The Buchan Trap, Interceptor or Water Syphon Trap was once regarded as a genius invention. These plumbing devices were once a common feature in many households’ sanitation systems. During the late 1800s and early 1900s, building regulations insisted that these devices were fitted to all new properties that were built which means that they can still be found in many buildings today. So, what is a Buchan trap? Do we need them? and whose responsibility are they when they get blocked or damaged? In this article, we will have a look at the Buchan trap in a little more detail and answer any questions that you may have.

Buchan Trap Drawing

What is a Buchan Trap?

The Buchan is a device made of fired clay that was installed as part of the domestic sewage system designed to help prevent the backflow of noxious gasses into the home from the public sewer system. Buchan traps (originally patented as Water Syphon Traps) also known as a Bristol interceptor, interceptor trap or disconnecting trap were first patented by Scottish sanitation engineer William Paton Buchan on April 23rd, 1875.

WP Buchan was lauded for this invention and attended many exhibitions where he received many awards.

What Does a Buchan Trap Do?

It was a widely accepted belief of the time that the noxious odours from sewer systems were a direct cause of diseases such as cholera and chlamydia. This is now known as the Miasma theory. WP Buchan designed his Syphon Water Trap to prevent these noxious odours (miasmas) from entering the household from the sewer system. The trap acts in much the same way as the common u-bend under your sink in that it creates an area filled with water creating an impenetrable seal that stops odours and gasses from coming back the other way.

Do Buchan Traps Work?

In a word, yes. Although they have not been without their problems, Buchan traps did exactly what they were designed to do. In fact, they were so successful in preventing foul odours from entering homes that they were quickly made part of new building regulations.

Do We Still Need Buchan Traps?

The answer is no. Buchan traps and other interceptors are no longer required and they are no longer installed unless as a direct replacement for an old or damaged one. This is due to advancements in plumbing and the invention of stack vents. The common s-bend and the u-bend are examples of traps that are still in use today. It is these individual traps attached to all plumbing fittings that complement the use of a stack vent that made the Buchan trap obsolete.

Buchan Trap Interceptor Trap
Buchan Trap Interceptor Trap

Buchan Trap Problems

In modern times, just the mention of a Buchan trap can set the nerves jangling of even the most seasoned plumber. With the disproval of the miasma theory and the advancement of the medical profession, the Buchan trap quickly became obsolete; more so when people realised that they can often cause more problems than they solve.


One of the main problems with the Buchan trap is that they are prone to blockages which can lead to several issues. Typically, the opening in a Buchan trap is very small and this can lead to blockages which can become extremely severe and cause the raw sewage to back up into the home. As time goes on, more and more stuff has been getting put in the toilet, such as ladies’ sanitary items, toilet paper and baby wipes etc.

All this extra material can easily cause blockages in the small opening which is not ideal. If you live in a home that still has a Buchan trap, try and limit the amount of material you put down there. Blockages are also common in households that do not have a great deal of pressure in the water system. A poor flow of water is going to have a harder time removing any lodged waste that may be blocking the trap.

Construction Works

It is well known that fireclay Buchan traps are practically bombproof and in fact, in all my years, I have never seen a broken one. A problem that many people face is that the Buchan trap moves or becomes dislodged due to ground movement or construction meaning that the trap no longer functions correctly. Again, this can cause a serious issue and will need to be rectified immediately as the raw waste will have nowhere to go but back upwards!

Whose Responsibility is The Buchan Trap?

Buchan traps are usually located on the edge of the property, and many are shared between households although it seems nobody wants to claim ownership and responsibility for one they are connected to. Who bears responsibility for a Buchan trap has been a contentious issue and subject of debate for friends and neighbours for many years.

Shared Trap – Public Sewer

The truth is that regardless of where the trap is located; if the trap is shared and goes into a public sewer, the water company own it and is solely responsible. If for any reason, you need to carry out any work on it (even on your own land), you will need the water company’s permission.

Shared Trap – Cesspit or Septic Tank

If the trap is shared between properties and discharges to a cesspit or septic tank, responsibility and ownership lie jointly with those that use it. If you are having problems with your trap discharging into a septic tank, it is a good idea to have it dug up and replaced with a straight pipe and then have a stack vent fitted.

Single Household Trap

Regardless of where your trap feeds into, whether it discharges into a public sewer or into a septic tank, the responsibility of the Buchan trap is yours and yours alone.


Buchan traps, a historical solution to a historical problem that now causes problems themselves. Once a must-have in all new buildings, they are now often just a pain in the backside. With modern plumbing systems and stack vents, Buchan traps have become a thing of the past although blockages and ownership disputes are still happening today.

Buchan Trap Problems and Why We Stopped Using Them Infographic

Plumbing Wizard Tips

“To prevent problems with your Buchan trap, it is a good idea to thoroughly flush it through at least annually!”

“If you have an old Buchan Trap, do not put baby wipes down the loo!”

“If your drains are backing up, call out a professional to help fix your problem!”

“If you have realised that you have a problem Buchan trap, have it replaced with a straight pipe and an up-to-date ventilation system!”

Frequently Asked Questions

How to unblock a Buchan trap?

A Buchan trap can be unblocked in a couple of different ways. It can be flushed with a high-powered stream of water or if there is something lodged in there, it can be rodded.

What is an interceptor trap for?

An interceptor trap is another name for a Buchan trap which was designed to prevent harmful odours from entering homes from the sewer system.

Who is responsible for a shared drain?

The water board are responsible for shared drains that go discharge into public sewers. If, however, the shared drains enter a shared septic tank, the drain’s responsibility is shared between those that are using it.

What are the dangers of a blocked Buchan trap?

A blocked Buchan trap can cause sewer gas to enter the building, which can lead to unpleasant smells and potential health hazards.

Can a blocked Buchan trap cause damage to my plumbing system?

A blocked Buchan trap can cause damage to your plumbing system if it causes the water to back up into the sink or other fixtures.


Lee Pearce is not just a master plumber; he’s a veritable Plumbing Wizard. With over 30 years of experience in the trenches of pipes and drains, Lee has become the go-to sage for DIY plumbing, saving homeowners thousands in potential call-out charges. As the founder of Plumbing Wizard, he’s dedicated to demystifying the complexities of home plumbing, offering easy-to-follow advice that stands the test of time and pressure. His online blog is a treasure trove of tips, tricks, and tutorials that empower everyday individuals to take charge of their home’s plumbing health. Lee’s practical wisdom is not just about fixing leaks; it’s about imparting confidence and self-reliance. When he’s not writing or elbow-deep in a plumbing project, Lee is passionate about educating the next generation of DIYers, ensuring that practical skills are passed down and preserved.

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