Gutter sizes

By Gutter Report Editorial. Last reviewed August 2026. How we research this.

In brief

1.0The method, in four steps

Step 1 is where people go wrong first. Measure the footprint, the flat area the roof covers, not the surface you would walk on. Berger warns directly that using the true sloped area of a pitched roof often leads to oversizing.

There is a fifth input almost nobody uses. It is in the plumbing code, section 1106.4. Add half the area of any vertical wall that sheds rain onto the roof. A two story wall above a lower roof pours onto that lower roof. The gutter below has to carry it.

2.0Step 2, the pitch factor

The Copper Development Association publishes this set in its Copper in Architecture Design Handbook. This Old House publishes it identically. Two sources agreeing on a whole table is rare in this trade, and worth noting.

A steep roof throws water further and faster off the same footprint, which is what the factor is for. A 1,000 square foot footprint under a 12 in 12 roof is a 1,300 square foot design area.

3.0Step 3, rainfall intensity, and it changes everything

This input is what makes national gutter advice close to useless. Here is the five minute intensity for nine cities, at a ten year recurrence. The figures are NOAA Atlas 14 point estimates at city coordinates.

Miami is about three times Los Angeles. Any published roof area figure has to be divided by this number, so the same gutter serves roughly three times more roof in Los Angeles than in Miami. That is why a national rule of thumb cannot be right in both places.

Two cautions. These are point values at city hall coordinates, and NOAA warns that intensity varies inside a metro area. And Atlas 14 does not cover Oregon or Washington at all, so those two states need a state source instead.

The design storm itself is a choice, not a fact. Fine Homebuilding uses the ten year, five minute burst for houses. SMACNA offers ten year or 100 year at five minutes. The plumbing code sizes storm drainage on a 100 year, one hour basis. A more severe storm means a bigger gutter.

4.0Step 4, and the number everyone misquotes

Gutter	Drainage area at 1 in. per hour	Source
	SQ FT of roof
5 in. K-style	5,520	Fine Homebuilding, basis stated
6 in. K-style	7,960	This Old House, basis not stated
5 in. half round	2,500	This Old House, basis not stated
6 in. half round	3,840	This Old House, basis not stated

Divide the figure by your rainfall intensity. That gives a real maximum roof area. Fine Homebuilding does exactly that in its own worked example. It takes 5,520, divides by 6.6 inches an hour in Washington DC, and gets 836 square feet for one 5 inch gutter with one downspout.

Run the same division elsewhere and you can see the country move. In Los Angeles at 3.64, the same gutter covers about 1,516 square feet. In Miami at 10.8, about 511. That arithmetic is ours, using the NOAA figures in section 3.0.

One caution on the half round rows. Those two figures trace to a different table from the K-style rows, so the profiles are not compared on one method. Treat half round as the smaller gutter at the same width, and see K-style against half round for the detail.

5.0What the code tables say

The plumbing code sizes gutters by flow rate in gallons per minute rather than by roof area. These are the semicircular rows of International Plumbing Code Table 1106.6, read on UpCodes.

Two things to take from it. Capacity roughly triples from a 5 inch to an 8 inch at the same slope, and doubling the slope from 1/16 to 1/8 inch per foot adds about 43 percent on the 8 inch row. Slope is a capacity input, not just a drainage nicety. See gutter slope.

Also worth knowing what the code does not do. It does not require you to have gutters. The IPC sizes them when they are used, and it generally does not apply to one and two family houses at all, which fall under the residential code. That code has no general gutter mandate either.

6.0What sizes exist

That roll formed limit is the practical one. If you want seamless, you are choosing between 5 and 6 inch, because those are the sizes the machines make. Anything larger is joined sections.

A floor exists too. Texas A&M AgriLife Extension states that as a general rule gutters should be at least 5 inches wide. Four inch gutter is a porch and canopy size.

7.0You may not need a bigger gutter

If you are here because a gutter overflows, size is the most expensive of the four possible causes and the least likely. A clog backs up at any size. A run pitched wrong holds water and spills at the low point. Too few downspouts is the most common cause of all.

Two changes buy more capacity than a size increase, for far less money. Fine Homebuilding states that going from one downspout to two doubles the roof area a section of gutter can drain, and that pitching a gutter at 1/8 inch per foot multiplies its watershed by 1.4. Compare that with the roughly 40 percent that moving from 5 inch to 6 inch buys, per Englert.

So work through how many downspouts and gutter slope before you price re-hanging the house. We can say that plainly because we do not sell gutters and we do not install them. If you are replacing anyway, the size decision is on the 5 inch against 6 inch page, and the money is on the cost calculator.

Rev

RevAugust 2026

Source

SourceCDA, NOAA Atlas 14, IPC via UpCodes, Fine Homebuilding, This Old House, Berger, Englert (read 2026-08-21)