DWV Plumbing System Issues: A Home Inspector’s Guide to Drain, Waste, and Vent Problems

Drain Waste Vents: The Unsung Hero of Plumbing

Written By: Sean Struckmeyer, Professional Home Inspector | Tech Inspect Home Services LLC

You use your plumbing every day, but do you know how it works without smells or clogs? Did you know it’s all thanks to that white or gray pipe sticking out of your roof?

Waste Management is all thanks to the DWV or Drain Waste Vent. It’s a network of pipes running throughout the home that is responsible for getting water and waste out of the home and to the sewer or septic system. While this system may be ‘simple’ compared to others in the home, it’s just as critical that it’s maintained properly and properly inspected.

The Three Critical Components: How the DWV Plumbing System Works

To understand how these systems work, lets define it’s base components:

1. 1. The Drain: Piping that carries water from a fixture (sink, shower, tub, toilet).
      1. All Drains have a P-Trap installed. The P-Trap is designed to hold water to act as a block between uses. The water that the P-Trap hold keeps sewer gases from coming back into the home. If you smell sewer gas from the drain, there’s a good chance there’s an issue with a P-Trap.
   2. The Waste Line: The larger pipes that carry the waste from the individual drains to the main sewer line or septic tank. These are generally 3 – 6” depending on the size of the home and how many drains are connected to it.
   3. The Vent: The vertical pipes that extend through the roof, often called the Vent Stack.
      • Crucial Function: The opening in the vent allows air into the system to prevent suction (siphonage) that would drain the P-traps, and relieves gas pressure. The vent is essential for proper drainage.
      • Different Styles: Vents come in different styles, they may be PVC with a rubber boot or they may be covered in lead to act as a sealer. While it’s okay for water to enter the vent, I do not want water to enter the attic structure around the vent.

      The following picture shows a side-by-side comparison of the two types of vents normally encountered on an inspection.

Top 3 DWV Plumbing Problems Found During a Home Inspection

1. 1. Blocked Vent Stack Symptoms: Slow Drains and Sewer Gas Smell: Often caused by bird nests, leaves, or snow/ice. Leads to slow drains, gurgling noises, and siphoned P-traps. Trap must be high enough above the roof to not be covered and clogged with snow. Too short of a vent stack can also be a concern.
   2. Improper Pipe Slope (Pitch) Leading to Clogs: Pipes must be angled correctly to allow gravity to work. Incorrect slope causes waste to sit, leading to clogs or corrosion.
   3. Leaks and Corrosion: Especially common in older homes with cast iron or galvanized steel pipes. The galvanized pipes will wear out at the bottom of the pipe over time. This is especially common on the line with the laundry or dishwasher as the detergents can be abrasive as they go through the pipe.

What do I look for during an Inspection?

During a home inspection, I look for a variety of things when it comes to a DWV system. It starts on the outside on the roof and goes into the home.

1. 1. Installation at the roof: I check for proper shingling around the vent flange, proper caulking and sealant of any nails used to install the vent flange. I check for tears around the boot and if it’s a lead wrapped pipe, ensuring there’s no gaps for water to get in around the pipe. I also check to ensure that it’s not clogged. Even if I are using the drone, I fly right over it and look straight down the vent pipe to make sure there’s no visible clogs (like birds’ nests or leaves) that could hinder the function of the system.
   2. Improper Pipe Slope (Pitch): I check the overall pitch and layout of any visible vent pipes. It’s important to keep in mind that I can’t see them all as much of the pipe is inside or behind walls. But what I can see usually gives us pretty good clues as to how the rest of it’s installed.
   3. Leaks and Corrosion: Type of Pipe, I check for PVC or if it’s galvanized. If it’s been repaired and for any active leaks at any of the joints.
   4. Flushing and Drainage, when I are doing the plumbing inspection, and checking toilets, showers, tubs and sink drainage, I pay attention to how they drain, this can indicate an issue with the system.

Video Demonstration: DWV Vent Boot Explained

Something I pay close attention to during the roof inspection phase of a complete home inspection is, the vent boot around the DWV roof penetration. In this video I perform a test on a purposely damaged boot to show how much water can easily enter the attic system if the boot is damaged (even just a small tear can spell disaster). Anytime the DWV boot is damaged or not sealing properly, it creates a path for rain to enter the attic and travel throughout the home. This can lead to significant damage and mold problems if left unfound and unchecked.

Troubleshooting DWV Plumbing Issues: Noise, Smell, and Speed Tests

Here are a few tips and indicators of a problem with the Drain Waste Vent, if you ever notice a change with how your plumbing drains seem to work, the vent is the first place to investigate.

1. 1. The Noise Test: Gurgling in one fixture when another is flushed or draining usually indicates a vent issue.
   2. The Smell Test: Foul odors in your home are a strong sign of a dry or siphoned P-trap due to venting failure (allowing seIr gas entry).
   3. The Speed Test: Significantly slow drainage in multiple fixtures (a sign of a significant obstruction in the waste line).

Conclusion

Your home’s Drain, Waste, and Vent (DWV) system is the unsung hero working silently behind your walls and through your roof to keep your plumbing flowing smoothly and your home odor-free. This critical network of pipes relies on a simple yet ingenious design: drains carry water away from fixtures, waste lines transport it to your sewer or septic system, and vent stacks extending through your roof allow air in to prevent suction while releasing sewer gases safely outside. When this system fails, whether from a blocked vent stack, improper pipe slope, or corroded pipes, you’ll notice the warning signs: slow drains, gurgling sounds, and unpleasant sewer odors invading your living space.

During a professional home inspection, I examine every accessible component of your DWV system, from the vent boot on your roof to the drainage performance of every fixture, because catching these issues early can save you from costly water damage, mold problems, and major plumbing repairs down the road.

Ready to ensure your DWV system is functioning perfectly? Schedule your full home inspection with Tech Inspect Home Services Today!