5-Inch or 6-Inch Gutters for a Phoenix House?

5-inch or 6-inch gutters for a Phoenix house?

Every gutter company in the Valley says “monsoon-ready.” Almost none of them show the arithmetic, and the arithmetic is what tells you which size you need. Here it is.

The math for a monsoon storm

Start with the rate. Phoenix gets most of its rain between June 15 and September 30, and the storms that matter for sizing are the ones that drop an inch in well under an hour. We size for an inch an hour as the working number, knowing that a strong cell can run harder than that for ten or fifteen minutes.

Then the roof. One inch of rain on one square foot is 0.623 gallons.

• A 1,500 square foot roof: 935 gallons per inch.
• A 2,000 square foot roof: 1,246 gallons per inch.
• A 2,500 square foot roof: 1,558 gallons per inch.

At an inch an hour, 1,246 gallons over sixty minutes is about 21 gallons a minute across the whole roof. That is not spread evenly. A hip roof drains to all four eaves, but the long rear slope on most Valley plans carries a third to a half of the roof, so the rear gutter is handling 7 to 10 gallons a minute on its own, and every gallon of it has to get to a downspout.

That last point is the one that matters. A gutter does not store water; it moves it sideways to an outlet. Capacity is really three things: how much the trough can carry to the outlet without spilling over the front lip, how fast the outlet and downspout can take it away, and whether the water lands in the trough in the first place.

Why 6-inch is our standard

It carries more water to the outlet. A 6-inch K-style trough is wider and deeper than a 5-inch, so for the same slope it moves more water before it tops the front lip. We are not going to put a percentage on that, because the honest figure depends on the slope, the outlet spacing and how clean the gutter is. More is the accurate word.

It catches the tile overshoot. This is the Phoenix-specific reason and the one no national sizing guide mentions. A concrete S-tile roof has a curved profile, and the water running down the troughs between the pans leaves the edge of the tile with some forward momentum. On a tile eave the stream lands an inch or two further out from the fascia than it would off a flat shingle edge. A 5-inch gutter, with its front lip closer to the wall, can have monsoon-rate water clear the lip entirely or hit the lip and splash over. The wider 6-inch trough puts its front lip out where the stream lands.

It takes a 3x4 downspout. A 6-inch gutter has a wide enough floor for a 3x4 outlet. That is the size that keeps up with the trough in a monsoon and does not plug on a mesquite pod.

It handles the debris load. Between palo verde blossoms, mesquite pods and a dry season of haboob silt, a Valley gutter carries a layer of material along its floor for most of the year. In a 6-inch trough that layer takes up a smaller share of the capacity. In a 5-inch trough it can be the difference between draining and overflowing.

Where 5-inch is the right answer

We would rather tell you this than upsell you.

A casita, a detached garage or a shed. A roof under about 600 square feet draining to a single eave with a downspout at each end does not need a 6-inch trough. Five-inch with 2x3 downspouts will drain it at monsoon rate.

A patio cover. Most aluminum patio covers have a built-in gutter in the front beam already, and where they do not, a 5-inch run matches the scale of the beam and drains the cover’s roof comfortably.

A short shingle run. A shingle or metal roof with a modest pitch and a run under 30 feet, with a downspout at the end, is fine at 5-inch. The overshoot problem is a tile problem.

Matching an existing 5-inch system. If the rest of the house is already in 5-inch and you are adding a run over the new patio, matching is a reasonable choice, as long as that eave is not the long tile one.

What we will not do is put 5-inch on the rear eave of a 2,000 square foot tile roof because it is a little cheaper. That is the eave that carries the most water, throws it the furthest, and floods the patio when it spills.

Downspouts: 2x3 versus 3x4

The downspout is half the capacity conversation and most people never ask about it.

A 2x3 downspout has an opening of six square inches. A 3x4 has twelve. That is twice the opening, and in practice it passes roughly twice the water. It is also far harder to plug: a mesquite pod that jams a 2x3 outlet drops straight through a 3x4.

Our planning rule on a 6-inch run is a 3x4 downspout at every low point, at every inside corner where two roof planes dump into one valley, and no more than 30 to 40 feet of gutter between outlets. Long runs with one downspout at the far end are the classic overflow: the water has to travel the length of the trough, and by the time it gets there the trough is full. Adding a second outlet in the middle does more for a struggling run than any change in gutter size.

Each downspout then needs somewhere to let go. Four to six feet from the wall, onto a splash block or into a buried line to a spot that drains. A 3x4 downspout that discharges at the stem wall has simply moved the monsoon closer to the slab.

Looks, HOAs and color

The other reason people ask about 5-inch is appearance. On a single-story house with a low fascia, 6-inch can look heavy in a catalog photo. On a tile roof it does not: the tile edge is thick and the eave is deep, and a 6-inch K-style profile reads in proportion to it. We run four colors, dark gray, bronze, white and tan, and in master-planned communities like Anthem the HOA rule is usually that the gutter matches the fascia or the stucco field. Bronze on a brown fascia and tan on a desert-tone stucco disappear from the street.

What it costs

Our published installed range is for 6-inch seamless aluminum, and it covers the trough, hangers, corners and the labor to hang it under tile. Five-inch on a small run is quoted the same way. The what seamless gutters cost in Phoenix post has the per-foot range and worked examples by house size, and the seamless gutter installation page covers how we hang it on a tile eave.

Houses in the North Valley, New River, Desert Hills and Cave Creek especially, tend to have large single-story tile roofs on big lots with long rear eaves, and they are the clearest case for 6-inch with 3x4 downspouts we see. If you would like the math run on your own roof, we do free estimates and will tell you which eaves need 6-inch and which do not.