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This barndominium gutter size calculator turns your roof size into gallons per minute at Tyler, Texas rain rates. It then compares that flow with standard gutter and downspout capacities. You enter the roof, the storm and the number of downspouts. It is a planning estimate, not a quote.
En español: Calculadora de canaletas y bajantes para barndominium en Tyler, Texas
Background on where roof water should go: barndominium gutters and drainage on East Texas clay.
Calculator: gutter and downspout size
| Your roof |
|---|
| Semicircular gutter, 1/8 in per ft slope | Capacity (gpm) | Downspouts needed per run |
|---|
| Round downspout | Capacity each (gpm) |
|---|
How the math works
- Roof area to one gutter run
Length times width gives the footprint. A gable roof drains half the width to each long eave. A single-slope roof drains all of it to one eave.
- Rain rate
Use the 1-hour rain depth in inches as inches per hour. Tyler's 100-year value is 3.96.
- Gallons per minute
Area × rate × 0.623 ÷ 60. One inch of rain on one square foot is 0.623 gallons, and an hour has 60 minutes.
- Compare with the capacity
Divide the run flow by the number of downspouts. The result has to be at or under the capacity of the gutter and the downspout.
Tyler rain rates and what they mean per 1,000 sq ft
NOAA Atlas 14 gives the rain that falls in one hour for each return period. The last column is plain arithmetic: 1,000 × rate × 0.623 ÷ 60.
| Storm return period | Rain in 1 hour (in) | gpm per 1,000 sq ft |
|---|---|---|
| 2 year | 1.85 | 19.2 |
| 5 year | 2.28 | 23.7 |
| 10 year | 2.65 | 27.5 |
| 25 year | 3.16 | 32.8 |
| 50 year | 3.55 | 36.9 |
| 100 year | 3.96 | 41.1 |
Rain depths: NOAA Atlas 14, Volume 11 (Texas), Precipitation Frequency Data Server, point 32.35 N, 95.30 W near Tyler, partial duration series, 60-minute row.
Standard capacities used in the calculator
These are the International Plumbing Code values for the sizes most used on houses. Gutters are at a slope of 1/8 inch per foot.
| Gutter diameter | Capacity (gpm) | Round downspout | Capacity (gpm) |
|---|---|---|---|
| 4 in | 39 | 3 in | 92 |
| 5 in | 74 | 4 in | 192 |
| 6 in | 110 | 5 in | 360 |
| 8 in | 247 | 6 in | 563 |
Values from the International Plumbing Code, 2021 edition, Tables 1106.3 and 1106.6. Check the table in the edition your building office has adopted.
Why the downspout count matters: a roof that sheds a lot of water can overload a small gutter, but adding downspouts cuts the flow each section carries. Compare a bigger gutter against more outlets before you pay for either.
What to ask your builder or installer
- Who supplies and installs the gutters? Shell bids and kits often leave them out. See our gutter and drainage guide.
- Which gutter size, shape and material? Get the size in writing, with the rain rate they assumed.
- How many downspouts, and where do they discharge? Water dumped beside the slab on clay soil is the problem. See the foundation on East Texas clay guide.
- Where does water go at the driveway? See driveway culverts in Smith County.
- What does it cost? Gutters and site drainage fall in the site and exterior lines of the budget line items guide; the barndominium cost calculator gives a whole-project range.
What the result leaves out
| Item | Why it is not in the number |
|---|---|
| Leaves and debris | They reduce real capacity. Guards and cleaning matter. |
| Wind-driven rain | Rain blown against walls adds water the footprint does not show. |
| Gutter shape and slope | Capacities here are for semicircular gutters at 1/8 in per ft. |
| Cost | Priced by the installer, not by this tool. |
To compare local gutter and drainage quotes, use the form on this page. We are a referral service that connects you with independent local builders and installers; we do not install gutters or give quotes. Read how to choose a barndominium builder first, and see the other barndominium calculators.
Sources
- NOAA Atlas 14 Precipitation Frequency Data Server, Volume 11 (Texas): 1-hour rain depths for Tyler, 1.85 to 3.96 inches for 2-year to 100-year storms, and 1.20 inches in 5 minutes for the 100-year storm.
- International Plumbing Code 2021, Chapter 11, Storm Drainage: roof drainage sized for a 1-hour, 100-year storm; Table 1106.3 round leaders and Table 1106.6 semicircular gutters.
- University of Nebraska Extension G2220: 0.623 gallons per square foot per inch of rain.
Frequently asked questions
What size gutter does a barndominium need?
It depends on the roof area draining to each gutter run, the rain rate you design for and how many downspouts share the run. This calculator turns your roof size into gallons per minute at Tyler rain rates and compares that flow with the semicircular gutter capacities in the International Plumbing Code table. A roof with long eaves and few downspouts needs a bigger gutter or more outlets. Your builder or gutter installer sets the final size.
What rainfall rate should I use for gutters in Tyler?
The International Plumbing Code sizes roof drainage for a storm of one hour and a 100-year return period. NOAA Atlas 14 gives 3.96 inches in one hour for that storm at Tyler. The calculator also offers the 2, 5, 10, 25 and 50-year values from the same NOAA table, and lets you type your own rate if your designer or supplier uses a different one.
Does roof pitch change how much water a gutter gets?
Not much for the flow calculation. Rain falls straight down, so what counts is the flat area the roof covers, called the footprint. A steeper pitch makes the roof panels longer but does not add rain. Pitch can still change how fast water reaches the gutter and how it splashes over, so ask your installer about that.
How many downspouts does a barndominium need?
There is no single number. The more downspouts on a run, the less water each gutter section carries. The calculator shows how many downspouts a run needs for each standard gutter size, so you can compare a larger gutter with more outlets. Ask your builder how many are in the bid and where they discharge.