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Why Is It Called a Bulkhead Fitting?
A bulkhead fitting gets its name from the word bulkhead, which means a wall or partition.
In plumbing and water-storage systems, a bulkhead fitting passes through the wall of a tank, barrel, or other container. It creates a sealed connection so you can attach a pipe, valve, hose, or other fitting without leaving an open hole.
The name comes from the same basic idea used on ships, where bulkheads are walls that divide the inside of the vessel into separate sections.
What a Bulkhead Fitting Does
A bulkhead fitting lets plumbing pass through a container wall while keeping the connection sealed.
For example, you might install one through the side of:
- A rain barrel
- An IBC tote
- A plastic water tank
- A cistern
- A filter housing
- An irrigation reservoir
Once installed, the fitting gives you a threaded or slip connection on one or both sides of the wall.
That makes it possible to add a valve, outlet pipe, overflow line, pump connection, or other plumbing component.
Why the Word "Bulkhead" Makes Sense
Think of the tank wall as a small bulkhead.
Instead of simply pushing a pipe through a hole and trying to seal around it with caulk, a bulkhead fitting is designed specifically to clamp through that wall.
A typical fitting has several parts:
- A fitting body that passes through the hole
- A flange on one side
- A gasket that creates the seal
- A nut on the other side that tightens the assembly
When the nut is tightened correctly, the tank wall is held between the parts of the fitting.
The exact design varies, but the basic idea is the same: make a secure plumbing connection through a wall.
Bulkhead Fittings Are Common on Rainwater Tanks
Bulkhead fittings are especially useful in rainwater systems because tanks often need several openings.
You may need a fitting for the:
Tank outlet
A low outlet lets water leave the tank and enter a hose, pump, or irrigation line.
Overflow
An overflow near the top lets excess rainwater leave the tank when it becomes full.
Tank-to-tank connection
Two rain barrels or tanks can sometimes be connected through bulkhead fittings so water can move between them.
Drain
A low drain can make tank cleaning and winter preparation easier.
Not every tank opening needs the same fitting size. The pipe diameter, expected flow, tank material, and wall thickness all affect what will work.
Why Not Just Thread a Pipe Through the Tank?
Most plastic tank walls do not provide enough material to hold pipe threads securely on their own.
Trying to screw a threaded pipe directly into a thin barrel wall can lead to:
- Poor thread engagement
- Cracking
- Leaks
- A loose connection
- Damage when a valve or hose places force on the wall
A bulkhead fitting spreads the load across a larger area and provides a gasket designed to seal against the tank surface.
Bulkhead Fitting vs. Uniseal
A bulkhead fitting is not the only way to pass plumbing through a tank wall.
A Uniseal-style rubber fitting uses a flexible rubber seal instead of a threaded body and retaining nut. A pipe is pushed through the seal, and compression around the pipe creates the watertight connection.
Bulkhead fittings are often useful when you want a solid threaded connection for valves or other plumbing.
Rubber seals may be convenient for some pipe-through-wall connections, especially when the pipe simply needs to pass through the tank.
Neither type is automatically better. The right choice depends on:
- Tank wall shape
- Wall thickness
- Pipe size
- Whether the connection carries weight
- Whether you need threaded plumbing
- Whether you can reach the inside of the tank
Does a Bulkhead Fitting Need Access Inside the Tank?
Many traditional bulkhead fittings do.
Factor in key facts about the difference between a boss fitting and a bulkhead fitting to evaluate the main trade-offs before the next design decision.
The fitting body or gasket may need to be positioned from inside the tank while the retaining nut is tightened from outside, or vice versa.
This can be difficult on closed tanks with small access openings.
Some fittings are designed for installation from only one side. These are useful when you cannot comfortably reach the inside of the tank.
Check the fitting's installation design before drilling the tank.
Which Side Does the Gasket Go On?
This depends on the fitting design, so the manufacturer's instructions should take priority.
On many standard bulkhead fittings, the gasket sits against the flange rather than against the retaining nut. The flange and gasket form the primary seal against the tank wall.
Adding extra gaskets or placing them differently does not necessarily improve the seal. It can sometimes make the connection less reliable.
Thread sealant or PTFE tape may be needed on plumbing threads attached to the fitting, but it normally does not replace the main tank-wall gasket.
How Tight Should a Bulkhead Fitting Be?
It should be tight enough to compress the gasket and prevent movement, but not so tight that the fitting distorts the tank wall or damages the gasket.
Overtightening can cause problems such as:
- Cracked plastic
- Warped tank walls
- A gasket being squeezed out of position
- Damaged fitting threads
- Leaks that appear after the tank is filled
Plastic rain barrels and tanks can flex, so more force is not always better.
After installation, fill the tank gradually and inspect the connection before putting the system into normal use.
The Hole Size Matters
Bulkhead fittings require a correctly sized hole.
The hole is usually larger than the nominal pipe size because the body of the fitting must pass through the tank wall.
For example, a fitting described as a 1-inch bulkhead does not necessarily go into a 1-inch drilled hole.
Do not estimate the hole diameter from the pipe size. Use the hole-size requirement for the specific fitting.
An oversized hole may prevent the gasket from sealing properly, while an undersized hole may damage the fitting or tank.
Flat Tank Walls Seal More Easily
Bulkhead fittings usually work best on reasonably flat surfaces.
A strongly curved rain barrel can make sealing more difficult because the flat gasket and flange may not sit evenly against the wall.
Small fittings can sometimes work well on gently curved surfaces, but larger fittings may need a flatter mounting area.
Avoid forcing a rigid fitting onto a surface that visibly distorts when tightened.
Frequently Asked Questions
What does bulkhead mean in plumbing?
In plumbing, a bulkhead is a wall or partition that a pipe or fitting needs to pass through. A bulkhead fitting creates a sealed plumbing connection through that wall.
Why are bulkhead fittings used on water tanks?
They provide a secure way to add outlets, drains, overflows, valves, and pipe connections without relying on the thin tank wall to hold plumbing threads.
Are bulkhead fittings waterproof?
They are designed to create a watertight connection when the fitting, gasket, tank surface, and hole size are suitable and the fitting is installed correctly. A damaged gasket, warped wall, wrong hole size, or overtightening can still cause leaks.
Can I install a bulkhead fitting in a rain barrel?
Yes. Bulkhead fittings are commonly used for rain-barrel outlets, drains, overflow lines, and connections between barrels. Make sure the fitting matches the barrel's wall shape and thickness.
Do bulkhead fittings work on curved tanks?
They can work on gently curved surfaces, especially in smaller sizes. Strong curvature may prevent the gasket from seating evenly and can make leaks more likely.
Do I need silicone around a bulkhead fitting?
Usually, the fitting's gasket is intended to make the tank-wall seal. Silicone should not automatically be used to compensate for a poor hole, damaged gasket, or unsuitable mounting surface. Follow the fitting manufacturer's installation instructions.
Is a bulkhead fitting the same as a tank connector?
Often, yes. "Tank connector" is commonly used for fittings that perform the same basic job: creating a sealed plumbing connection through the wall of a tank or container.
