In short
- Roof runoff is simple math: footprint sq ft x inches of rain x 0.623 gallons. One inch on a 3,000 sq ft roof is about 1,869 gallons.
- Tyler's normal year is 42.10 inches of rain with no dry month, so that water arrives all year.
- Gutters, downspout extensions and finish grading are often not in a shell bid. Ask who owns each in writing.
- Site work is already a big budget line: $10,000 to $60,000 with driveway on our budget guide.
On this page
- How much water does your roof send to the ground?
- Calculator: roof runoff in gallons
- Gutter sizing: questions, not a spec
- Slope away from the slab and where downspouts discharge
- Driveway culverts are part of the same water plan
- Who does finish grading, and is it in the bid?
- Checklist for the bid
- Sources and method
- Frequently asked questions
Good barndominium gutters and drainage keep roof water from soaking the clay beside your slab. A metal roof delivers a lot of water to a few edges, and the bid rarely says where it goes. This guide gives the runoff math for Tyler, the questions to ask, and the line items that get skipped. We are a referral service, not a builder.
En español: canaletas y drenaje de barndominium en arcilla del este de Texas.
Planning estimate, not a quote or engineering advice. Your engineer's drainage notes and local rules come first.
Rain figures: NOAA NCEI U.S. Climate Normals 1991 to 2020, Tyler Pounds Regional Airport, as shown on our best time to build page. Normals are averages, not forecasts.
How much water does your roof send to the ground?
One inch of rain on one square foot is one twelfth of a cubic foot. A cubic foot holds 7.48 gallons, so the answer is 7.48 / 12 = 0.623 gallons per square foot per inch. That is the most a roof can shed; real systems lose a little. The University of Nebraska Extension publication G2220 on rainwater harvesting uses the same factor.
| Roof footprint | 1 inch of rain | Tyler's year |
|---|---|---|
| 2,000 sq ft | 1,246 gal | 52,457 gal |
| 3,000 sq ft | 1,869 gal | 78,685 gal |
| 4,000 sq ft | 2,492 gal | 104,913 gal |
Arithmetic only: footprint x inches x 0.623, using the 42.10 inch normal. Footprint means the flat area the roof covers, overhangs included.
Calculator: roof runoff in gallons
| Line | Gallons |
|---|
Gutter sizing: questions, not a spec
We do not size gutters here; the right size depends on roof area, pitch and the rainfall rate your designer chooses. Ask these and get the answers on paper:
- Are gutters in the bid? Some shell and kit prices stop at the roof panels and trim.
- What size and how many downspouts? Ask what storm intensity the sizing assumes.
- How is the gutter fastened? Ask how it attaches at a metal eave and whether panels shed straight into it.
- Who cleans and maintains them? Pine needles and leaves are part of East Texas ownership.
- Do both long eaves get water control? Water from a 60 ft roof edge adds up fast, as the table above shows.
Slope away from the slab and where downspouts discharge
The ground beside a slab should fall away from it so water does not sit at the edge. The International Residential Code has a drainage requirement for the grade around foundations. Ask your builder or engineer for the exact slope on your plan and for the code section and edition your permitting office enforces. Our foundation on clay guide explains why wet edges matter on swelling soil.
| Item | Ask for | Who |
|---|---|---|
| Grade at slab edge | Slope shown on the plan, all four sides | Engineer |
| Downspout outlet | Extension or pipe that carries water away | Builder |
| Low spots | Final check after trades leave | Dirt contractor |
| Neighbor runoff | Where water enters and leaves your lot | Engineer |
Watch out: a downspout that drops onto bare clay beside the slab sends the whole roof's water to the one place you want dry. Ask where each outlet ends, not just whether downspouts exist.
Driveway culverts are part of the same water plan
If your driveway crosses a roadside ditch, the culvert is part of how water leaves the lot. On a Smith County road the county's order sets a 15 inch minimum pipe and says larger may be needed depending on natural flow. Our driveway, culvert and 911 address guide lists who approves it and what the order says. Settle the culvert before the pad, so the roof drainage plan and the ditch agree.
Who does finish grading, and is it in the bid?
Finish grading is the last shaping of the ground around the finished building. It often belongs to a dirt contractor, not the shell builder, and it can fall between two bids. Our budget line items guide puts site prep (clearing, pad, fill) at $10,000 to $60,000 with driveway and names the site contractor as the quoter. Ask whether drainage and finish grading sit inside that figure.
Shell builder includes it
- One contract
- Check that it names slope and downspout outlets
Best if: the bid lists drainage as a line.
Separate dirt contractor
- Own quote and schedule
- You coordinate the handoff
Best if: the builder says site work is excluded.
Checklist for the bid
- Gutters and downspouts: included, excluded or allowance?
- Downspout outlets: extensions, pipe or splash blocks, and where they end.
- Finish grading: who, when and what slope?
- Culvert size and approval, in writing.
- Who fixes settling or low spots after the first heavy rain, and for how long (see builder warranty)?
- Does the engineer's plan show drainage notes?
Compare bids with our how to choose a builder guide, then use the form below to get quotes from independent local builders.
Sources and method
Rainfall: NOAA NCEI U.S. Climate Normals 1991 to 2020, station USW00013972, Tyler Pounds Regional Airport. Conversion: 7.48 gallons per cubic foot divided by 12 inches, 0.623 gallons per sq ft per inch; see University of Nebraska Extension G2220. Culvert and cost items: the Smith County and budget pages linked above. Results are theoretical maximums; gutters, splash and wind change real flow.
Frequently asked questions
Does a barndominium need gutters?
Nothing on this page says it must. A metal roof sends a lot of water to the eave line in one place, and on clay that water ends up beside the slab. Gutters or another planned way to carry roof water away are worth asking about, and the engineer's drainage notes decide what your lot needs.
How much water comes off a metal roof?
Plain arithmetic: roof footprint in square feet times inches of rain times 0.623 gallons. One inch on 3,000 square feet is about 1,869 gallons. Use the calculator on this page with your own footprint.
How steeply should the ground slope away from the slab?
Ask your builder or engineer for the grade on the plan and for the code section it follows. The International Residential Code has a drainage rule for the ground around foundations, and the permitting office that covers your parcel confirms which code edition applies.
Who does finish grading on a barndominium?
Often a dirt or site contractor, and often outside the shell bid. Ask each bidder in writing whether finish grading is included and who is responsible for final slope after the other trades leave.
Do I need a culvert with a new driveway?
On a Smith County road, the county's driveway culvert order sets a 15 inch minimum pipe. See our driveway and culvert guide for the full list and who to call.