A homeowner in Madison called us last spring about water pouring over his gutters every time it rained. Not a drizzle — any real rain, and water was shooting right past the gutters onto the ground below.
His house was about eight years old. Nice place, 3,200 square feet, steep roof pitch on the front elevation. The builder had installed 5-inch gutters with 2x3-inch downspouts. Standard stuff — the cheapest option that meets code.
Problem is, standard doesn't mean right. His roof was pushing water faster than those gutters could catch it. We upgraded him to 6-inch gutters with 3x4-inch downspouts. Same house, same roof, same rain — no more overflow.
The builder saved maybe $250 going with smaller gutters. The homeowner spent eight years with water pooling at his foundation before he figured out what was wrong.
Here's the thing about gutter sizing: it's not one-size-fits-all, and the charts you find online don't tell the whole story. Let me give you the chart you came here for, then explain what it actually means for your house.
The Quick Reference Chart
Here's what you need to know at a glance:
| Gutter Size | Capacity Per Foot | Handles Roof Area Up To* | Typical Use |
|---|---|---|---|
| 5" K-Style | 1.2 gallons | 5,500 sq ft | Standard residential |
| 6" K-Style | 2.0 gallons | 7,960 sq ft | Larger homes, steep roofs |
| 5" Half-Round | 0.7 gallons | 2,500 sq ft | Historic homes, aesthetics |
| 6" Half-Round | 1.2 gallons | 5,500 sq ft | Larger historic homes |
| 7" K-Style | 3.0+ gallons | 10,000+ sq ft | Commercial, large estates |
*Assumes proper downspout sizing, moderate roof pitch, and average rainfall intensity. Keep reading — these assumptions matter.
Why That One Inch Matters More Than You Think
Look at the capacity column again. A 5-inch K-style gutter holds 1.2 gallons per foot. A 6-inch holds 2.0 gallons per foot.
That's not 20% more. That's 67% more capacity from one extra inch of width.
Let's put that in real numbers. Say you have 150 linear feet of gutters — pretty typical for a 2,000-2,500 square foot house. With 5-inch gutters, your total system capacity is about 180 gallons. With 6-inch gutters, it's 300 gallons.
During a heavy North Alabama thunderstorm — and we get plenty of those — your roof might be shedding 50+ gallons per minute. The difference between 180 and 300 gallons of buffer capacity is the difference between your gutters handling the surge and water pouring over the sides.
Here's something else that chart doesn't show: it's not just about holding water, it's about catching it in the first place. A 6-inch gutter has a wider opening. On a steep roof where water comes off fast, that wider mouth gives the water somewhere to go. A 5-inch gutter on a steep roof — especially if it's mounted a little too far from the roof edge — can have water overshooting it entirely before there's any chance to catch it.
How to Figure Out What Size You Actually Need
The charts give you general guidance. Here's how to get specific for your house.
Step 1: Calculate Your Drainage Area
For each section of your roof that drains to a gutter run, calculate the square footage. Length times width for each section, then add them up.
On a simple gable roof, you've got two sections — front and back. On a hip roof or a house with multiple levels and roof lines, you might have five or six separate drainage areas.
Don't overthink this. You're looking for a ballpark, not an engineering spec. If your roof section is roughly 40 feet long and 20 feet from ridge to gutter edge, that's 800 square feet of drainage area.
Step 2: Adjust for Roof Pitch
Steeper roofs shed water faster. That matters for gutter sizing because faster water is harder to catch and creates higher peak flow rates.
Here's the multiplier to use based on your roof pitch:
| Roof Pitch | Multiply Sq Ft By |
|---|---|
| 4/12 or less (low slope) | 1.05 |
| 5/12 to 6/12 (moderate) | 1.1 |
| 7/12 to 8/12 (steep) | 1.2 |
| 9/12 to 12/12 (very steep) | 1.3 |
Not sure what your roof pitch is? Stand back and look at it. If you can easily walk on it, you're probably 4/12 to 6/12. If it looks steep and you'd be nervous walking on it, you're probably 7/12 or higher. If it looks like a ski slope, you're 9/12 or steeper.
