How to avoid crossflow

A common issue for installers is overcoming multiple waste pipe connections in a confined space. Of course, there are many fittings available to overcome most problems. However, although a particular fitting may appear to be the solution, incorrect application can create problems.

Approved Document, Building Regulation Part H should be the starting point for installers to adhere to the regulations when a cross connection of waste or soil pipes needs to be factored into the system design and installation.

How to use fittings correctly

Figure 1 is an image of a 110mm soil pipe and a no-connection zone of 200mm when a branch exceeds 65mm to stop crossflow irrespective of stack diameter.

Where the branch pipe is less than 65mm (Figure 2), the no connection zone is 110mm deep on a 100mm soil pipe. If the soil pipe is 150mm diameter, then it is 250mm deep, as can be seen in the image above right.

Another area to avoid when back-to-back connections might be required is when a double branch connection is applied as can be seen in Figure 3.

Do not use this horizontally as one WC will flow into the other. You should not add connections in the 200mm zone, but you can use the cut-out boss instead.

A branch pipe should not discharge into a stack at less than 450mm above the invert level, at the foot of the rest bend (see Figure 4). This is found at the base of the soil stack on single dwellings that are no more than three storeys high; for taller buildings, the distance should be increased to more than 450mm.