Learning Center · Sizing
Gutter Sizing and Downspouts for Texas Rain
A gutter is sized to the roof above it and the rain that falls on it. This guide puts NOAA’s own rainfall numbers for Round Rock, Dallas and Houston next to the plumbing code’s gutter math, so you can see when a 5 inch gutter is enough and when a 6 inch one is the right call.
Updated October 12, 2026
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The short version
- NOAA Atlas 14 puts a once in 100 years hour of rain at 4.58 inches in Round Rock, 3.69 in Dallas and 4.85 in Houston.
- The International Plumbing Code turns roof area and rainfall into gallons per minute, then rates each gutter size and slope.
- A 5 inch gutter at 1/8 inch per foot of fall is rated at 74 gallons a minute, and a 6 inch at 110.
- Most overflow we see comes from too few downspouts or a flat gutter, not from the gutter width alone.
To have your gutters measured against your roof, start with Get Help or call or text 512-575-5052.
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Direct Answer
Do I need 5 inch or 6 inch gutters in Round Rock?
Size the gutter to the roof area feeding each run and the rainfall, and 6 inch wins on big or steep roofs.
The short version
It depends on how much roof drains into each run. By the plumbing code method and NOAA’s 100 year hourly rainfall for Round Rock, a 5 inch gutter with proper fall handles roughly 1,500 square feet of roof per run, and a 6 inch roughly 2,300. Long runs, steep roofs and two slopes meeting over one gutter push toward 6 inch.
What the gutter has to do
One inch of rain on a 1,500 square foot roof is about 935 gallons. A gutter does not store that water. It has to move it sideways to a downspout as fast as it arrives, and a Texas storm can arrive fast. That is why width, fall and downspout count all matter together.
NOAA Data
How hard it rains in Round Rock, Dallas and Houston
NOAA Atlas 14 gives the hourly rainfall depth by city and by how rare the storm is.
The numbers engineers use
NOAA Atlas 14 is the federal rainfall study that codes and engineers rely on. It gives, for any point in Texas, how much rain falls in a given time for a storm of a given rarity. The table shows the one hour depth, which is also the inches per hour rate, for three cities. A once a year hour of rain in Round Rock is about 1.6 inches. A once in 100 years hour is 4.58 inches, and that is the number the plumbing code sizes drainage to.
Short bursts are harder than the hourly number
Gutters overflow in bursts, not averages. The same NOAA data puts a once in 10 years 5 minute burst in Round Rock at 0.77 inch, which is a rate of about 9 inches an hour while it lasts. A gutter that keeps up with that burst keeps up with the storm. That is the reason a run with one downspout at the far end spills over in May even though it drains fine in a steady rain.
| City | Once a year, 1 hour | Once in 10 years, 1 hour | Once in 100 years, 1 hour |
|---|---|---|---|
| Round Rock | 1.60 inches | 2.84 inches | 4.58 inches |
| Dallas | 1.56 inches | 2.55 inches | 3.69 inches |
| Houston | 1.90 inches | 3.22 inches | 4.85 inches |
Plumbing Code
The gutter sizing method in the plumbing code
Multiply roof area by inches per hour by 0.0104 to get gallons per minute, then read the gutter table.
Step one: rainfall to gallons per minute
The International Plumbing Code sizes drainage from the 100 year hourly rainfall for the site. Its Equation 11-1 is simple: gallons per minute equals the rainfall rate in inches per hour, times the roof area in square feet, times 0.0104. For a 1,500 square foot roof section in Round Rock, that is 4.58 times 1,500 times 0.0104, or about 71 gallons a minute. The same section in Dallas is about 58, and in Houston about 76.
Step two: read the gutter table
Table 1106.6 of the code rates gutters by size and fall. A 5 inch half round gutter set at 1/8 inch of fall per foot is rated at 74 gallons a minute. A 6 inch at the same fall is rated at 110. An 8 inch at 1/16 inch per foot is 172, and at 1/8 inch per foot it is 247, which shows how much the fall matters. The table lists half round and box shapes, not the K-style profile most Texas homes have, so treat it as a planning guide and let the installer size the real run.
