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A bulkhead fitting lets a pipe, valve, or hose connection pass through the wall of a rain barrel, tank, cistern, or other container while keeping the opening watertight.
For water-storage systems, the three most useful types to know are threaded bulkheads, slip bulkheads, and combination bulkheads. The difference is mainly how the plumbing connects to each side of the fitting. Common commercial configurations include thread-to-thread, slip-to-slip, and mixed thread/slip designs.
The Three Main Types of Bulkhead Fittings
| Type | How It Connects | Best For | Main Limitation |
|---|---|---|---|
| Threaded | Pipe or adapters screw into threads | Systems that may need to come apart | Threads must match |
| Slip | PVC pipe is solvent-welded into a smooth socket | Permanent PVC plumbing | Harder to change later |
| Combination | Threaded on one side and slip on the other | Connecting different plumbing styles | You must check which side has which connection |
1. Threaded Bulkhead Fittings
A threaded bulkhead has female or male pipe threads on one or both plumbing connections. A common version has female threads on both sides.
You can screw a matching nipple, valve, hose adapter, or other fitting into it. This makes threaded bulkheads useful when you expect to remove or change part of the system later.
For example, a rain barrel could have:
rain barrel → threaded bulkhead → threaded adapter → valve
This setup can make valve replacement easier than a permanently glued connection.
Threaded bulkheads are also useful when connecting a tank to removable pump or irrigation plumbing.
The important part is thread compatibility. A fitting labeled 1 inch does not tell you everything you need to know. You also need to know the thread standard.
For example, NPT and BSP are different pipe-thread systems. Parts from the two systems should not be assumed to fit or seal correctly just because their stated sizes look similar.
Avoid forcing a threaded fitting that does not turn together normally. Plastic threads can crack or deform if they are cross-threaded or overtightened.
2. Slip Bulkhead Fittings
A slip bulkhead has a smooth socket made to accept compatible pipe or a fitting.
With PVC plumbing, the connection is normally made using the correct PVC solvent-cement process. Once joined, it is intended to be a permanent connection. Slip-to-slip bulkheads are a common configuration.
A simple setup might look like:
tank → slip bulkhead → PVC pipe → valve or irrigation line
Slip connections work well when the plumbing layout is unlikely to change. They can also provide a compact connection without threaded adapters.
The disadvantage is serviceability. You cannot normally unscrew the pipe later. Changing the layout may require cutting the pipe and installing new fittings.
Material compatibility also matters. Do not assume that any plastic pipe can be glued into any plastic bulkhead. Check the bulkhead material and follow the fitting manufacturer's joining instructions.
3. Combination Bulkhead Fittings
A combination bulkhead uses a slip connection on one side and a threaded connection on the other.
Two common arrangements are:
- Slip × thread
- Thread × slip
The order matters. Manufacturers commonly describe the flange side first, so a thread × slip fitting is not necessarily interchangeable with a slip × thread fitting in a tight installation.
Combination fittings can be useful when you want permanent plumbing on one side but removable plumbing on the other.
For example:
tank interior → bulkhead → glued PVC pipe outside → threaded valve assembly
Or the arrangement may be reversed depending on the tank and plumbing layout.
This flexibility is useful around rain barrels, storage tanks, filter housings, pump lines, and irrigation systems where one section needs to stay fixed while another needs occasional removal.
What Parts Make Up a Bulkhead Fitting?
A typical bulkhead fitting has several basic parts:
- A fitting body that passes through the tank wall
- A wide flange
- A gasket that creates the water seal
- A retaining or lock nut
The tank wall is held between the flange and retaining nut. Tightening the fitting compresses the gasket against the tank surface.
The gasket is what provides the seal. Silicone or other sealant should not automatically be treated as a substitute for the correct gasket or proper installation.
Most conventional bulkheads place the sealing gasket against the flange side, but always follow the instructions for the exact fitting you are installing.
Threaded vs. Slip vs. Combination: Which Should You Choose?
Choose based on how the rest of your plumbing needs to connect.
A threaded bulkhead is usually the better choice when you want to remove valves, adapters, or pipe sections without cutting them.
A slip bulkhead makes sense when you are building permanent PVC plumbing and do not expect to change the connection.
A combination bulkhead works well when one side of the tank needs permanent PVC plumbing and the other side needs a removable threaded fitting.
Connection style is only one part of the decision. You also need to check size, material, tank-wall shape, and available installation space.
Bulkhead Size and Hole Size Are Not the Same
Details about key facts about a watertight bulkhead help assess this choice within the overall harvesting layout.
One of the easiest mistakes to make is drilling a hole based only on the stated bulkhead size.
A 1-inch bulkhead does not normally require a 1-inch hole.
