Cast iron was the standard drain material in American houses for most of the twentieth century, and houses from the 1950s through the 1970s often have it everywhere: the building drain, the stacks, the branches and sometimes the lateral. It is heavy, quiet and, when new, very durable. It also corrodes from the inside over decades until the bottom of the horizontal runs channels through, and a buyer of a house of that age should assume the cast iron is somewhere on that path and find out where.
Quick answer: prioritise the horizontal building drain, especially under a slab, where channeling does the most expensive damage; then the stack bases, where rust-through leaks into the structure; then the lateral if it is iron. On camera, look for scale that narrows the bore, rough tuberculated walls, and channeling where the pipe floor is gone and the flow runs on soil. Light scale is a descaling candidate; channeling is replacement. Price the house on which it is.
How cast iron fails
Hydrogen sulphide from the sewage attacks the iron above the waterline and makes scale; the constant wet at the bottom of horizontal runs corrodes the wall thin until it channels. Stacks rust through at the base where they are wettest. Hubs crack and leak at the lead and oakum joints. None of it is visible from outside until a leak appears or a drain slows. Cast iron vs PVC on camera shows what the inside looks like.
Priority one: the building drain
The horizontal drain under the floor collects everything and carries it out, and it is the run that channels first. Under a slab, its failure is a floor repair; in a crawlspace, a day’s work. Camera it from the main cleanout toward the street and, where access allows, back toward the stacks. Record the worst sections by distance. Slab homes and crawlspace homes explain how the foundation changes the stakes.
Priority two: stack bases
Where each vertical stack meets the building drain is the wettest cast iron in the house and the first to rust through. A leaking stack base soaks the floor or slab beneath it and can go unnoticed for years. Look at each base in the basement or crawlspace, and camera into it from a base cleanout if there is one.
Priority three: the lateral
An iron lateral is less common than an iron building drain, but where it exists it channels like the rest. Camera to the main and note the material changes; many houses have iron inside and clay outside.
Reading the camera picture
Scale appears as a rough, dark, bumpy wall that narrows the bore; heavy scale catches paper and gives a picture of a pipe much smaller than its nominal size. Tuberculation is scale in nodules. Channeling appears as a flat or missing pipe floor with the flow running in a trough, sometimes on visible soil, with the walls above still standing. A cracked hub shows as a dark line around the joint. Cameras for cast iron covers equipment that shows these well.
Descale or replace
Scaled cast iron with sound wall thickness can be descaled with chain or cable cutters and then lined, restoring bore and sealing the wall for decades. Channeled cast iron has no wall to line against along the bottom and is replaced. The camera shows the surface; it cannot measure wall thickness, so where the decision is close a contractor may descale first and camera again before committing to a liner. Descaling, coating and replacement and descaling before lining explain the methods.
Branch lines
Iron branches to kitchens and bathrooms are smaller and often inaccessible to a 23 mm head. Where they can be reached from a pulled toilet or a trap, a small camera shows whether they share the drain’s condition. A house with channeled branches and a sound main is unusual; the main usually goes first.
Pricing it
A descale-and-line of a building drain is a significant but bounded cost; a replacement under a slab is a major project priced per room opened. Get a quote from the footage, from a contractor who does both, and negotiate it. A seller who argues that the drains still work is right until the first channeled section opens. Negotiating repairs covers the process.
Owning it
Cast iron that is scaled but sound is managed with gentle cleaning, no acid drain cleaners, and a camera every two or three years to watch the trajectory. How often to inspect and drain cleaning frequency for older homes cover the schedule.
Acid cleaners and what the kitchen tells you
Cast iron corrodes faster where chemical drain cleaners have been poured down it for years, and the kitchen branch is where that happens most. A kitchen drain that has been dosed with acid or lye products every month for a decade can be far worse than the rest of the system, and the camera from the kitchen trap arm shows it. Ask the seller what drain products have been used and look under the sink for the bottles. A house full of cast iron with a shelf of chemical cleaner is a house where the branches deserve as much attention as the main, and the new owner should switch to mechanical cleaning on day one.
Common questions
How long do cast iron drains last?
Fifty to seventy-five years in most houses, less with aggressive water or acid cleaners. Houses from the 1950s to 1970s are at the end of that range now.
Can cast iron drains be lined?
If they are scaled but not channeled, yes, after descaling. Channeled sections are replaced.
Does a camera inspection cover the whole cast iron system?
It covers what it can reach: the building drain, the lateral and some branches. Stack bases are inspected visually where exposed.
Related guides
See clay sewer pipe priorities, whether a 100 ft camera reaches 100 ft, and measuring lateral length.