3 May 2019 · By Geoff Wilkinson
Geoff Wilkinson looks at the regs relating to basic sanitary drainage design Caustic Soda Shower Drain
When designing a kitchen or bathroom it is easy to get caught up in the clients demands for the latest fashion, whether that’s a free-standing bath, or an island sink, without thinking of the practical aspects of the drainage that will serve it. This month I look back at some of the basics of sanitary drainage and point out some of the common traps (pardon the pun) that architects fall into.
One of the first considerations is the distance of the proposed fitting from the nearest foul water pipe or manhole.
In most cases, fittings are connected to a soil and vent pipe (SVP) but in some older properties there may still be a two-pipe system where ventilation is provided separately to the wastewater disposal. Assuming it is a combined SVP, then the building regulations set out maximum distances that a fitting such as a bath, WC, sink and so on can be from that SVP. This can be very restricting in the design of the kitchen or bathroom so this is the first thing that should be taken into account when thinking about the design.
The trend for placing a sink on a central island presents more of a design challenge when thinking about wastewater
The limits can be very restrictive and are as little as 1.7m for a 32mm waste pipe to a wash hand basin. By increasing the size of the pipe, you can place the fitting further away – typically between 3 and 4m away for a 50mm pipe. If you exceed these dimensions there is a risk of the trap ‘pulling’, wastewater, which would allow foul smells into the room through the empty trap. To prevent this, it is possible to fit a resealing trap or air admittance valve.
If the pipe is run in a straight line it is relatively easy to clear any future blockage by simply removing the trap.
However, in many cases it is necessary to turn the pipe in one or more directions in order to get it to the SVP. In such cases it is important to provide rodding points to enable the pipes to be cleared. In most modern designs the pipework is hidden in the walls of the kitchen and bathroom, and often this is then tiled. This makes it very difficult to access the pipes in future without damaging the tiling and therefore access panels in the tiling should be provided to enable access to the rodding points.
Traditionally kitchen sinks were placed underneath a window in an external wall, this meant that they could easily be taken to an external gulley, which then connected to a manhole on the main drain.
However, in recent years the trend has been to place the sink on a central island in the middle of the kitchen, which presents more of a design challenge when thinking about the disposal of the wastewater.
A common solution is to provide a stub stack within a kitchen unit and to connect the waste to that. A stub stack is a cut-off version of an SVP, which is fitted with an air admittance valve to prevent foul smells. The stub stack should still be a 100mm diameter pipe.
Rules on the use of stub stacks are commonly forgotten. Firstly, the floor served by the stub stack should be no more than 1.3m high from the invert of the drain that it connects to. Secondly, the stub stack should be no more than 2m high to its highest connection. If the invert of the drain/sewer is already over 1m deep, this will mean it is unlikely that it can be used in conjunction with a stub stack.
Where these distances limit the design of the kitchen or bathroom, all is not lost, as it is possible to use a pumped macerator-type system to extend the distances and install the fittings in the chosen location. Remember, though, that there should always be at least one gravity-fed toilet for use in the event of a power cut.
Geoff Wilkinson is an approved inspector and managing director of Wilkinson Construction Consultants www.thebuildinginspector.org
Tags Building Regulations Drainage Kitchens and Bathrooms
With respect, a stub stack does not necessarily have to have an air admittance valve fitted. It can be fitted with screwed access plug. Such an arrangement is subject to the limitations noted in the article. If an air admittance valve is fitted however, the stack becomes a “ventilated discharge stack”, according to AD H1 1.33, so overcoming many of the limitations of a stub stack.
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