For a precise measurement: hold a level horizontal against the roof, measure 12 inches out from where it touches, then measure straight down to the roof surface. If that measurement is 6 inches, you have a 6/12 pitch.
Step 3: Factor In Local Rainfall Intensity
This is where most online calculators fail you. They use national averages or don't account for rainfall intensity at all.
Rainfall intensity isn't about annual totals — it's about how hard it rains when it rains. The National Weather Service tracks maximum rainfall intensity: the most rain that could fall in a 5-minute burst, expressed as inches per hour.
For North Alabama — Huntsville, Madison, Decatur, Athens — that number is around 5.5 to 6.5 inches per hour. We get serious storms. Compare that to somewhere like Denver at 2.5 inches per hour or Seattle at 2.0. Our gutters have to handle more than twice the peak flow rate.
The formula: take your pitch-adjusted square footage and multiply by your local rainfall intensity.
Putting It Together: A Real Example
Let's say you have a house in Huntsville with a roof section that's 1,000 square feet draining to one gutter run. The roof has an 8/12 pitch.
Step 1: Drainage area = 1,000 sq ft
Step 2: Pitch adjustment = 1,000 × 1.2 = 1,200 effective sq ft
Step 3: Rainfall intensity = 1,200 × 5.5 = 6,600 adjusted sq ft
Looking back at the chart, 5-inch K-style gutters handle up to 5,500 adjusted square feet. This roof section needs 6-inch gutters.
And that's just one 1,000-square-foot section. A 2,500 square foot home might have 2,000+ square feet draining to the front gutters alone. Do the math for your own roof and the numbers get real pretty quick.
K-Style vs Half-Round: What's the Difference?
Most residential gutters you see are K-style — they have a flat back that mounts flush against the fascia and a decorative front profile that looks a bit like crown molding.
Half-round gutters are exactly what they sound like: half of a tube. They hang from brackets rather than mounting flat, and they have a completely different look.
Here's what matters for sizing:
K-style holds more water per inch of width. The shape is more efficient — a 5-inch K-style holds about 1.2 gallons per foot, while a 5-inch half-round holds only about 0.7 gallons. That's why you'll see recommendations to go one inch wider on half-round to match K-style capacity.
Half-round drains cleaner. The smooth, curved interior doesn't have corners where debris can catch and accumulate. In theory, they clog less easily. In practice, the difference is modest if you're doing regular maintenance.
Half-round costs more. The brackets add material and installation time. Half-round is often used for historic homes or high-end copper installations where aesthetics matter more than cost efficiency.
For most homes in North Alabama, K-style is the practical choice. It holds more water, costs less, and handles our storm intensity well. Half-round makes sense when you're matching historic architecture or going with copper — but budget accordingly.
Downspout Sizing: The Part Everyone Forgets
Your gutters are only as good as your downspouts. You can have 6-inch gutters with plenty of capacity, but if your downspouts can't drain them fast enough, water backs up and overflows anyway.
Here's the downspout capacity breakdown:
| Downspout Size | Handles Roof Area | Typically Paired With |
|---|---|---|
| 2" × 3" rectangular | Up to 600 sq ft | 5" K-style (minimum) |
| 3" × 4" rectangular | Up to 1,200 sq ft | 5" or 6" K-style (recommended) |
| 3" round | Up to 700 sq ft | 5" half-round |
| 4" round | Up to 1,200 sq ft | 6" half-round |
The rule of thumb: one downspout for every 40 feet of gutter run. But that's a minimum. Valleys, corners, and areas with heavy water concentration often need additional downspouts.
Here's what we see constantly: homes with 6-inch gutters and 2x3-inch downspouts. The gutters can hold plenty of water, but the downspouts can't drain it fast enough. During heavy rain, water backs up at the outlets and eventually overflows the gutters right at the corners — exactly where you don't want it.
We default to 3x4-inch downspouts on almost every install, even with 5-inch gutters. They cost a few dollars more and they handle real storms without backing up. The 2x3 size might be fine in Portland. It's marginal in Alabama.
What North Alabama Weather Means for Sizing
Let me be direct about our local conditions because this is where generic advice fails.
