the code converts a rainfall rate in inches per hour and a roof area in square feet into a flow in gallons per minute, using its Equation 11-1. You can check the order of magnitude yourself from the U.S. Geological Survey's figure of 27,154 gallons per inch of rain on an acre. That comes to about 623 gallons per inch on 1,000 square feet, or roughly 10.4 gallons per minute for every inch per hour of rain on that area.

A worked example: 1,200 square feet of projected roof draining to one gutter run in New Orleans.

• At the 100-year, 60-minute rate of 5.34 inches per hour, the run carries about 67 gallons per minute.
• At the 10-year, five-minute rate of 10.3 inches per hour, it carries about 128 gallons per minute.
• In Mobile, the same area gives about 61 gallons per minute at the hourly design rate and about 148 during a 10-year five-minute burst.

Step 4: match the flow to gutter and leader capacity

With the flow in hand, the code's tables do the rest. Section 1106.6 sizes horizontal gutters by the flow from the roof surface and caps it at the capacity listed in Table 1106.6, which gives gallons per minute for each gutter size at several slopes. Section 1106.3 sizes vertical leaders, the downspouts, by the flow coming from the horizontal gutters, capped by Table 1106.3. Consult the tables on the ICC site rather than a copy; your jurisdiction may have adopted a different edition or local amendments.

Two practical levers come out of the method. A steeper gutter slope raises capacity in the gutter table, and more outlets split the roof area into smaller pieces, which cuts the flow each downspout has to carry.

Designing past the minimum

The code's hourly figure is the design basis it names, and the five-minute rates show how much higher real bursts run. In New Orleans, the 10-year five-minute intensity is nearly double the 100-year hourly rate; in Mobile and Tampa it is more than two and a third times as high. A gutter sized exactly to the hourly figure can overflow briefly during an intense burst. Whether that matters depends on what sits below it: a flower bed, or a door threshold and a slab.

Getting it built

Gutters, fascia and roof edges are linked, so gutter sizing is worth raising with whoever works on your roof. You can find a roofer in any state on the National Roofing Directory. For what hurricanes add to the picture, see the hurricane rain guide; for the five Gulf cities in detail, see Gulf Coast gutters.

Frequently asked questions

What rainfall intensity should gutters be sized for?

The International Plumbing Code bases storm drainage sizing on the 100-year hourly rainfall rate from its maps, or on rates from approved local weather data. NOAA Atlas 14 gives that 100-year, 60-minute figure for a specific location.

Does roof pitch change the gutter size?

Not directly. The plumbing code's sizing language refers to projected roof area, the footprint seen from above, rather than the sloped surface. What does change the numbers is a vertical wall that sheds rain onto the roof: half of its area is added to the drainage area.

How do I find the rainfall rate for my house?

Open NOAA's Precipitation Frequency Data Server, click your location on the map and read the intensity table for the duration and return period you want.

How many downspouts does a gutter need?

Enough that the flow reaching each one stays within the leader capacity in the code's Table 1106.3. Divide the run's flow by the capacity of the leader size you plan to use, and add outlets if the numbers do not fit.