The nominal bulkhead size usually refers to the pipe or fitting it connects to. The body that passes through the tank wall is larger, so the installation hole must also be larger.
Do not estimate the hole diameter from the nominal pipe size.
Check the exact manufacturer's required hole size before drilling. Bulkheads with the same nominal plumbing size can have different body dimensions.
This matters because an oversized hole may prevent the gasket from sealing correctly, while an undersized hole may damage the tank if you try to force the fitting through.
Check the Tank Wall Before Installing a Bulkhead
A bulkhead seals best against a suitable surface.
Look for an area that is:
- Flat enough for the gasket to sit evenly
- Free of deep scratches and molding seams
- Accessible from the side needed for installation
- Thick enough for the fitting's intended wall range
- Away from structural ribs unless the fitting is designed for that location
Rain barrels and some polyethylene tanks have curved or molded walls. A standard flat gasket may have trouble sealing against a sharply curved area.
Do not keep tightening the nut in an attempt to flatten a tank wall. Excess force can deform the tank, damage the gasket, or crack the fitting.
Bulkheads and IBC Tote Connections Are Not Necessarily the Same
IBC totes need extra care because the factory outlet may use a large proprietary or industry-specific thread.
An adapter that fits the existing IBC outlet is different from a bulkhead fitting installed through a newly drilled hole in the tank wall.
If you are connecting to the original IBC valve, identify the actual outlet thread before buying adapters. Do not choose by diameter alone.
If you are adding a new bulkhead elsewhere on an IBC tote, check whether the tank wall at that location is suitable for the fitting and whether drilling the container is appropriate for its intended use.
Does Bulkhead Material Matter?
Yes.
Common bulkhead materials include PVC and ABS, while other systems may use polypropylene or metal fittings.
Choose materials based on the stored liquid, connected pipe, outdoor conditions, and fitting manufacturer's requirements.
For a rainwater system, also think about exposure to sunlight, freezing conditions, physical strain from attached plumbing, and whether the water will be used for irrigation, household non-potable purposes, or another application.
Non-potable means water that is not intended for drinking.
If collected rainwater will be considered for drinking, selecting a bulkhead is only one small part of the system. Roof runoff should not be assumed safe to drink. Potable use requires appropriate collection materials, contamination control, treatment, maintenance, current water testing, and compliance with applicable local requirements.
Common Bulkhead Mistakes to Avoid
Many leaks come from the surrounding installation rather than from choosing the wrong basic bulkhead type.
Watch for these problems:
- Drilling before checking the required hole diameter
- Mixing incompatible thread standards
- Cross-threading plastic adapters
- Overtightening the retaining nut
- Installing over a molding ridge or badly curved wall
- Using a damaged, twisted, or dirty gasket
- Letting heavy pipe pull sideways on the fitting
- Gluing a slip connection before checking the whole plumbing layout
- Assuming two fittings with the same nominal size have the same outside dimensions
Supporting long plumbing runs can be especially important. A bulkhead is meant to seal the tank penetration. It should not have to carry the weight of an unsupported pump, valve assembly, or long pipe run.
Frequently Asked Questions
What are the three main types of bulkheads?
For tank and water-system plumbing, they can be grouped into threaded, slip, and combination bulkheads. Threaded versions accept screw-in fittings, slip versions accept pipe that is normally solvent-welded in place, and combination versions use one connection style on each side.
What is a threaded bulkhead fitting?
A threaded bulkhead has pipe threads that accept a matching threaded adapter, nipple, valve, or fitting. It is useful when the plumbing may need to be removed or changed later.
What is a slip bulkhead?
A slip bulkhead has a smooth socket designed for compatible pipe or fittings. In PVC systems, the joint is normally solvent-welded, creating a connection intended to remain in place.
Is a slip × thread bulkhead different from a thread × slip bulkhead?
Yes. The connection styles are on opposite sides. Manufacturers commonly list the flange-side connection first, so check the fitting diagram before ordering or drilling.
Does a 1-inch bulkhead need a 1-inch hole?
No. The nominal size normally describes the plumbing connection, not the hole through the tank. The installation hole is larger. Always use the hole diameter specified for the exact fitting.
Can I install a bulkhead on a curved rain barrel?
Sometimes, but the sealing surface needs to work with the fitting and gasket. A strongly curved, ribbed, or uneven wall can prevent an ordinary bulkhead from sealing evenly. Choose a suitable location and follow the fitting manufacturer's requirements.
Should I use threaded or slip fittings for a rain barrel?
Use threaded fittings when easy removal or future changes matter. Slip fittings are better suited to permanent compatible PVC plumbing. A combination bulkhead can provide both connection styles in one fitting.