Step three: check the downspouts
Table 1106.3 rates the downspout, which the code calls a leader. A 2 1/2 by 2 1/2 inch leader is rated at 54 gallons a minute, and a 3 by 4 1/4 inch leader at 192. The common residential sizes are 2 by 3 and 3 by 4, which sit near those rows. Our 71 gallon Round Rock example needs more than one 2 by 3 outlet, or one larger outlet, on that run. Our roof edges and drainage guide explains where the outlet sits in the whole system.
What Actually Overflows
Why the fall and the number of downspouts matter more than width
A flat gutter and a single far outlet overflow long before a 5 inch gutter runs out of room.
Fall
The code numbers assume the gutter falls toward the outlet. At 1/8 inch per foot, a 10 foot run drops 1 1/4 inches. A gutter that has settled flat holds water after every rain and carries far less than its rating. Fascia movement and loose hangers are the usual reasons a run goes flat, and the fix is often to rehang it, not replace it. Where the run is joined and sealed is covered in our seamless versus sectional guide.
Outlets
A long run with one downspout at the end asks all the water to travel the full length before it leaves. Splitting the run with a second outlet halves the distance and the load on each one. On the homes we look at around Round Rock, a second downspout fixes more overflow than a wider gutter does. Downspout count is part of our gutter price, never a separate line, so adding one is a design decision, not an upsell.
Roof shape
Where two slopes drain into one inside corner, that corner takes twice the water. Steep roofs shed water faster than low ones, so the burst arrives sooner. Both cases argue for 6 inch gutter and a downspout close to the corner. The gutter service page shows how we measure a roof before we size the run.
A Round Rock House
A worked example: a 2,400 square foot roof in Round Rock
Four gutter runs, each sized to its own roof section, with the downspouts placed by the math.
Divide the roof into runs
Say a one story home has a simple gable roof of about 2,400 square feet, with a long eave on the front and back. Each long eave drains about 1,200 square feet. At 4.58 inches an hour, that is 1,200 times 4.58 times 0.0104, or about 57 gallons a minute per run. A 5 inch gutter at 1/8 inch per foot, rated at 74, handles it. One 2 by 3 downspout does not, since the nearby table rows sit at 54 and 92 gallons a minute, so each run gets two downspouts, one near each end.
When the same roof wants 6 inch
Now put a hip at each end so two slopes feed each long run, and make the sections 1,800 square feet. That is about 86 gallons a minute, past the 5 inch rating of 74. The run moves to 6 inch, rated at 110, with 3 by 4 downspouts. That is the pattern we see on the large ranch style homes around Round Rock and Georgetown: the roof area, not the house size, decides the gutter.
Common Questions
Questions homeowners ask
Are 6 inch gutters worth it in Texas?
On large or steep roofs, yes. The plumbing code rates a 6 inch gutter at 110 gallons a minute against 74 for a 5 inch at the same fall, and a 1,800 square foot roof section in Round Rock produces about 86 gallons a minute in a 100 year storm. On a small roof section with enough downspouts, 5 inch is fine.
How many downspouts does a gutter need?
Enough that the gallons per minute from the roof section never exceed what the outlets can take. Using the code tables, a 1,500 square foot section in Round Rock makes about 71 gallons a minute, which is more than one 2 by 3 downspout handles. Long runs do better with an outlet at each end.
What slope should a gutter have?
The code capacity tables for 5 and 6 inch gutters assume 1/8 inch of fall per foot, about 1 1/4 inches over 10 feet. A gutter that holds water after rain has lost its fall and needs rehanging.
How much rain can a gutter handle?
A 5 inch half round gutter at 1/8 inch per foot is rated at 74 gallons a minute in the International Plumbing Code, and a 6 inch at 110. Whether that is enough depends on the roof area feeding the run and the local rainfall rate from NOAA Atlas 14.
Does a 2 by 3 downspout match a 5 inch gutter?
Often, when there are enough of them. The code rates a 2 1/2 by 2 1/2 inch leader at 54 gallons a minute, close to a 2 by 3. A 3 by 4 downspout carries far more and is the usual match for a 6 inch gutter.
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