You think you might have an issue with the drain lines under your home, but don’t know much about them. How are they set up? How do they work? What usually goes wrong with them? And finally, how can you tell if there’s a problem?
At Anchor Foundation Repair, we’ve been around drain lines for 40+ years working under foundations and doing under-slab tunneling to access plumbing issues. We have worked with countless BCS plumbers and seen the damage and repair processes as they unfold for Brazos Valley homeowners. We might not be certified plumbers, but we’re experts in providing solutions for under-slab drain issues.
This article describes the basic drain line set-up for a slab-on-grade foundation home. Then we review how drain line systems work and how we commonly see them fail.
Basic Sewer Drain Set-Up and Function Under a Slab Home
Understanding your drain system setup is the beginning of understanding how drain lines work. A drain system consists of 6 basic components:
- Draining fixtures,
- Vertical lines,
- P-traps,
- Horizontal lines,
- Connection to a sewer or septic system, and
- Air vents.
So let’s talk about each component and visualize how this is all structured and connected. We’ll start with the familiar fixtures you can see in your home and proceed *down the line* to where the waste eventually ends up.
1. Draining Fixtures
Every plumbing fixture in your home has a drain that takes used wastewater *and other stuff* away from it. These are the normal items you think of in your home: toilets, sinks, bathtubs, showers, and washing machines. Dishwashers are tied to your kitchen sink, so they are not quite the same as these other fixtures and appliances.
Some draining fixtures you probably don’t think of are the AC condensation line and the drain pan line under your water heater.
All these draining fixtures are attached to your drain line system. With some help from gravity, air, and water pressure, the fixtures first flow into vertically oriented drain lines . . .
2. Vertical Lines
Vertical drain lines run inside your home from the bottom of the draining fixtures and then go through your slab foundation to the underside of your home. Vertical lines are typically embedded into walls or cabinetry and then penetrate the floor and your foundation.
Usually made from cast iron or PVC, the vertical lines essentially connect the draining fixtures to a point under the slab, where they then turn to the horizontal. But first, you need a P-trap somewhere along the way . . .
3. P-traps
A P-trap is a curved section of plumbing line designed to keep sewer gases out of your home. Sewer gas is bad. Sewer gas makes you sick. You can easily spot the telltale curvy pipe under your kitchen and bathroom sinks. You don’t necessarily see them on all the other draining fixtures, but they are there somewhere.
Some P-traps appear above floor level (i.e. on sinks and toilets) and some are positioned below floor level (think tubs and showers). On a toilet, the P-trap is actually part of the toilet itself and explains the funny snaking base you see below the seat on most commodes.
In case you’re wondering, these are called a P-traps because they are kinda shaped like the letter P. I think it looks more like an S, but maybe that’s just me . . .
Anyway, a small amount of water constantly remains in the curved part of the P-trap section of the pipe. This magical *barrier of water* blocks sewer gas from coming through the rest of the pipe and into your home. Every draining fixture must have a P-trap on its drain line path.
Even though it has a funny name that makes you giggle, you want this thing on every drain line in your home and you want them working properly. Though there could be some exceptions, most P-traps are positioned before a drain line turns to the horizontal . . .
4. Horizontal Lines
Once the vertical lines reach under the slab, they have to turn to the horizontal. Now, they don’t run on a perfectly horizontal path, it’s more of a general direction. These drain lines should have a slight slope to encourage wastewater flow in the right direction, i.e. AWAY from your home.
As the lines from each of your fixtures turn to the horizontal, they meet up with other lines along the way gathering into larger drain lines.
It’s sort of like a bunch of straight tree branches that eventually connect to a tree trunk. They even call it that. A main trunk line then finds its way out from under your house.
At this point, you might see a “cleanout” pipe that comes up to ground level or sometimes a little higher. Most homes have cleanouts to help clear clogs, but occasionally there could be an older home without one.
Seeing a cleanout lets you know where your sewer lines come out from under your home and where they are heading – it’s often somewhat “downhill”. Sometimes homes have more than one clean-out location (perhaps near a kitchen), but there’s always a primary one on the trunk line.
The main trunk line then proceeds through your yard in the direction of a municipal sewer or septic system . . .
5. Connection To a Sewer or Septic System
If you live in a city or town, your drain lines will join up with the greater municipal sewer system somewhere on the edge of your property. Often, the lines head towards the street. Then the wastewater joins up with other people’s wastewater and it’s all carried off to a treatment plant, but we won’t go any further down the line.
Just know that the sewage is leaving your property and going off to a better place – *wastewater heaven* is what we’ll call it.
If you live in a rural area, your drain lines will likely terminate at your septic system. In the country, no shared community sewer system is run by a city, so everyone has their own personal one.
I’m also not going to get into what septic systems do – because that’s too much *doo* to talk about . . .
