Plumbing Vent Stacks for Commercial Roofs: Do’s and Don’ts

Plumbing vent stacks for commercial roofs are a vital component of a building’s plumbing system. After all, no landlord or tenant wants the hassle, discomfort or expense of a vent stack that causes a slow draining toilet, foul indoor smells or roof leaks.

This article explains what plumbing vents do, their parts and how to install them. We will discuss risks and common problems to avoid.

What are plumbing vent stacks?

Let’s start with a quick definition of plumbing vents stacks. These vertical pipes extend from the building’s internal plumbing system through the roof. By allowing air to enter the plumbing system, vent stacks prevent gurgling pipes, prevent sewer gas backup and maintain proper trap levels.

Understanding plumbing vent stack components

While each roof is unique, their vents stacks components are the same:

Top grommet or cap

This component prevents rain, debris or animals from entering the vent.

Body or pipe

The body of the stack may be aluminum, galvanized steel or stainless steel. When choosing the material, the roofer must balance cost, weight, strength, durability, corrosiveness, the environment and other factors.

For example, a lightweight and affordable material like aluminum could be a good choice. However, keep in mind that it can dent easily. If low cost is essential and there is a low risk of corrosion, consider galvanized steel. Stainless steel is the most durable, but it is heavier and more expensive.

Insulation

High-quality insulation inside the pipe prevents condensation, reducing the risk of freezing or dripping inside the roof assembly.

Bottom grommet

The grommet creates a tight seal between the pipe and the roof flashing. This seal helps keep water out and absorbs movement. It can handle changes, like contraction and expansion, from the freeze-thaw cycle.

Here are two different types of plumbing vent stacks: SJ-24 and SJ-VPC.

Older hub-style plumbing vent stacks versus the newer straight style

Are you working with an older vent stack or a modern one?

If you’re unsure, you may end up using incompatible flashing, adhesives, and sealants that will result in leaks or UV damage.

What’s the difference between older and newer systems?

Older systems use copper pipes that are inserted into larger “hubs” and sealed with materials like lead and oakum. Modern systems use straight-through pipes made of ABS plastic.

Advantages of newer vent stack systems:

Newer systems require smaller penetrations and an easier sealing process. Also, seals are less likely to dry out or crack. ABS is less expensive, lighter, less corrosive and less heat-sensitive. In short, they reduce leaks, make for easier installation, and fit better with today’s roof flashing systems.

Risks of plumbing vent stacks for flat roofs

All roofs need vent stacks, but each type of roof has its own set of challenges:

• Flat roofs shed water more slowly and tend to pond, creating greater risk if vents aren’t high enough (ideally 8” above the top of roof membrane) or sealing is poor.
• Pitched roofs with smooth slopes shed water more easily. However, they require pipe boots or rubber flashings integrated into the membrane for waterproofing.
• The uneven surface of corrugated metal roofs requires flexible flashing that conforms to the ribs to avoid leaks at the valleys and to accommodate thermal movement.

10 Steps to installing a plumbing vent stack for commercial roofs

Here’s important advice: Proper vent stack installation requires careful planning and execution every step along the way. Without going into the weeds, here’s a brief list to get you started:

1. Prime the aluminum vent flange for strong adhesion to the roofing membrane, fewer leaks and less risk of eventual membrane failure.
2. Remove stone ballast, if present. Shovel back any loose ballast and set it aside for reuse after installation.
3. For proper bonding, clean, dry and prime the membrane where the vent stack will be installed.
4. Select a membrane patch compatible with the material and matches the existing roof system.
5. Cut the membrane patch at least 6 inches larger than the vent flange on all sides to allow for adequate sealing.
6. As you install the vent stack, ensure it is plumb and at the required height.
7. Securely adhere the patch to the vent flange and the surrounding roof membrane using the appropriate adhesive or heat-welding technique. Press or roll the membrane patch to eliminate air pockets and ensure full, even contact for a watertight seal.
8. After the membrane is sealed, carefully return the previously removed ballast around the vent area (if applicable).
9. Place a proper cap or hood on the vent stack to prevent rain, debris, or animals from entering the pipe.
10. Once installation is complete, inspect all seals, the membrane patch, and the vent height to ensure code compliance with local codes, water tightness, and long-term durability.

As a final word, schedule periodic plumbing inspections and schedule twice-yearly roofing inspections to maintain a well-performing roof. To prevent problems from growing, quickly contact a plumber if you smell sewer gas or a roofer if you see a leak.

4 Common plumbing vent stack mistakes on commercial roofs

Like all roofing projects, there are common mistakes you’ll want to avoid during installation and as part of your maintenance program.

• Low vent terminals: If the vent terminal is too low, then leaks are possible.
• Cracked plumbing vents: If you smell sewer gas, report it immediately. You may have a cracked rubber boot at the base of the vent stack, which can also cause leaks.
• Uninsulated vent stacks: Condensation from poor-quality, insufficient or non-existent insulation will drip and eventually cause a leak, especially in winter.
• Loose or improperly sealed caps/grommets: At the end of the process, when installing the cap, double-check that they are properly sealed to prevent leaks and the potential for a back up of sewer gas.

Lower your risk of plumbing vent stack problems in commercial buildings

When you install a plumbing vent stack, careful planning is important. This will help prevent problems like leaks, bad indoor air quality, or fines for breaking codes. Choose a high-quality roof for installation and to provide thorough roof inspections.

If your building has a flat roof, be aware of special considerations such as water ponding. In addition, Toronto roofs must consider the implications of cold weather. Get the #Elite advantage for quality roofing services in the Greater Toronto Area.