Soil Vent Pipes

These are the pipes that run vertically from the below ground drainage system to just above your roof gutter level, the SVP allows smells and odours from the drainage system to vent in an area that wont offend anyone and at the same time allows air into the system to prevent siphoning of water from sinks, baths and shower traps. Older systems are typically constructed from cast iron pipe work but we do occasionally come across asbestos type pipe work with modern day systems being constructed from polypropylene.

A soil vent pipe (SVP) or soil stack pipe can be just that, a pipe to vent the system however they would normally be utilised to carry foul waste from upper floor toilets, sinks, baths and showers down to the below ground drainage system.. External SVP`s are handy when there is no direct access into a drainage system and the gully pot traps are tight and restrictive, by making access into the soil vent pipe you have direct access into the system without the need to pass through a trap. Many modern SVP`s have access points fitted and you can buy access doors that can be fitted to the stack after you have made a hole into it to clear the system.

On older properties you would tend to have a soil vent pipe that picked up the first floor WC, the waste pipes from the bath and hand basin would discharge into a separate vertical pipe that in turn discharged into a gully at ground level as would the kitchen sink waste pipe, on modern plastic systems both internal and external waste pipes can often connect directly into the SVP

Internal Soil Vent Pipes

For some reason certain house builders decided that this practical and vital part of the drainage system should be hidden away from view and placed within the property, which is fine when all is working well however when things go wrong major disruption ensues because they are usually boxed into the corner of the kitchen with the kitchen units built around it, so if there is a defect on the stack or the rest bend at its base they can be a sod to get to. Soil vent pipes are often built around when a property is extended which usually means that you will also have a certain amount of below ground drainage beneath the property as well, again this is fine as long as there is suitable access for maintenance but if things do go wrong it can be a big job to sort them out.

The worst case scenario is the rest bend at the base of the soil vent pipe collapsing or blocking for one reason or another, if you can not get direct access from a downstream chamber it is common practice to cut into the now blocked and full soil vent pipe. This is a tricky enough operation when outside the property because you are cutting into a vertical blocked pipe and trying not to get covered in the brown stuff at the same time, on an internal SVP its a nightmare and usually time to get the mop buckets out.

Many internal soil vent pipes do not actually extend to above the roof line of a property, they finish in the roof space and have an air admittance valve fitted that allows air into the system when required but prevents smells and odours from venting within the property.

Air Admittance Valve

When an upstairs toilet is flushed a plug of water can be formed as it drops down the soil vent pipe, this plug of water can momentarily leave a vacuum behind it as it drops unless there is a sufficient air flow through the system. The vacuum will empty internal sink, bath and shower traps leaving a direct vent into the property.

As most properties have external soil vent pipes that vent above the eaves this is not an issue however internal SVP`s often finish beneath the roof line and an air admittance valve is fitted. The spring loaded valve opens to allow air into the system when required but remains closed and air tight for the remainder of the time in order to prevent internal venting, they can also be used in en suite bathrooms and additional washrooms when it is not practical to either connect into the existing SVP or to install a new one.

These valves are for internal use only as they fail when subject to a heavy frost of freezing weather, they are also known to occasionally fail in loft and roof spaces due to a build of of dust or insulation fibres within the valve itself but they can be easily dismantled and cleaned before being replaced.

Drainage Stub Stack

A stub stack would be typically found where an additional downstairs WC has been installed and it is often easier to bring the pipe work through the external wall then excavate the floor inside the property, it is always prudent to install some kind of access point on a stack and the one shown allows access into the drainage system and also direct access to the back of the pan should a blockage ever occur.

The image to the right shows a sub stack that serves a toilet only, if a waste pipe from a hand basin was connected to the pipe work an air admittance valve would be required inside the property. We do find air admittance valves fitted externally but they are prone to failing during heavy frosts and freezing weather and it is not generally recommended.

What Can Go Wrong With A Soil Vent Pipe

There`s not much that can go wrong with a soil vent pipe in itself other than the wife knocking it off the gable wall when reversing onto the drive, plumbers have been known to push a waste pipe too far into a SVP causing the odd blockage. Air admittance valves can and do fail due to dust, fibres and frost (if installed outside) and kids like to drop things down them like stones, twigs, branches and tennis balls when they can access a flat roof at a school.

The most common problem related to soil vent pipes is the 90 degree bend (also known as a rest bend) that sits at the base of the SVP failing, this is usually due to poor installation along with the constant pounding of waste water hitting it over many years. Once there has been movement and water loss from the bend the problem accelerates as the pipe work is undermined, this leads to stepped and displaced joints and sometimes a complete collapse of the bend itself.

The image to the right shows the base of a SVP where the rest bend has settled leaving a gap between the cast stack pipe and the below ground vitrified clay pipe work, the sand and cement haunching around the base of the SVP is always a dead give away that something is amiss.

Replacing A Soil Vent Pipe

The original cast iron soil vent pipes can corrode over time and need replacing, they are prone to impact damage and often people just want to move them as part of new build works around the house. We tend to install plastic soil vent pipes these days and they come in gray and black and the plastic is designed to withstand all that nature can throw at it including sunlight unlike below ground drainage pipes which we often see used as a cheaper alternative. Cast iron pipe work is still available for period and listed properties and many salvage and reclamation yards can supply original pipe work.

The old cast iron pipes are extremely heavy and dismantling a soil vent is a specialist job with all kinds of health and safety implications not to mention the damage a falling pipe can do to your windows, car and driveway, when moving or installing a new SVP consideration should be given to how this will affect the opening of windows and air flow from bathroom extraction fans and heating and gas appliance vents.