An outdoor sink is a useful addition to any outdoor living space. It provides a convenient place to wash hands, clean dishes, or rinse off items outside the house. When installing an outdoor sink, one decision that needs to be made is whether or not the sink needs a trap. A trap is a curved piece of pipe installed under sinks to hold water and prevent sewer gases from coming back up through the drain. For indoor sinks, a trap is an absolute necessity. But for outdoor sinks, the rules are a bit different. There are pros and cons to adding a trap to an outdoor sink drain that must be considered when making this decision.
Page Contents
What is the Purpose of a Trap?
A trap holds water in the curved portion of the pipe to block sewer gases from rising up through the drain. Gases like hydrogen sulfide and methane naturally occur in sewer systems due to the breakdown of waste materials. These gases can be smelly and even toxic in high concentrations. A trap forms a water barrier to keep these gases from wafting up through drains and into living spaces where they can be inhaled.
Traps also serve another important purpose – they prevent the loss of water from the drain pipe. As water flows down a sink, it can siphon out water sitting in the drain. The curve of a trap retains some water to prevent this siphoning action. This helps keep the water seal intact and functioning.
So in summary, the main jobs of a sink trap are:
- Block sewer gases from entering occupied spaces
- Maintain a water barrier to prevent siphonage
Both of these functions are vital for indoor sink drains that connect to sanitary sewer systems.
Do Outdoor Sinks Need a Trap?
For outdoor sinks, whether or not a trap is required depends on where the sink drains to. Outdoor sinks generally drain into one of two places:
- A sanitary sewer line
- The ground surface
If an outdoor sink connects to a sanitary sewer system, then a trap must be installed. It needs to provide the same functions as a kitchen or bathroom sink draining indoors. A trap prevents sewer gases from venting up through the drain and maintains the water barrier in the drain pipe.
However, if an outdoor sink simply drains to the ground surface or into a leach field, a trap may not be necessary. Since it does not connect to a sanitary sewer system, there are no sewer gases to contain. And the open environment outside dissipates any odors so they are less concern.
Let’s look in more detail at outdoor sinks draining to these two locations.
Outdoor Sinks Draining to a Sewer Line
If an outdoor sink ties into the household drainage system and drains into a sewer or septic line, code requires that it has a functional trap. This includes sinks on covered patios, in pool houses or cabanas, or anywhere else outdoors that connects to the main plumbing.
| Location | Trap Required? |
|---|---|
| Covered patio | Yes |
| Pool house | Yes |
| Outdoor kitchen | Yes |
The trap maintains the water barrier between the sewer system and outdoor space. It prevents any sewer gases from venting up through the drain. The outdoor environment does not change the need for a trap on these sinks draining indoors.
Outdoor Sinks Draining to the Ground
For a simple outdoor sink that drains directly to the ground surface or leach field, a trap may not be required by code. These types of sinks include:
- Rinse sinks on patios or decks
- Potting shed utility sinks
- Garage, workshop, or barn utility sinks
With no connection to a sanitary sewer system, there are no sewer gases to contain. The outdoor environment also dissipates any limited odors that may come from the drain water. So a trap is not always mandated.
However, it is still a good idea to install a trap on these sinks for a couple reasons:
- Helps avoid any backflow if standing water accumulates in drain line
- Maintains water in drain to prevent foul odors
So even if not required, a trap is still beneficial. It provides extra protection from potential backflow and odors.
Installing Traps on Outdoor Sinks
Any traps installed on outdoor sinks must be the type approved for outdoor use. Standard indoor sink traps are not suitable as they can crack from temperature changes and exposure to the elements. Here are some tips on selecting and installing proper trap types:
Choose UV-Resistant ABS or PVC Traps
Look for trap models made of UV-resistant ABS or PVC material. The UV inhibitors prevent damage from sunlight exposure over time. Both ABS and PVC stands up to weather extremes and temperature swings better than standard indoor metal traps.
Deep Seal Traps
Select a deep seal trap, ideally with a 4-inch or 5-inch water column. The deeper water barrier prevents evaporation from wind and dry conditions. It also keeps more water in the trap during freezing temperatures to avoid freeze-up issues.
Install According to Manufacturer Instructions
Follow the trap manufacturer’s specifications for proper installation and orientation. Most require the trap be installed with the vent branch on top to prevent water from pooling. Secure with appropriate fittings and glue or fasteners.
Insulate Vulnerable Areas
Use insulation wraps or tapes on any exposed drain pipes or trap bodies prone to freezing. Make sure the water seal itself is not obstructed. Proper insulation prevents freeze damage and keeps the trap working year round.
Maintain Proper Slope
The drain line must slope continuously downward away from the trap to allow complete drainage after use. A level or sagging line allows water to pool in the trap, increasing the risk of freezing.
Other Considerations for Outdoor Sinks
A few other tips for installing outdoor sink traps include:
- Provide access – Use a cleanout fitting near the trap for easy access if clearing clogs.
- Check local codes – Some jurisdictions may require traps even on ground-draining outdoor sinks.
- Rodent proof – Secure trap openings with metal mesh screens to keep out rodents.
- Backflow prevention – Install a backflow preventer valve if required by code.
Taking these factors into account helps ensure a functional, problem-free outdoor sink installation with proper drainage. The trap plays an important role in keeping things flowing smoothly.
Conclusion
While not always required, installing a trap on an outdoor sink is recommended in most cases. For sinks connecting to a sewer line, a trap is absolutely mandatory to prevent gases and odors from venting up the drain. But even for sinks draining to the ground, a trap adds protection from backflow, freezing, and foul odors. The key is using a trap rated for outdoor use. With the right trap installed following manufacturer guidelines, an outdoor sink can drain properly in all seasons while avoiding the issues that can come with an open drain line. Considering the minimal effort and expense involved, including a trap in the installation is an advisable safeguard for reliable performance.