How a shared soil stack works
A soil stack runs vertically through the building, usually in a service duct next to the bathroom or kitchen. On each floor the branch pipes of one or more homes connect via a T-piece or reducer. At the top the pipe continues to above the roof for ventilation, at the bottom it bends into the horizontal underground drain beneath the ground floor or in the cellar.
Larger blocks often have separate soil stacks for kitchens and bathrooms, and sometimes a separate pipe for rainwater from the roof. The usual diameter is 110 mm, in older buildings sometimes slightly smaller. The more homes connect to one pipe, the more important good venting becomes: with a full flush on the top floor the water falls at considerable speed and moves a lot of air.
Typical symptoms with soil stacks
Because all the water comes together at the bottom, blockages often form in the bend at the base or in the horizontal pipe after it. There the falling water loses speed, giving grease, paper and wet wipes the chance to settle. If only one home is affected, the cause is often in its own branch pipe. The symptoms of a blocked soil stack are easy to recognise:
- water coming up in the shower, floor gully or toilet of the lowest home on that stack;
- gurgling drains on several floors at once, especially when someone upstairs flushes;
- slow draining water for several residents in the same line of the building;
- a sewer smell in the stairwell, the storerooms or a bathroom that is rarely used.
A kitchen soil stack often becomes gradually fouled with grease from all the kitchens above. That layer sticks to the wall and narrows the bore further over the years. At some point a small amount of food scraps is enough to block the pipe completely, often on a public holiday or at the weekend when a lot of cooking is done.
Materials: from cast iron to PVC
In walk-up flats and apartment blocks from the early and middle decades of the twentieth century, soil stacks are often cast iron. That material dampens sound well, but rusts from the inside, making the wall rough so grease and dirt stick more easily. In later buildings PVC is the standard, sometimes in a sound-dampening version. During renovations the engineer regularly sees combinations: cast iron soil stacks with new PVC connections per home.
Those transitions between materials need attention. A poorly fitting transition can leak or form a ridge where dirt gets caught. A leaking soil stack in the duct can also go unnoticed for a long time, until damp patches appear on a ceiling or wall of the neighbours below. Leak detection and a CCTV survey pinpoint such a leak precisely.
Access and cleaning
A soil stack is reached via access fittings: fittings with a removable cover, often at the bottom of the stack and sometimes on each floor. Where these are missing, cleaning can also be done via the roof vent or a fixture connection. The service duct should therefore always have a hatch or removable panel. If the bathroom is renovated, do not let the access fitting disappear behind tiles or a false wall, as that makes every future cleaning more laborious.
To clean soil stacks an engineer uses a spring cable machine with a suitable head or high pressure jetting with a nozzle that flushes the wall clean all round. With hard scale in cast iron, cutting is sometimes needed, carefully and in stages to spare the wall. After cleaning, the engineer checks with the inspection camera that the pipe is clean and clear along its full length.
Responsibility and maintenance by the owners' association
In an apartment block the soil stack is usually a communal part and then falls under the owners' association (VvE). The branch pipes from the connection point to your own bathroom or kitchen are often private. Exactly where the boundary lies is set out in the deed of division and the association's regulations. In rented homes, maintaining the soil stack is usually the landlord's task. Clear agreements prevent discussion at the moment when quick action is needed.
A maintenance plan with periodic cleaning prevents many emergency call-outs. It is common to jet kitchen soil stacks preventively, for example every one to two years, depending on the number of homes and their use. A CCTV survey every few years gives insight into the condition of old pipes, so an owners' association can plan renovation or relining in good time.
How Bosmans helps with soil stacks
Bosmans Rioolservice works on soil stacks for owners' associations, landlords and residents: from clearing an acute blockage, including outside office hours via the 24/7 emergency service, to preventive jetting, CCTV surveys and relining of cast iron or PVC soil stacks. Relining is done from the inside, without breaking open ducts, and comes with a 10-year guarantee. For an owners' association Bosmans can schedule maintenance on a structural basis, so all residents know where they stand.