Territory Intelligence · drainage
Water risk lives outside the floodplain.
Cook County carries two flood layers on the public record and they disagree by an order of magnitude. One is federal, drawn, and legally binding on the homeowner. The other is modeled, published, and covers nearly ten times as many homes. Every figure below is computed from the county flood file joined to the assessor's characteristics file on 1.11 million residential parcels.
11,100Homes in a FEMA Special Flood Hazard AreaCounty flood layer
108,315Homes at modeled flood factor 5 or higherCounty flood layer
103,170Of those, outside the mapped floodplainComputed join
82,499Of those, with a full or partial basementAssessor + flood layer
01 · The gap
One map covers 1% of the county. The other covers 10%.
FEMA's Special Flood Hazard Area contains 11,100 Cook County residential parcels. That is 1.0% of the 1,111,156 parcels carrying a flood record. The First Street flood factor, a 1-to-10 modeled scale published alongside it, puts 108,315 homes at 5 or higher: 10.0% of the 1,081,687 parcels with a factor on file. The second number is 9.8 times the first.
The gap has a commercial shape, and it comes from paperwork. An SFHA designation arrives with a lender letter, a mandatory insurance premium, and a flood zone printed on a closing document. Homes inside that line have been told, repeatedly, for decades, by an industry with every reason to keep telling them. The other cohort gets none of that. No letter, no premium line item, no closing disclosure. Nothing in the mail all year.
So the mapped floodplain is the most heavily worked 1% of the county, and the modeled risk outside it is close to untouched.
The read for contractors
Sizing a Cook drainage territory off the FEMA layer gives you 11,100 homes county-wide, and every competitor with an insurance-adjacent list is already on them. The addressable middle is the cohort with real modeled exposure and no federal document telling them about it.
Two ways to count water riskCounty flood layer · assessor
02 · The distribution
The scale is lopsided, and the second hump is the market.
Four homes in five sit at the floor. 862,155 Cook parcels carry a flood factor of 1, 79.7% of everything scored. Then it thins out fast: factors 2, 3 and 4 hold 111,217 homes between them, a tenth of the county.
And then it climbs again. Factor 6 alone holds 71,032 homes, more than factor 3, 4 or 5 taken individually and more than six times the entire mapped floodplain. Past that the tail is thin without being empty: 10,017 at factor 7, 1,793 at 8, 4,772 at 9, 1,564 at 10. Add the top two rungs together and you get 6,336 homes at the model's most severe grades, of which 5,711 are outside the FEMA line.
That shape is useful. A smooth gradient would force you to defend an arbitrary cut point. This one has a cliff in it, and the cliff sits between 4 and 5.
The read for contractors
If you want one number to size a Cook drainage territory, factor 6 and up is a defensible one: 89,178 homes, concentrated, and mostly invisible to the insurance channel.
Flood factor, all 10 gradesn=1,081,687 scored parcels
| Factor | Homes | Share |
|---|---|---|
| 1 | 862,155 | 79.7% |
| 2 | 10,446 | 1.0% |
| 3 | 49,972 | 4.6% |
| 4 | 50,799 | 4.7% |
| 5 | 19,137 | 1.8% |
| 6 | 71,032 | 6.6% |
| 7 | 10,017 | 0.9% |
| 8 | 1,793 | 0.2% |
| 9 | 4,772 | 0.4% |
| 10 | 1,564 | 0.1% |
03 · What the layer is
Say plainly what a flood factor is, because the honesty is the argument.
The flood factor is a model output. It estimates the probability and severity of water reaching a property over a multi-decade horizon from terrain, rainfall, and hydrology. It is not a record of a house that took water. Nobody inspected these basements. Nothing on this page says any specific home has flooded, and we will not build a campaign that says it either.
The honest version cuts against us too. The two layers disagree in both directions. Of the 11,100 parcels inside the federal floodplain, only 5,145 also carry a modeled factor of 5 or higher. The other 5,908, which is 53.2% of the FEMA cohort, score 4 or below on the model, and 47 have no factor at all. Treat the model as ground truth and you have to accept that it quietly downgrades more than half the homes the federal map flagged.
Read them as two independent opinions about the same ground, from institutions with different vintages and different incentives. Where they agree, confidence is high. Where they disagree, you have a question worth asking at the door rather than an answer to assert on a postcard.
The read for contractors
Model language belongs in your targeting and never in your pitch. "This area models high for water risk" is defensible and checkable. Anything stated about a specific basement is neither.
Inside the FEMA floodplainn=11,100 SFHA parcels
04 · Where the water would go
82,499 homes: modeled risk, no federal letter, and a basement.
Risk only becomes a job when there is something below grade to protect. So join the outside-the-floodplain cohort to the assessor's basement field. Of the 103,170 homes at factor 5 or higher outside the SFHA, 103,148 have a basement type recorded. 63,353 have a full basement and 19,146 have a partial one. That is 82,499 homes, 80.0% of the cohort, and 7.4 times the size of the entire mapped Cook floodplain.
