Tips on installing waste and soil pipes

Article written by Lizzy

Installing domestic drain and soil pipes seem to be an easy job, no water pressures to account for or non-return and other valves to think about and regulation seems to be comprehensible to any construction worker. Just connect your pipes from point A, always go downwards and make sure you end up in the sewage and that’s it. Will it work? …It might. Will it work well? That’s a totally different ballgame. Although sticking some pipes together to drain the water seems fairly simple, a lot can go wrong when drain pipes are not installed the way they should be. Common problems with drain and soil pipes are clogs and smells and in order to avoid those providing sufficient air and keeping your waste flow under control is key.

For every litre of water waste we need 6 litres of air if we want to avoid the risk of creating under-pressure. Surely everyone has tried to stick a straw into a glass of water, close the top part with a finger and pull the straw up. As long as the finger stays on the straw the water will remain, the moment you release the finger the water drops back into the glass. You try the same trick with a tube of a wider radius and you will find out that eventually the water will drain as the air will fight its way in from underneath. You will notice however that the water drains a lot slower than if you just let go of your finger and let the air come in from the top. The wider the radius of your tube the faster your water will drop into your glass.

The construction and mechanism of drain and soil pipes is very similar with the difference that our pipe is not vertically positioned but almost horizontally and that our finger is replaced by an inverted siphon or trap. The easiest way for water to flow is to draw air from the site of our finger. Air does not have to fight the water coming down, and subsequently the speed of the water is not slowing down. When water passes through a trap and subsequently through the drainage pipes it should not slow down. When water slows down the small waste particles may settle and slowly built up sedimentation. This may encourage or even be the source of a future blockage or clog. Secondly water that slows down is most often the result of an under-pressure. And if the under-pressure is big enough it will try to suck air through the trap. Depending on the under-pressure and the type of trap this may result in the release of unpleasant smells through the trap or worse, result in the trap being sucked dry. All this however can be avoided by properly venting the drainage system. Depending on the country you live in various systems are applied. Most system though will work with a vent stack which goes through the roof. Air admittance valves are another option but have their drawbacks as I’ll explain later.

Venting is of course common knowledge within the plumbing industry, nevertheless do we still find many systems which are not properly vented. Below I will touch on a few points to watch out for.

Venting can be done at each trap. It provides air right at the point where the under-pressure could occur. Considering the diameters of the pipes are properly calculated and big enough for the potential waste flow rate, venting at the trap is probably the most ideal way to vent the system.

Drainage with separate venting cirquit

This way of venting will result in a separate vent circuit however requiring more pipe and labour and consequently drive up cost considerably. So very often the main vent shaft through the roof may be the only venting shaft you will find in the building.

Drainage with no separate venting cirquit

In those cases having the pipe system well designed is especially important. We know that we need to supply air to the trap if we want to avoid problems. Although we have an abundance of air at the entrance of the trap, the site of our appliance, this is of course air we can’t use since it would mean the air would have to travel trough the trap. We need air at the exit of the trap. In order to provide that, we will need to make sure that air can get to the trap the same way the waste needs to exit, and this in a way that waste and air do not interfere with each other. Key to all this is to make sure the calculations of the pipe diameters are correct and the pipe can handle the maximum flow rate of the attached appliance only using 50% of its capacity. This means that 50% of the pipe should be empty at all times allowing air to easily reach the exit of the trap. This most often results in a pipe size which is 2 sizes wider than that of the trap.

Correct tube size Wrong tube size

We will also have to consider the way the waste pipe connects to the stack. Often I find a 45 degree angle at which the pipe enters the stack. Odd as it may seem a 90 degree angle is a better choice in this case. A 45 degree shoot may actually interfere with proper airflow.

Correct connection Poor connection

45 degree entrance angles are used to keep the flow of the waste going and this is definitely important at the end of a gravity fall. When connecting to the vent pipe however and at the beginning of a gravity fall a 45 degree angel is going to make little difference in the flow rate.

On the other hand at the end of a gravity fall we want a double 45 degree bend to ensure a smooth flow. A 90 degree bend or t-section will induce a serious slowdown which could result in waste to be left behind in the pipe.

Correct connection Poor connection

T traps are for the same reason not a recommended choice since water and waste do not pass the trap in a fluent continues motion.


So the flow rate is definitely an issue to consider in order to avoid clogs. I often read that the recommend drop on a soil or waste pipe is 2%. Personally I don’t agree. You put a 2 degree drop on the soil pipe of a dual flush toilet and you are bound to have problems in the long run. Some people make mistakes and may misuse the dual flush. Or some may notice when they look in the pot that their package clears even when using the small flush, and will continue doing so since it saves water. What they often don’t realise is that the water needs to carry their package all the way through the pipes until it reaches the sewer or septic pit. You will find that at a 2% drop water will run faster than the package, and since there was too little water to start with the package will be left behind with no vehicle to carry it all the way down. We want the water to stay with the waste and not run away, so in order to have a decent carrier a fall of a little more than 1 to 1,5% will do the job just fine.

Air admittance valves which only let air in but will block the air to come back into the room have their use, although personally I am not fond of them. If they are used to replace the main vent stack which normally goes through the roof I would say “bad idea.”
Waste and excrements may ferment and build up an air pressure inside your waste circuit. If there is no open vent or no vent at all the pressure will finally push its way through one of the traps and make a burp, an unpleasant smelly part of air from the drainage system due to the internal air-pressure that was building up due to fermentation.
Local admittance valve placed at the trap itself can solve the problem of under-pressure and subsequently the problem of a trap being sucked dry. Keep an eye on those however since over time seals of many of these admittance valves start to fail and will also release air in the other direction leaving a bad smell in the room. So you may want to change those at a determined interval.