Last reviewed 3 January 2019

In this article Alan Field explains how to identify drainage assets and be aware of both the safety and environmental issues that their ongoing maintenance could entail. For some sites this may be straightforward. However, this cannot be assumed. At some sites subsurface drainage services will require further investigation and levels of risk assessment.

Below ground

Each site will have unique drainage arrangements. It should never be assumed what these are or that they will prove to be trouble free.

While older sites might present more complex drainage assets, even modern buildings may have unexpected subsurface infrastructure, sometimes due to cost expediencies before or during construction. Also, the site may have environmental considerations, such as receptors, ie protected species or aquifers, that may eventually be negatively impacted upon by known (and unknown) drainage pathways. The so-called “source-pathway-receptor” approach helps a facility management professional understands the consequences of drainage — both those that are planned and unplanned.

This is one reason why risk assessment is so important. A part of the risk assessment process should be to understand the drainage assets. After this, the environmental and safety risks connected with these assets can be assessed, including those connected with planned and preventive maintenance.

Drainage, is not of course, the only underground service on many sites. The Health and Safety Executive’s HSG47 document should be read in connection with the way different services can interact with one and other, particularly from a safety perspective.

Even where drainage arrangements are understood on a site, these could be compromised by, say, subsidence, or other consequences of poor (or simply non-existent) maintenance. So, these pathways might also cause pollution on the way and create an unplanned (and often unknown) receptor, unless obvious signs of pollution arise at surface level.

Unplanned events can also lead to health and safety issues not considered or planned for. In other words, always consider the risks arising from unexpected drainage activity, eg backflows or the risks involved in resolving drainage issues and methane gas build ups in private sewers or septic tanks.

Not being able to resolve drainage issues quickly can make the risk of prosecution for environmental and/or safety issues more likely. It also unnecessarily increases the risks of incurring civil liabilities and, possibly, provoking insurance companies into asking awkward questions about why the incident could have arisen in the first place. Also, there is the risk of bad publicity for the organisation’s brand, let alone the risk to human life and safety. In short, ignorance is never bliss with drainage.

Drawing the line

A facility management professional should always be aware of their site drainage plan. This could be the architect’s original drainage plan or one prepared later after a drainage survey. The older the plan then the greater the risk there have been changes made to the drainage assets at the later date. The other common risk is that the original drainage plan was never fully executed during construction or, occasionally, was executed completely differently. Sometimes drainage pipes and private sewers may be shown on the plan but not the other drainage infrastructure, eg interceptors or sumps. Some drainage plans show straight lines when the underground services actually snake. Equally important, some drainage plans are unclear about the depth of installations.

In other words, assumptions should not be made about the accuracy of any drainage plan unless there is other evidence to establish this. It may be necessary for the occupier or landlord (depending on their responsibilities) to pay for a drainage survey. In this way, both safety and environmental risks can be better understood.

Sometimes non-standard drainage arrangements might be found — anything from dodgy DIY (for example, on a small site or a farm building converted into offices) or, on an industrial estate discovering that a neighbour has made illegal drainage connections through the site’s drainage (often many years previously and without the current neighbour’s knowledge).

Where the site has both foul drainage and then storm water drainage runoff (eg from car parks) may flow to unknown receptors. Unless it is a very modern site, it may be difficult to determine exactly where the stormwater receptors are. This needs to be understood to avoid potential pollution risks, especially where car parks are also used for storing fuel oil tanks or other liquid pollutants that could end up in stormwater drainage, say, due to tank failure.

Some sites will either not be mains drainage at all or only partially so. The distance from the nearest public sewer is sometimes a guide to the original design decision on this point. Sites which are not connected to mains drainage at all (or only partially connected) will usually have septic tank arrangements (or more complex arrangements might be classified as package treatment plants). The next question is then to determine how these interact with the site drainage. A package treatment plant may handle the effluent for neighbouring properties as well; this needs to be verified. Also, all such assets need to be assessed from a safety perspective, eg subsidence; gradual failure of the tank or a dangerous build-up of methane gas. These are just a few examples where health and safety concerns might arise and are sometimes connected with pollution incidents.

Some drainage assets may be redundant and, if so, can still present health and safety risks, eg a redundant interceptor might collapse or develop a build-up of dangerous gases (this is especially true if the underlying ground conditions are polluted with VOCs or other gases that can ingress into an empty tank that is starting to fail).

What to do next

Once the drainage assets are understood, then planned maintenance, where necessary, can be budgeted and scheduled. Drainage maintenance should not just be reactive, as emergency scenarios will tend to mean either operational disruption and/or potential legal issues involving public authorities or neighbours.

Some maintenance checks can be relatively straightforward for experienced hard services operatives to carry out, eg routine checking of interceptors and the cleaning of gulleys. Other routines may need more specialist advice or intervention.

Some drainage survey and repair techniques can require a number of specialist techniques or tools. But never be blinded by science. It shouldn’t just be left to the contractor — in other words, the contractors should explain these tools and techniques as a part of their own risk assessments and safe systems of working. The facility management professional should always ensure that they understand the safety implications of such work and any pollution implications.


  • Never assume what the drainage infrastructure is, for a building or larger site.

  • In particular, never assume a site is on mains drainage.

  • Drainage assets may need to be surveyed to determine location and current condition.

  • It is surprising how common it is that assets such as septic tanks, interceptors or sumps may have been installed at different times.

  • Drainage plans give essential information to help prevent pollution and safety-related issues.

  • Risk assessment of drainage assets should always consider unplanned events, eg subsidence and build-up of methane gas in private sewers.

  • Pollution leads to all manner of legal and reputational risk.

  • Some drainage assets may be redundant and, if so, can still present health and safety risks.

  • Never be blinded by science. Drainage work often requires specialist contractors. A facility management professional should always ensure that they understand the safety implications of such work and not just leave it to the contractors.