The Drain Risk Index — Where America’s Drains Work Hardest

Three things measurably shape how often a drain line gives trouble, and all three vary enormously by state: how deep the ground freezes, which drives the joint movement that lets roots in; how hard the water is, which drives the scale that narrows a pipe over decades; and how old the housing is, which is the best available proxy for what the buried pipe is actually made of.

We combined published figures for all three into a single 0–100 score across the 37 states where we have coverage and all three data points. Higher means more pressure on buried drain and sewer lines.

The index

Methodology

1. Frost depth is the published state frost-line figure in inches, normalised linearly from 0″ (score 0) to 80″ (score 100).
2. Water hardness is the midpoint of the published state hardness range in ppm, normalised linearly from 20 ppm (score 0) to 275 ppm (score 100).
3. Housing age is the median across the towns we cover of the US Census median year structure built (ACS 5-year 2023, table B25035), normalised so that a median of 1940 scores 100 and 2000 scores 0. Older housing scores higher because it is more likely to sit on jointed clay or cast-iron laterals rather than plastic.
4. Composite is an unweighted mean of the three sub-scores. We chose equal weighting because we have no defensible basis for preferring one over another — and we would rather say that than invent a weighting that looks more scientific than it is.
5. States missing any of the three are excluded rather than estimated.

What the third factor changed

Adding housing age moved the table substantially, and in a direction worth understanding. Iowa rises sharply on a median build year of 1950 — the oldest housing stock in our coverage — despite unremarkable water. New York and New Jersey rise for the same reason. Arizona and Texas fall: the water is genuinely hard, but the housing is new enough that most of it is on plastic pipe that does not care. That is the point of including it. Hard water attacking a PVC lateral laid in 1998 is a much smaller problem than soft water around jointed clay laid in 1935.

How to read it, and how not to

A high score means the environment and the likely pipe are harder on a buried line, not that your drain will block. Tree planting, how the line was originally laid and bedded, and what has gone down it since all matter, and none of them is in here.

Download the data

The full index is available as a CSV: drain-risk-index-v1.csv — rank, state, composite, frost depth, water hardness, median year built and all three sub-scores. Published under CC BY 4.0, so you may republish or adapt it, including commercially, with attribution.

Every state also has its own page with the numbers behind its ranking. Click any state in the table above.

Licence and reuse

This index is published under Creative Commons Attribution 4.0. You are free to use, adapt and republish it, including commercially, with attribution to DrainDispatch. If you use it, we would appreciate a link back, though the licence requires attribution rather than a link specifically.

Sources: state frost-line depth reference tables; published state water-hardness data; US Census Bureau, American Community Survey 5-year estimates 2023, table B25035. All cited in full on the methodology page.