Soil and waste pipes do a vital job in getting rid of our mucky water, and they’ve got to do it right!

 What’s the difference between a soil pipe and a waste pipe?

Water that goes through waste pipes is sometimes called “grey water”. Think sink water, shower water, washing machine or bath water, all carried away into a soil and waste stack and then to the sewer.

Soiled water has more of the “ugh” factor – it’s what goes down toilets, bidets and urinals. It passes through soil pipes then into the stack, before arriving in sewer-land.

If the contents of waste and soil pipes both end up in the sewer, are the pipes themselves any different? I’m glad you asked…

Soil and waste pipe size

Soil pipes are wider than waste pipes, ‘cos they need to carry solids. Lovely. They’re typically 110mm, whereas waste pipes are more likely 21.5mm, 32mm or 40mm, depending on exactly where they’re used. The soil and waste stack will also be 110mm.

 Soil and waste ventilation

Soil and waste pipes need ventilation. After all, non-one wants bathroom smells to linger permanently, do they? Ventilation is part of the Building Regs, and you’ll see the stack (basically a big vertical pipe), which goes way up to the top of your house and is open at the top. It lets the air in and keeps all the nasty gases well out! In multi-story buildings you use aerators too.

You can also buy traps (for under sinks and baths) with in-built air admittance valves, which help to keep the fresh air flowing!

Another reason air-admittance is important is that a body of water flowing down a waste pipe can cause a slight vacuum – resulting in slow drainage. Us plumbers use a product know as a ‘Durgo’ to allow air into the stack but no smell out. You’ll often find these in flats where the stack is terminated in a loft, or in box work by the toilet. Durgo’s need air vents to supply the air needed to break the vacuum…so don’t box them in an airtight space!

If you want to vent the stack through the roof but don’t want a big, horrible pipe sticking out, check out tile vents online. They can vent the stack but in a more aesthetically pleasing way!

Soil and waste pipe system design and installation

The most important bit of obvious advice ever: soil and waste pipes need to be on a downhill gradient! The “fall” or “drop” should be between 1/40 (1cm down for every 40cm across) and 1/110. Too steep (1/10) then the water runs quicker than the solids so doesn’t wash them away (ugh!). Too shallow (1/200) and there won’t be enough gradient to get rid of last night’s curry. Both can cause a blockage!

You can find out how to calculate the fall gradient in the video below.

Everything else about soil and waste pipe installation comes down to good plumbing practice: Use pipe clips and brackets, deburr your pipe, branch and tee your waste pipes into the soil and waste stack using bosses. Oh, and use that solvent glue somewhere ventilated – it’s pungent stuff! Watch how to install a 110mm soil and waste pipe in my video:

Rodding points

Trust me, you’re gonna need a rodding point; or at least to know where yours is. It’s often the manhole cover on your property, and it’s where you can access the drain to poke a big stick around and clear any blockage. If you’re fitting waste pipes for a new bathroom or basin, it helps to add a tee and mini-rodding cap just in case. The more access to a potential blockage problem, the better. Prepare for smells though. You’ve been warned!