How Do I Know Where My Drain is Blocked

Blocked Drain Issues. When and Where.

How do you know if you have a blocked drain on its way.  Where is my drain blocked, and can I work out where?

We will aim to answer some of these questions for you.

Australian drainage Standard are among the best in the world. When you travel around the world you very often smell sewage when you walk the streets and stay at high quality accommodation. In Australia though this is a absolute NO NO. If you ever smell the sewer then something is wrong. It is a blocked drain in the making or the drain has now already blocked up.

What can cause smells in and around the house?

One reason could be that there has been a storm or we have experienced high winds. High winds can cause the water seal or the water that remains in the toilet and sink “S” bends or traps” to reduce in it’s water level and when the water in the S Bend lowers it allows the sewer smells to pass between the drainage system and the bathroom, kitchen or the overflow trap outside that window.


Solution to a wind storm drainage smell is to just simply ran water in the sink, toilet or outside drain and the water level will rise. A good thing to check is that the main house drainage vent or stink pipe is not blocked. If it is blocked then unblock this drainage vent as soon as possible.

Venting of a Drainage System

As a illustration of how drainage venting works please picture a glass of water and a straw. Put your thumb on the straw that is in the water and keep it there and lift the straw out of the water. Notice that the water is still in the straw. The water is BLOCKED. Now venting that straw or allowing air to enter the top of the straw allows a venting process to occur and the water quickly falls back into the glass.

Drainage is exactly like this. Instead when air can not get into the drainage system it will take or draw air through the S Bends and Traps. Now the water that was in the traps has now been sucked out the smelly sewage air can freely enter into the room. Venting is the most important aspect of any drainage system.


Inspect that the drain vent is not blocked in any way. If for some bizarre reason you do not have a vent. Get a plumber to install one for you. I have also seen a vent terminated into the roof space between the ceiling and the roof tiles. This caused the sewer smell to enter into every room of the house. This was a result of a handyman bathroom renovation.

Smells Outside the Building

Every building that has the plumbing connected has a Overflow Relief Gulley. This gulley is set at a level that is lower then the inside of the building so if the sewage overflows it with overflow outside first and not through the inside shower or bathroom floor drain. You may smell the drainage and it could be that the drain is blocked and the waste water has now flowed over and is all over the ground. Sometimes these overflow gullies are covered by shrubs and garden plants. They are mainly outside a bathroom or a kitchen or laundry. Pool back wash pipes also connect to these gullies if that helps in locating it.

If you have overflowing water from these gullies well that’s a 100% sign that the drain is blocked. Sometimes the Councils or the Main Sewerage service drain could be blocked and it is overflowing in your place first. Maybe you are near to the bottom of the hill or the blockage in the main sewer occurred just below your sewer connection to the main drainage system. The other cause could be that the sewerage has blocked within your own property.

How do you know the difference between a sewer blockage in or outside your property boundary. Keep in mind that if the blockage is outside your property it is the councils or Local Water Authorities responsibility to fix it.  

Easy Test

Make sure that you do not allow any water from your property to enter into the sewer drainage system. Just do not use any water is the best trick. The best way to check if the system is overflowing is in the morning before any water is use at your house. If the drain is overflowing then the problem is caused by the water that your neighbours are using and the blockage is in the main drain pipe. This is a responsibility of the Local Water Authority so call them and they can come out and unblock it. Understanding this test will help you explain to them that you know that it is their problem.

On the flip side if no water is overflowing when you do your check it is most likely that you are either the first one up in your area, the main drain has not had enough time to fill with water or the blockage is in your property and will start to overflow as your household or business starts to use water. If it is your property, just don’t use the water. Call the plumber and hopefully it is just a small blockage that can be overcome in the hour.

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