Short answer: The free USDA Web Soil Survey shows the mapped soil under any Austin address. Draw a box around your lot, open the Soil Map, then go to Soil Data Explorer → Soil Properties and Qualities → Soil Physical Properties → Linear Extensibility to see shrink-swell potential. High numbers (deep clays like Houston Black, common on the east side) mean roof water should land well away from the foundation; thin soils over limestone (like Brackett, common on the west side) mean runoff moves fast and needs to be spread out. It’s a planning map, not a test of your yard.
What to do
- Open Web Soil Survey and go to your address.
- Draw an area of interest around your lot only.
- On the Soil Map tab, write down the map unit name(s) and their share of your area.
- In Soil Data Explorer, check Linear Extensibility and Dwellings Without Basements.
- Read the result against the table in What your results mean for downspouts.
- Treat it as a reason to ask better questions, not a foundation diagnosis.
Why soil decides how much gutters matter
A gutter only collects water. Where it ends up is what affects your foundation, and how the ground handles that water depends on the soil. On deep clay, roof water dumped at one corner soaks one side of the slab while the rest stays dry. On thin soil over rock, the same water runs downhill, cuts channels and heads for whatever is below. Same gutter, different plan. That’s why we check soil before we decide where downspouts go, and why it feeds into the bigger question of whether your house needs gutters at all.
What Travis County’s soil survey shows
We pulled the Travis County survey data (NRCS SSURGO, data dated September 3, 2026) to see which soils cover the most ground. Acres are for map units whose name includes that soil; many map units are mixtures, so the totals overlap a little.
| Soil | Acres in Travis County survey | What NRCS describes | Highest typical shrink-swell (linear extensibility) | NRCS slab rating, sample map unit |
|---|---|---|---|---|
| Brackett | ~173,000 | Shallow soil over limestone, mainly Glen Rose, on the eastern Edwards Plateau | ~3% (low to moderate) | Somewhat limited: slopes, soft bedrock within 20 in |
| Houston Black | ~117,600 | Very deep clay of the Blackland Prairies; its type location is in Travis County | 12% (very high), up to 17% in one urban map unit | Very limited: high shrink-swell |
| Eckrant | ~80,000 | Very stony clay, hard bedrock about 8 inches down in the survey data | ~4–5% (moderate) | Very limited: large stones |
| Speck | ~39,200 | Shallow over hard limestone, bedrock about 14–20 inches | ~6% (moderate to high) | Somewhat limited: shrink-swell |
| Austin | ~36,300 | Silty clay | ~5% (moderate) | Somewhat limited: shrink-swell |
The single largest map unit in the county is Brackett-Rock outcrop complex, 1 to 12 percent slopes (about 110,000 acres). The second is Houston Black clay, 1 to 3 percent slopes (about 71,300 acres). That matches the broad pattern Austin homeowners already talk about: limestone and thin soils to the west, deep clay to the east. One r/Austin commenter put it simply: “east of 35 you are sitting on top of clay.” But the map is patchy, and your lot is what counts.
Watch for “Urban land.” Many developed areas are mapped as complexes like Houston Black soils and Urban land or Brackett soils and Urban land. “Urban land” means the surface has been altered, paved or built over, and it has no soil ratings. Use the named soil in the complex as your clue, and assume there may be fill.
Step-by-step: Web Soil Survey for an Austin lot
NRCS describes Web Soil Survey as four steps: define an area of interest, view the soil map, explore soil data, and check out a report. Here’s what that looks like for one house. Menu names can change; these match NRCS’s own guide to the tool.
Step 1: Go to your address
Open Web Soil Survey and, under Quick Navigation, open Address to jump to your address, or zoom the map in until you can see your roof and lot lines on the aerial photo.
Step 2: Draw your area of interest
On the Area of Interest tab, use the rectangle or polygon tool to draw around your lot. Keep it tight. A big box that takes in half the subdivision will average your soil with the neighbors’.
Step 3: Read the Soil Map
Click the Soil Map tab. You’ll see map unit symbols drawn on the photo, plus a table with each map unit’s name and its percent of your area. Write down the full name. Houston Black clay, 1 to 3 percent slopes and Brackett-Rock outcrop complex, 1 to 12 percent slopes tell you different stories before you open anything else.
Click the map unit name for its description. In Travis County’s data, for example, the Houston Black clay, 1 to 3 percent slopes map unit is 80% Houston Black; the rest is other soils.
Step 4: Check shrink-swell (linear extensibility)
Click Soil Data Explorer, then Soil Properties and Qualities. Open Soil Physical Properties and choose Linear Extensibility. View the rating. The map shades your area by percent.
Linear extensibility is how much a clod of soil shrinks as it dries from moist to oven-dry. NRCS groups it into shrink-swell classes:
| Linear extensibility | Shrink-swell potential |
|---|---|
| Under 3% | Low |
| 3 to 6% | Moderate |
| 6 to 9% | High |
| Over 9% | Very high |
For a horizon-by-horizon table, use Soil Reports → Soil Physical Properties → Physical Soil Properties.
