California ranks #16 of 37 in the DrainDispatch Drain Risk Index, with a composite score of 42 out of 100 — around the middle of the index. The median across all 37 states is 40.
What is driving the California score
The largest of the three contributors in California is water hardness.
Freeze depth: 5 inches (sub-score 6/100)
At roughly 5 inches, the California frost line is shallow and frozen laterals are rare. That removes one of the two common causes of joint movement, which means recurring blockages here are more often about grade, grease or root pressure than about winter.
Water hardness: 100-300 ppm (sub-score 71/100)
Municipal supplies across California typically test 100-300 ppm — moderately hard to very hard. Hard water deposits scale on the inside of pipe over years. It rarely blocks a line by itself, but it narrows the effective diameter and gives grease and solids something to anchor on, which shortens the interval between blockages.
Housing age: median year built 1971 (sub-score 48/100)
Across the 263 California towns we cover, the median home was built in 1971 (US Census ACS 5-year 2023, table B25035). That straddles the change-over from cast iron and clay to plastic. A house from this period can have either, and it is common to find a plastic replacement indoors still running out to an original clay lateral — which means the weakest section is usually the one nobody has looked at.
What this does and does not tell you
It describes an environment, not a property. A high score means buried drain lines in California face more pressure on average. It says nothing about your house. Your own pipe material, how the lateral was originally laid and whether there are mature trees over it all matter at least as much — and none of those is in the index, because none of them can be sourced state by state.
Housing age here is a state median, not your house. It is the median across the 263 California towns we cover, so it is a reasonable proxy for the pipe most homes in the state were built with and a poor one for any single address. A 1961 house on this street may have been repiped in 2015.
Version 1.0 uses three factors. Tree canopy and storm intensity are planned for 2.0. We would rather publish three factors we can source than five we cannot.
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States either side of California
Full index, all 37 states, with methodology →
Questions
What does a score of 42 mean for California?
It places California at #16 of 37 covered states for environmental pressure on buried drain lines — around the middle of the index. It is a relative measure across states, not a prediction for any individual property.
How is the score calculated?
An unweighted mean of three normalized sub-scores: frost line depth (0 inches scores 0, 80 inches scores 100), median municipal water hardness (20 ppm scores 0, 275 ppm scores 100), and housing age from the US Census median year structure built (a median of 2000 scores 0, 1940 scores 100). California scores 6, 71 and 48 respectively.
Why unweighted?
Because we have no defensible basis for preferring one factor over another. We would rather say that than invent a weighting that looks more scientific than it is.
Can I republish this?
Yes. The index is published under Creative Commons Attribution 4.0, so you are free to use, adapt and republish it, including commercially, with attribution to DrainDispatch.
Where does the data come from?
Three published sources: state frost-line depth reference tables, state water-hardness data, and US Census ACS 5-year 2023 table B25035 for median year structure built, aggregated across the 263 California towns we cover. All three are cited on the methodology page.
Does a high score mean my California drains will block?
No. It means the environment is harder on buried pipe on average. Your own pipe material, how the lateral was laid and whether there are mature trees over it matter at least as much, and none of those can be sourced state by state, so none is in version 1.0.