North Alabama gets about 55 inches of rain annually. That's 45% higher than the national average. And it doesn't fall gently — we get thunderstorms that drop 2+ inches in under an hour, multiple times every spring and summer.
Those intense storms are what gutters have to handle. It doesn't matter if your gutters work fine during a light drizzle. What matters is whether they work when it's really coming down.
Our recommendation for this area: 6-inch gutters with 3x4-inch downspouts on any home over 2,000 square feet of roof area, or any home with roof pitches steeper than 6/12. The cost difference is modest — maybe $200-400 more for a typical house — and the performance difference during real weather is dramatic.
If your gutters have been "fine for years" but you've noticed more overflow lately, you might not be imagining it. Rainfall patterns have shifted. What worked in 2005 may genuinely not handle what we're getting now.
The Builder-Grade Problem
Here's something we see constantly, especially in Madison and the subdivisions that went up during the building boom of 2005-2015.
Builders install the minimum that meets code. For gutters, that typically means 5-inch K-style with 2x3-inch downspouts, spaced to pass inspection — not optimized for actual performance.
The cost difference to the builder between 5-inch and 6-inch gutters is maybe $200-300 per house. On a $350,000 home, that's nothing. But multiply that by a hundred houses in a subdivision, and the builder saves $25,000.
The homeowner lives with undersized gutters for years, watches water pool at the foundation during every storm, and eventually pays someone like us to fix it — usually for more than the builder would have spent doing it right in the first place.
We see this pattern in Providence, Bradford Creek, the developments along County Line Road, and a dozen other Madison and south Huntsville neighborhoods. If your house was built between 2005 and 2015 in this area and you've got water problems during storms, undersized gutters are a likely culprit.
Signs your builder-grade gutters are undersized:
Water overshooting during rain. If water is going over the front edge of the gutter rather than into it, the gutter isn't catching it — either too small, mounted wrong, or both.
Pooling at the foundation. Some pooling is normal if your grading is off, but if it's happening directly below overflow points, your gutters aren't handling the flow.
Erosion under downspouts. If your downspouts are blasting water hard enough to dig trenches, they might be undersized and creating high-velocity discharge.
Debris washing out the ends. If you find debris on the ground at the gutter corners after a storm, water is flowing fast enough to push material over the edge.
How to Measure Your Existing Gutters
If you want to know what size gutters you currently have, here's how to measure:
For K-style gutters: Measure from the back of the gutter (where it touches the fascia) to the front lip. That's your gutter size. A 5-inch gutter is 5 inches from back to front. Note that the opening at the top is smaller than the overall width — a 5-inch gutter has about a 4.25-inch opening.
For half-round gutters: Measure the outside diameter at the widest point.
For downspouts: Measure the face dimensions. Rectangular downspouts are described by width × depth — so a 2x3 is 2 inches by 3 inches. Round downspouts are measured by diameter.
If you're not comfortable on a ladder, the gutter and downspout sizes are usually easy to eyeball once you know what to look for. Five-inch and 6-inch gutters look noticeably different in proportion to the fascia board. Two-by-three and 3x4 downspouts are obviously different sizes.
When It Makes Sense to Upgrade
If you're getting overflow during storms that shouldn't be overwhelming your gutters, something's wrong. Either the gutters are undersized, the pitch is off, the downspouts are blocked, or some combination.
Upgrading gutters isn't cheap — you're looking at $1,500-3,500 for a typical house, depending on size and complexity. But compare that to the cost of foundation repairs, which can run $5,000-15,000. Or siding damage from constant water exposure. Or landscaping that keeps washing away.
Proper gutter sizing is insurance. The math almost always works out in favor of doing it right.
Need Help Figuring It Out?
If you're looking at your gutters and not sure whether they're sized right for your house, give us a call at (256) 616-6760. We'll come out, look at your roof, check your current system, and tell you what we think — whether that's an upgrade, a simple repair, or "you're fine, don't spend the money."
We've been doing this across North Alabama for over twenty years. We've seen what works in our weather and what doesn't. No charge for the assessment, no pressure to buy anything. Just straight answers about what your house actually needs.