Counter-evidence belongs here, so take it now: this cohort is not basement-enriched. County-wide, 82.2% of Cook homes with the field populated have a full or partial basement (915,331 of 1,114,049), which is slightly more than the 80.0% inside the high-factor group. High modeled water risk in Cook does not select for basements. Its structure is ordinary. Size and silence are what make it worth working.
The stock does skew old. 86.5% of the cohort predates 1980 against 84.6% county-wide, and its median build year is 1952 against 1954 for the county. Old drain tile, old sump arrangements, old lot grading. And 82.9% are owner-occupied, so the person who would authorize the work usually sleeps above the basement in question.
The read for contractors
Two public files, one join, and the addressable market moves from 11,100 homes to 82,499. Nothing exotic went into that: the county publishes both layers, and almost nobody joins them.
Basement type, factor 5+ outside FEMAAssessor · n=103,148
05 · Township concentration
Eleven townships have no floodplain at all. Some of them have the most risk.
Eleven of Cook's 38 townships contain zero parcels in a Special Flood Hazard Area. Lake Township is one of them, and it also holds the largest single block of modeled risk in the county: 18,687 homes at factor 5 or higher, none of them federally mapped. West Chicago Township, 10,464 and zero. South Chicago, 4,583 and zero. A drainage contractor working those three townships off the FEMA layer is working from a blank sheet.
Concentration is heavy. The five biggest townships hold 52,897 of the county's 103,170 invisible-risk homes, 51.3% of the total, and Lake plus Jefferson alone account for 30,606. By share rather than raw count the ranking flips: South Chicago leads at 29.1% of its homes, then Evanston at 23.4%, Rogers Park 22.5%, Stickney 20.0%, Palos 19.9%.
The read for contractors
Those are two different campaigns. Lake Township is a volume play: 18,687 qualifying homes, 13,156 of them with a basement, spread across a big footprint. South Chicago is a density play, where nearly three homes in ten qualify and a tight canvass route hits the cohort without a mailing list at all.
Factor 5+ outside the floodplainTop 8 townships by count
| Township | Homes | 5+ outside | Share | w/ basement |
|---|---|---|---|---|
| Lake | 142,497 | 18,687 | 13.1% | 13,156 |
| Jefferson | 111,677 | 11,919 | 10.7% | 10,598 |
| West Chicago | 65,464 | 10,464 | 16.0% | 7,343 |
| Hyde Park | 63,935 | 6,039 | 9.4% | 5,633 |
| Thornton | 53,022 | 5,788 | 10.9% | 4,431 |
| South Chicago | 15,761 | 4,583 | 29.1% | 3,717 |
| Worth | 43,259 | 3,102 | 7.2% | 2,253 |
| Evanston | 12,945 | 3,029 | 23.4% | 2,635 |
06 · Direction of travel
Two-thirds of the cohort is modeled as getting worse.
The flood file carries a direction flag next to the factor. Inside the 103,170 invisible-risk homes, 70,108 are flagged as increasing risk (68.0%), 29,419 are flat, and 3,643 are modeled as improving. Whatever you think of the model's precision, its own account of itself says this cohort grows.
County-wide, 155,077 parcels carry an increasing flag, and 80,413 of those currently sit below factor 5 and outside the floodplain. On the model's own logic that is the queue behind the queue.
Narrow to the intersection that actually pays and you land on 56,642 homes: outside the mapped floodplain, factor 5 or higher, flagged increasing, with a full or partial basement. Five times the entire federal floodplain, sitting in public files, mailed to by almost nobody.
The read for contractors
Direction is what turns a list into a standing market. The floodplain barely moves between map revisions. A modeled risk layer re-scores every cycle, which means the same territory can be re-worked with a new cohort instead of a repeat mailing to the same 11,100 addresses.
Modeled risk directionFactor 5+ outside FEMA · n=103,170
Method & sources
Figures are computed from official public records covering Cook County residential parcels: the county's flood universe file (FEMA Special Flood Hazard Area status and First Street modeled flood factor, 1 to 10, plus a risk-direction flag) joined by parcel identifier to the Cook County Assessor's property-characteristics file (basement type, build year, ownership). 1,111,156 of the assessor's 1,114,335 parcels carry a flood record; 1,081,687 of those carry a scored flood factor. Flood-factor percentages use the 1,081,687 scored parcels as the denominator. Basement percentages use only parcels with the basement field populated (103,148 within the outside-floodplain cohort, 1,114,049 county-wide) and exclude blanks. Build-year and ownership percentages likewise exclude parcels with those fields empty.
The flood factor is a modeled estimate of future flood probability and severity, not a record of observed flooding at any property. Everything on this page is aggregate, at township level or higher. No individual home is identified, and nothing here is a claim about any specific property's condition, history, or need for work. The same layers, scored neighborhood by neighborhood, are what a Territory X-Ray runs on. This brief is recomputed as the underlying county files update.
FEMA · Special Flood Hazard Areas
First Street · modeled flood factor
Cook County Assessor · basement type, build year, ownership
County open-data universe file · parcel join
This is the county read. Your towns have their own.
The Territory X-Ray runs the flood layers, basement stock, and ownership on the specific townships you work: ranked neighborhoods, reason codes, and the first drainage campaign worth your money. Start with the Territory Intelligence hub for the county-level briefs.
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