In August 2010, I reported that the Scottish Government were soon to revise their Building Regulations to deliver water efficiency into Scotland’s buildings and expressed the hope that they would specify maximum allowable flow rates and cistern volumes, rather than use the widely discredited water calculator that is required for Part G of the Building Regulations in England and Wales.

Well, Section 7 of the Scottish Building Regulations has now been published. It covers a wide raft of sustainable requirements for both domestic and non domestic buildings and explicitly states that it counts the carbon emissions from distributing, processing and heating water. There are six levels; Bronze (the baseline level that demonstrates the building has met Sections 1-6 of the 2010 Standards), Bronze Active, Silver, Silver Active, Gold and Platinum1 . The water efficiency requirements are what interest us. Applicable to dwellings only, they are mandatory if the dwelling is to attain the silver or gold level. The stated requirements are clear and it can be seen at a glance where the Scottish Government thinks that water consumption can best be reduced.

Water efficiency requirements at silver and gold level

At silver level the following minimum requirements must be met: WCs with a maximum average flush volume of 4.5 litres2; flow at basin taps no greater than 6 litres/minute; flow at shower heads no greater than 8 litres/minute; and a water butt with a minimum capacity of 200 litres if there is a garden3. At gold level the requirement for a water butt remains and then a further three out of five water efficiency measures have to be met. These are: a water meter; WCs with a maximum average flush volume of 3.5 litres4 , flow at basin taps no greater than 4 litres/minute, and flow at kitchen or utility sink taps no greater than 6 litres/minute; flow at shower head no more than 6 litres/minute; rainwater harvesting system or greywater recycling system to provide water for toilet flushing.

How refreshing. There is no water calculator with a virtually impenetrable method of “calculating” average daily water use, no random number of times that the average person will use the appliance, no percentage breakdowns of the number of times a person may use the shower instead of the bath, no “normalisation factor” to make the calculated (to two decimal places!) average daily consumption fall within Code and Part G parameters. But simple and straightforward maximum flushing volumes and maximum flow rates which align with the performance bands within the BMA Water Efficiency labelling scheme. It is a success for sensible thinking, due in no small part to the AECB Water Standards.

Suggested improvements

However, there is one glaring discrepancy if the stated aim of Section 7 to reduce water use and the carbon emissions from heating water is to be met. While at silver level a maximum flow rate of 8 litres/minute for showers is mandatory, at gold level the developer could choose ultra-efficient WCs, install a meter, reduce flow rates at basin and sink taps (thus meeting the three out of five requirement) and then install a power shower. This loophole needs to be closed quickly, so that showers in homes that meet silver or gold level have a flow rate at the shower head of no greater than 8 litres/minute.

There should be a requirement for water meters (if installed) to be sited internally so that underground leaks are not charged to the customer, and to enable easy reading of meters by the building occupants (especially important for older or disabled customers). I would also have preferred a limit on bath sizes to a maximum 220 litre volume (standard bath size), and the flow rate at basin taps to be 4 litres/minute at silver as well as at gold level.

Overcoming limited drain carry

Section 7 also makes reference to a requirement to consider pipe diameter and gradient of drains, to ensure they are self cleansing even at low flow rates. The reduction in drain carry has been a concern for some engineers and developers since 2001 when flush volumes were reduced to as little as 4 litres for a full flush. The WRc (Water Research Centre) carried out an interesting piece of research in 2004/5 showing that WC flush volumes as low as 1.5 litres would not cause blockages in drain pipes from dwellings if carrying pooh and toilet paper. Tests showed that, although a flush of 1.5 litres does not (as expected) take the WC contents as far down the drain as greater flush volumes, a hydraulic head of water (from the shower, washing machine etc) builds up behind the blockage, eventually overcomes the frictional resistance and pushes the contents further along the pan. This process is repeated until the drain meets the sewer.

I have been specifying and using 4/2.5 litre flush WCs since 1998 and have never had a problem with drain carry. However, I appreciate that engineers and developers require certainty that drainage will deliver as designed and that actual falls on drains once laid may not meet the design requirements. Therefore I was really pleased to see that the Drainwave (an Australian invention) is now available on the UK market. Launched at Ecobuild, it was one of the few things worth reporting on the water front from this year’s event. Sited at the head of the drain run, the Drainwave is, essentially, a tipping bucket that, when full, releases 10 litres of water over five seconds resulting in a double wave action at an equivalent flow rate of 120 litres/minute through the drain thus ensuring efficient carry. With no power requirement, and a list price of less than £500, there should no longer be any arguments against installing the many 4/2.5 litre flush WCs now available in the UK.


  1. Platinum level is concerned with CO2 emissions only)
  2. This equates to a 4.5 litre single flush or 6/4 litre dual flush WC
  3. This requirement is excluded if there is no external rainwater pipe within the curtilage.
  4. This equates to a 4/2.6 litres dual flush WC

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Water Efficiency and Drain Carry under the revised Scottish Building Regulations