This post gives advice and tips for replacing cast iron shower drains on a concrete slab foundation. Hammering into an old concrete foundation to replace a drain is the trickiest and most hazardous part of remodeling a vintage midcentury shower. read on for tips on how to properly remodel your vintage home bathroom.

One of the downsides to buying an otherwise fabu vintage home is inheriting the geriatric plumbing. It takes skill and precision to properly waterproof a tile shower using even the most modern waterproofing materials. Unfortunately, those materials didn’t exist sixty years ago. And, plumbers and tile installers of the 1960s were just as likely to be lazy as any rando contractor today. A shower remodel on these slab foundation midcentury modern homes can open a pandora’s box of plumbing trouble.

The Problem

Modern conventional shower drain assemblies are made of PVC plastic and look like this:

The bottom piece attaches to the 2-in drain pipe, the middle piece makes a watertight seal with a vinyl or fiberglass waterproofing ‘pan’, and the top piece screws into the middle piece to adjust for height and make the drain grate (the only part that you actually see once the shower is finished) flush with the shower floor tile:

It wasn’t until the early 1980s that PVC drain pipes became the default for residential home plumbing. Homes built before the 1970s all used cast iron drain pipes that connected to cast iron shower and bathtub drains. If you own a vintage home with original 60+ year old cast iron drain plumbing, then you have a remodel problem.

Replacing a modern PVC shower drain attached to modern PVC drain piping is relatively easy even if the drain is set in a concrete foundation slab. Since PVC doesn’t rust, the drain bolts can be unscrewed and the whole assembly can be taken apart down to the base flange. There are then lots of options for attaching different drain types to this standard base flange, since the 4-bolt pattern is standardized to work with lots of other modern drain assemblies and adapters.

Replacing cast iron shower drains, on the other hand, is a real pain. Cast iron rusts over time. After doing many shower projects on midcentury homes in Austin, I’ve seen the whole spectrum of how an old-fashioned cast iron shower drain goes from shiny new to decrepit corroded mess over 50+ years of use. And even the occasionally well-preserved 1950s cast iron drain has a 3-bolt pattern that will not match any modern drain adapter kit:

There is literally nothing that you can bolt, glue, or otherwise attach to an old cast iron drain for an easy re-do of a shower floor. The only thing you can do with an old cast iron shower drain is rip it out and start over by booting a modern PVC drain to an intact section of the bare cast iron metal pipe underneath.

So here are the four basic steps for replacing cast iron shower drains:

1. Get access to the pipe below the corroded cast iron drain assembly
2. Cut the pipe with a grinder or hacksaw (being real careful to not crack it)
3. Boot new PVC pipe to the old cast iron pipe with a no-hub rubber coupling
4. Replumb using a modern PVC conventional or Kerdi drain set to proper height

Seems pretty simple, right? Nope, not on a concrete slab it isn’t.

The Danger

You can easily get access to the drain piping underneath an old cast iron shower drain if your vintage midcentury modern Austin ranch home is built on a pier and beam foundation. Unfortunately, most postwar homes in Austin were built on concrete slab foundations.

Getting access to drain piping in a concrete slab foundation means jackhammering through the concrete. And hammering into an old concrete slab will risk popping pipes and creating even more plumbing problems.

Remember that plumbers of the 1950s were just as likely (probably more likely) to be just as dodgy as any rando contractor today. In addition, there were no residential building inspections in 1950s Austin. So you can never know what might be lurking in an old concrete foundation slab before hammering into it.

For example, modern water supply piping run under a slab foundation actually runs UNDER the concrete slab. Also, modern plumbing inspection residential construction codes place minimum distances between supply pipes and drain pipes. This is all to ensure that supply pipes won’t get broken in case a homeowner has to have unexpected drain or other work done on a slab foundation.

In the 1950s, it was apparently common for plumbers to run copper water pipes right in the middle of concrete slabs. It must have been convenient for lazy midcentury plumbers to strap copper pipe directly to the concrete rebar. Also, I’ve encountered several examples of copper water pipes run right next to drain pipe flanges (like, within 6-in!!!). Penny pinching midcentury Austin plumbers also apparently liked to save on copper by taking real direct routes from water mains to water faucets.

Minimizing Risk

When hammering into a residential concrete slab foundation that was plumbed and poured in the 1980s or later, you can predict with good certainty where the water supply pipes are and therefore avoid accidentally popping them with just a little bit of care and common sense.

However, concrete slab foundations plumbed and poured in the 1950s and 1960s are like a lost lid box of chocolates. Hammering into them always carries risk of popping a water supply pipe no matter how careful you are.