6. Air Vents
Air vents are being listed last because I didn’t want to confuse you by talking about them earlier in the mix. Within the vertical part of the drainage system that starts in your walls when the wastewater is flowing down, there’s also a vent system that runs in the opposite direction – up instead of down.
Sewer vent pipes that let out through your roof help to facilitate the flow of the drain lines. Fluid will not flow well or at all in a sealed system and the vents encourage flow as well as protect against siphoning effects.
I’m not getting into a full explanation and all the science here, but it’s the same principle as having a little air vent on a water cooler. The water comes out of the tap way better when you open up the vent.
How Drain Lines Commonly Fail Under Slabs
Drain lines are well protected above ground in your home, but once they get below the house, it’s rough out there. The toughest environment for your drain lines is in the ground under your slab. Running through expansive clay soil is no picnic, and your drain lines are subject to degradation, and seasonal settlement and rebounding based on ever-changing moisture levels.
When I say fail, I don’t mean a little blockage or clog that is easy to fix and the pipe remains intact. We’re talking about complete plumbing line failure that requires full-access repair or replacement of some section of the line.
Here are the most common ways that drain lines fail under your home:
- Cast iron deterioration,
- Separation of PVC, or
- Soil or foundation movement creating bellies or breaks.
We’ll go into more detail on each of these common drain line failings next.
Cast Iron Deterioration
Older homes may have cast iron drain lines, and guess what . . . iron deteriorates, especially in a moisture-laden environment like the dirt under your home. The couplings could degrade or the drain line walls themselves can thin, become weak, and eventually rust through.
This is especially true if a lot of harsh chemicals have been poured down the drain over the years to release clogs.
They used cast iron a long time ago because it seemed strong and durable like it could last forever. Realistically, cast iron drain lines have about a 70 to 80-year lifespan. So, depending on the age of your home and drain lines, cast iron lines will fail at some point.
Separation of PVC
PVC is the newer preferred option for under-slab drain lines. It’s a plastic material that is inexpensive as well as relatively durable and easy to work with.
Imagine the weight difference alone of a long span of PVC line compared to a cast iron line . . .
Bonding various pieces of PVC together must be done properly. If for some reason the priming and adhesive applications are not sufficient or correct during construction, the PVC joints could separate under your home at any of the many connection joints. There are a lot of connections and therefore a lot of opportunities for potential failure.
Because of the unique under-home environment, under-slab lines should be done a certain way. Sometimes, they don’t get done properly for other reasons as well.
Soil Movement Creating Bellies or Breaks
The expansive clay soil environment I was talking about earlier can wreak havoc on your drain lines. The soils in our Central Texas area (and many other areas around the state and country) consist of soil that reacts quite strongly to varying moisture levels.
The soil expands when wet, and contracts when dry. Expansive soil moves so much that it can lift your house, or cause it to sink. All that soil activity subtly shoves your house and your drain lines around in different directions, potentially causing a couple of problems: bellies and breaks.
- “Bellies” happen when a low spot is created in a drain line. The low spot then impedes flow and causes accumulation and build-up because the line is not sloped correctly in a continuous downward direction anymore.
- Breaks happen when the force of conflicting home and drain line movement simply ruptures the line. You get thin cracks that start small but can continue to advance and crack more and more, opening up the drain lines to full-on fracture.
What Can Be Done About Under-Slab Drain Line Failures?
Unfortunately, when you look at the three common failures of under-slab lines, there’s not much you can do to prevent issues. You can’t magically keep cast iron from degrading. You can’t know if the PVC was properly affixed during construction or comes apart along the way. And you can’t control the behavior of expansive clay soil.
All you can really do is hope these things don’t happen and handle them well when they do.
I hinted earlier that failed drain lines need to be accessed and repaired or replaced. There are a couple of ways to approach these situations. Some are quick fixes that don’t fully address the issue, but if a plumber needs to access under-slab drain lines then they can either break through the slab or use under-slab tunneling to reach the problem for a fix.
Signs of Leak Problems With Your Under-Slab Lines
Now you know all about drain lines and how they are set up, along with common causes of failure. You’re probably wondering how you can confirm that under-slab leaks are your issue. Things like slow-draining fixtures, gurgling lines, and frequent back-ups are big indicators of drain line leaks.
At Anchor Foundation Repair, we’ve seen firsthand the long-term damage that can result from prolonged issues with under-slab drain lines. Get these faulty lines handled before allowing them to cause bigger issues, like foundation problems. We’ve been tunneling under Brazos Valley homes for years and years to make way for plumbers to fix leaks.
Want more descriptive detail on the signs of drain line leaks and ways to take care of them? Check this out: Signs You Have An Under-Slab Plumbing Leak + Ways to Solve It.