Step 5: Check the building-site rating
Go to Suitabilities and Limitations for Use → Building Site Development → Dwellings Without Basements. The rating says “Not limited,” “Somewhat limited” or “Very limited,” with the reasons, such as shrink-swell or depth to bedrock. For Travis County, the same list also has a Texas rating, Dwellings On Concrete Slab (TX), the closer match if your house sits on a slab.
Step 6: Save the report
Add what you viewed to the Shopping Cart tab and download the free report. Keep it with your house papers and bring it to any gutter, drainage or foundation conversation.
What your results mean for downspouts
| What you found | What it suggests for roof water |
|---|---|
| High or very high shrink-swell (over 6%), deep clay | Discharge well away from the slab, at least the 5 feet in the 2024 model code’s roof-drainage section, onto ground that slopes away. Use solid pipe to a daylight outlet where you can. |
| Moderate (3–6%) | Still discharge away from the foundation; evenness around the house matters. |
| Shallow to bedrock, rock outcrop, steep slopes | Little soil to absorb water. Spread and slow the flow with splash pads or rock, and think about who is downhill. Trenching may hit rock. |
| A map unit name that says “frequently flooded” | That’s a creek-flooding question first. Check your address in the City’s flood map; see our Onion Creek drainage post. |
Clay: plan for both extremes
A USDA sheet on Houston Black says water enters these soils rapidly when they’re dry and cracked and very slowly when they’re moist. In August, the first storm pours down open cracks next to the slab. In a wet May, the same soil can’t take water fast enough and it ponds. Building Science Corporation’s foundation report also warns that suddenly drying expansive soil around a foundation can cause movement. The aim is steady, even moisture near the house, with storm water carried away.
That’s also why a gravel pit or “drywell” next to the house is a poor default on clay: water that soaks in slowly sits there, near the foundation. Any infiltration design in slow soil needs storage, infiltration and overflow checked, not just a perforated pipe. And PNNL warns against tying roof drains into a perforated foundation drain at all.
Limestone: plan for speed
On Brackett-type soils, NRCS lists runoff as medium on 5 to 20 percent slopes and high on 20 to 60 percent slopes. Roof water concentrated at one downspout on a hillside becomes a small stream. Spread it, slow it, and keep it on your lot. On the west side, see West Lake Hills and Oak Hill; for clay country, see Pflugerville.
What Web Soil Survey can’t tell you
NRCS calls Web Soil Survey a planning resource and says onsite investigation is needed in some cases, including certain engineering applications. Limits to keep in mind:
- Map units are mixtures. A unit that’s 80% Houston Black can still have a different soil on your lot.
- Builders move dirt. Cut, fill and imported select fill under slabs aren’t on the map.
- It doesn’t know your foundation. Depth, design and past movement need a professional look.
- It can’t diagnose cracks. Plumbing leaks, trees and drainage all play a part. Gutters reduce concentrated roof wetting; they can’t promise a clay foundation won’t move.
If you’re worried about the foundation itself, hire a geotechnical or structural engineer. If you want to know whether a code drainage rule applies, see which Austin roof-drainage rules apply to your property.
FAQ
What type of soil is in Austin?
Both clay and limestone soils, often within a few miles. In Travis County’s survey, the largest map unit is a shallow Brackett soil over limestone, and the second largest is Houston Black clay. Check your own address, because the pattern is patchy.
Where can I find an Austin soil type map?
USDA’s Web Soil Survey is free and covers Travis, Williamson and Hays counties. Draw your lot as the area of interest and open the Soil Map tab.
What does linear extensibility mean?
It’s the percent a soil clod shrinks as it dries from moist to dry, a measure of shrink-swell potential. NRCS calls under 3% low and over 9% very high. Houston Black’s typical value in Travis County’s data is 12%.
Do French drains work in clay soil?
They can clog or drain slowly in clay, which is why a French drain shouldn’t double as a roof-water outlet. Carry downspout water in its own solid pipe to a daylight outlet or emitter instead. See downspouts and drainage.
Can Web Soil Survey tell me if my foundation will move?
No. It shows mapped soil properties for planning. Fill, foundation design and site conditions can differ from the map, so foundation questions need an onsite evaluation.
Sources
- USDA NRCS: Web Soil Survey
- USDA NRCS: Soil Data Explorer tab guide (PDF)
- USDA NRCS: Soil Data Access (SSURGO), Travis County TX453
- USDA NRCS: SSURGO database
- USDA NRCS: Technical Guide Section II, engineering interpretations (shrink-swell classes)
- NRCS Official Series Description: Houston Black
- NRCS Official Series Description: Brackett
- NRCS Official Series Description: Speck
- USDA: Houston Black soil sheet (hosted by TWDB)
- PNNL: gutters and downspouts
- International Code Council: 2024 IRC Chapter 8, Section R801.3
- Building Science Corporation, BA-1015
When you’re ready for the gutters themselves, see seamless gutters.
Request a gutter estimate. Bring your Web Soil Survey report if you have one. We’d check the slope away from the house at every downspout and lay out where each one can discharge on your lot.