My own record (so far) on this is 11-1, which is something that I’m quite proud of. But I have experienced one popped pipe loss so far. Here’s a picture montage of what happened on that job:

As you can see, this shower had an old corroded 1950s cast iron drain that needed to be cut out. Unfortunately for me, there was a buried 1/2-in copper hot water pipe running right next to it right in the middle of the 6-in thick concrete slab. I did not predict that level of plumber laziness. And, there was no way on earth that a jackhammer wasn’t going to find and pop that pipe.

WINNING REQUIRES LEARNING FROM LOSSES

Here is my most recent cast iron shower drain replacement project. This example shows how real careful jackhammering can pull a win from the watery jaws of potential popped pipe defeat. A past homeowner had already tried to “fix” this 1952 plumbing problem by replacing just the shower base:

You can tell from the mismatched tile work that someone tried to make the original leaky shower base “waterproof” sometime in the past by demolishing just the base, adding either a fiberglass or vinyl liner, and then re-tiling.

However, you can also tell just by looking at the drain grate that they didn’t replace the original cast iron drain. So, unsurprisingly, this shower was still leaking water into the walls and had to go. And this required finally replacing the old original cast iron shower drain.

Demolishing the floor tile and drypack underneath revealed lots of water damaged wall framing. Demo also revelated an old iron shower drain in all of its grungy glory:

This is where the hammering begins. Replacing cast iron shower drains on slab requires making a hole through the concrete around the drain with enough clearance to carefully cut through the metal drain pipe with an angle grinder. So, you need to jackhammer through the slab out to about 12 to 16-in from the drain.

Here are some tips for doing this carefully:

TIP #1: TRY TO PREDICT THE SUPPLY PIPING LAYOUT

You can never know exactly where the horizontal water pipe runs are in an old concrete slab foundation. But you can at least make some educated guesses by seeing where the pipes come up through the slab into the bathroom walls.

In this case, there were hot and cold 3/4-in manifolds in the wall underneath the bathroom sink. There were only single 1/2-in hot and cold copper pipes in the wall for the shower. So, I figured that even the worst lazy plumber couldn’t have done any worse than simply running hot and cold pipes diagonally from the 3/4-in manifolds directly to the shower valves:

I therefore figured there was a pretty big triangular space in the slab (outlined in white) that would be safe for jackhammering.

TIP #2: USE A SMALL HAMMER

Demo folks normally use a big 70-lb “breaker” jackhammer for making holes in concrete slabs.

These actually aren’t terribly large as jackhammers go. They will make a nice big hole in even a rebar-reinforced foundation slab in no time flat. But they’ll also hammer a copper water pipe flat as a traintrack penny in about two seconds.

So, you really REALLY want to use a small 20-lb demo hammer for tricky jobs with unknown pipe locations:

A 20-lb demo hammer can still definitely pop a pipe even if you’re working slowly and carefully (ask me how I know!). But it’ll at least offer some added caution and safety compared to a full size breaker hammer.

TIP #3: GO SLOW (and cut off the water before starting!)

Demo for replacing cast iron shower drains on slab will take awhile. It will also be loud, dusty, and a literal pain. I worked six full hours on this job just to make this relatively small opening in the 6in thick slab foundation:

Going inch by inch through old hardened concrete takes awhile. First, you make a shallow donut-like excavation all around the drain out to about 6-in. You want to be VERY careful for this step. Do not hammer directly on or adjacent to the cast iron drain or drainpipe itself. Old cast iron is very brittle. Treat it very gently. Only use a hand hammer and cold chisel to carefully knock out the concrete right around the pipe.

It’s the concrete slab further out from the drain that you can jackhammer more aggressively. But again, BE CAREFUL and GO SLOW. For example, my piping layout guess on this job proved to be completely wrong:

Turns out there was a big ‘ol 3/4-in water main right at the apex of the triangular area that I’d guessed would be completely clear of pipes. And it was right in the the slab, where the original plumber had hung it just below the rebar. A fullsize breaker hammer would have gone straight through it. But I was able to avoid popping it by using a small demo jackhammer, going slow, frequently using just a hand hammer and cold chisel, and treating every bit of uncovered rebar as a potential water pipe.

Slow and Careful for the WIN!

Replacing an old cast iron shower drain is simple once you’ve successfully hammered out a hole through the slab large enough to maneuver an angle grinder or simple hacksaw.

I use Kerdi for waterproofing custom tile shower installations. With the cast iron drain out of the way, installing a Kerdi drain was a simple matter of carefully cutting the original 2-in cast iron drain pipe at the appropriate depth and booting it to the PVC plastic Kerdi drain assembly:

Now it’s just a normal tile shower install project. I’m right now in the process of getting the shower tile ready (the whole project will be the subject of a future blog post). But here’s a sneak peek of the custom shower floor slope install and the Kerdi waterproofing:

Hopefully this has been a helpful deep dive on replacing cast iron shower drains for tile shower renovations on concrete slab foundations. It’s not a project for the faint of heart. But it’s not brain surgery. Cheers!