PVC drain pipe sizes: which drain connects to which pipe?

PVC drain pipes are referred to by their outer diameter and named in millimetres. As a general rule, the drains in a flat are connected with roughly the following sizes.

A 70 mm pipe is most often seen in collecting sections where several small drains meet. This intermediate size is useful in layouts where the bathroom washbasin and shower join in a shared pipe before connecting to the toilet line or the shared stack.

When the pipe diameter is too small for the amount of water a drain releases at once, the water fills the pipe completely and pushes the air ahead of it. This causes gurgling, water being sucked out of the trap, and odours. On top of that, as the water slows down, the grease, hair and soap residue in it cling to the pipe wall more easily.

Connecting a kitchen sink to a 40 mm pipe is a typical example. This size is fine for a washbasin, but it struggles to carry the hot, greasy water a dishwasher discharges, and blockages can start within a few months. The reason a toilet should not go below 100 mm is also that solid waste catches in a narrowing pipe.

The opposite can cause trouble too. On long runs where a small washbasin is connected to a pipe wider than necessary, a small amount of water spreads across the bottom of the pipe and flows shallowly. That shallow flow struggles to carry hair and soap residue, so a wider pipe is not always the safer choice.

Two PVC pipes of different diameters are joined with transition fittings called reducers. The basic principle is that, in the direction the water flows, the pipe either stays the same or gets larger. If a 50 mm pipe discharges into a 40 mm pipe, the transition point becomes a permanent bottleneck, and blockages will always start there.

For side connections, angled fittings are preferred over right-angle ones. When water enters the main pipe at a gentle angle, the flow is maintained and collisions with the water coming from the opposite direction are reduced. When a PVC pipe is connected to an old cast iron stack, a special gasket or coupling suitable for both materials is used; if the joint is sealed with sealant alone, it will leak over time.

Even a pipe of the right diameter causes problems if its slope is wrong or the line has no air. If water pools in one section of the pipe, even the widest pipe will fill with sediment over time. If the vent end of the shared stack that runs up to the roof is blocked, gurgling and odours start in the drains.

For this reason, when assessing diameter, we also look at how long the pipe runs horizontally, how many bends it passes through, and where it connects to the shared stack. A long kitchen sink line with many bends may deserve a wider pipe than a short, straight one.

Balcony and terrace drains are a separate case. Rainwater arrives in large amounts in a short time and brings leaves, soil and sand with it, so choosing a narrow pipe for these drains increases the risk of overflow in the first heavy downpour.

In most older apartment buildings in Istanbul, the shared stack and the building outlet are made of cast iron, while the drains inside the flats were later converted to PVC during renovations. That is why it is normal to find pipes from different periods in the same building, with sizes that do not match. Because the inside of a cast iron pipe thickens with rust and scale over the years, its diameter on paper may stay the same while the cross-section the water actually passes through has narrowed.

In such a building, enlarging only the pipe inside the flat does not always solve the problem. If the stack opening where the new PVC pipe connects has narrowed, or an intermediate piece was left at the old size, the bottleneck simply moves. Looking at the junction with the stack before replacement makes it clear which section needs renewing.

When renovating a bathroom or kitchen, the choice of pipes is usually discussed far less than the tiles and cabinets. Yet changing a pipe diameter after the tiles are laid is much more laborious. Asking these questions at the start of the job prevents blockages later on.