Surveying equipment and tests

Leaking water from mains, drains or other services can put the base of house walls at risk of significant dampness problems (see The 334-day wall watch, Buried hearth, buried pipes, buried truth and The 'weeping stones' of Lough Neagh). In the moisture categories used throughout this section, leaks located below ground level outside are classified as ‘below-ground moisture sources’; other leaks are generally internal plumbing leaks. The case studies illustrate how, with patience and perseverance, you can track down these problems. But there will inevitably come a time when you have to recommend further tests.

Testing water mains

Old mains pits in pavements are often full of a build-up of dirt, sand, and earth. It is wise to clear this out to provide easy access to the valve in an emergency. (As with all valves, they need to be turned on and off occasionally or they might seize up.) A pit filled with water could be an indication of a leak or a high water table.

Most water supply companies offer a free service in leak detection, which is worth using if you need to eliminate the water main as the likely source of a dampness problem. There are simple tests to establish whether a main is leaking: you can use a listening stick (or stethoscope); or there are more sophisticated tests using electronic equipment (such as an electronic correlator) that can often confirm quite accurately the exact location of larger leaks. Another simple technique to check whether a mains water pipe is leaking involves the principle of siphonage.

If there is a water meter, check the meter gauge to establish whether the gauge is moving. If the meter continues to move after all appliances and domestic services inside the property have been closed off, then there could be a leak on the potable mains supply pipe from the meter position to within the curtilage of the property.

If there is no water meter you need to establish if the property is served by its own separate mains supply pipe or whether it is part of a communal supply (common in local authority properties – in particular interwar cottage estates). The local water provider should be alerted to check the water main supply pipe for soundness where a leakage of water is suspected.

Simple siphonage test Turn water main on in street. Place glass under cold tap spout, fill it with water. Support the glass; e.g. an upturned washing-up bowl. With spout now immersed in water, turn off tap. Turn off water mains. Turn on tap. Observe if water level in glass reduces.

The repair or renewal of the potable water main supply pipes from the isolation valve (ISV) to within the curtilage of the property is the responsibility of the owner(s) of the property.

Note: in older former council-owned dwelling houses on, for example, cottage estates the water main supply pipe is often shared with adjoining dwellings in groups of up to 12 houses. Therefore the common water supply pipe is shared among each of the owners in the terrace of properties. This has, since the introduction of the right to buy initiated under the Housing Act 1980, caused difficulties as there are now multiple owners sharing one common water main supply pipe and each owner bares an equal share in the repair and/or replacement of the defective pipe, irrespective of where the defect or leak is located within the terrace of properties.

Some owners are resorting to making application with the local water providers to have their own separate supply run into the property, removing themselves from the communal supply.

Example  The relevant water authority, prior to privatisation, served a 28-day ‘waste of water’ notice on the owners of a development of ‘retirement homes’. The property owner was both the freeholder and landlord.   At the site, the authority’s inspectors could not establish the exact location of the leaking water main. They visited the site during the early hours of the morning (when there is low water usage) to check for any leakage. They used a ‘listening stick’ to check several ISVs in the street to establish if there was a ‘noise’ created by vibration on the supply indicating a leak. They tried to locate the leak by a process of elimination – the source would be where the noise was loudest - but this was not possible because the pipe had been laid under the access roadway for the development.   A correlator was then used to pinpoint the exact location of the broken section of pipe. In this case it saved a great deal of labour time and expense to excavate along the line of the pipe buried under the road.   The excavation revealed that the water main pipe had been laid unprotected in the ground. A sample of the perforated pipe and a sample of the soil water and soil were taken to the National Physical Laboratory at Teddington, where a chemist and metallurgist confirmed that the pipe had corroded prematurely.   The zinc coating on the pipe had eroded away (dezincification) because of the ‘aggressive’ soil surrounding it. (There is a national soil register detailing soil characteristics from samples extracted and tested at different parts of the country.)

Testing drains

An underground drainage leak may be the source of a dampness problem for the base of house walls and floors and, in order to eliminate this as a potential moisture source, drains investigation and testing may be required. Possible tests include:

• visual inspection;
• performance test;
• use of dyes;
• smoke test;
• air, gas and water tests; and
• CCTV inspection.

The further the drain is away from the building and the deeper the drain, the less likely it is that drain leakage is contributing to dampness of walls or floors. The nature of the subsoil and the height of the water table are also critical factors, as are the materials of construction of walls and floors.

If possible, question the occupier regarding the timing and occurrence of the dampness symptoms or staining, because sometimes it is possible to link the dampness problem with use of drains, drains blockages, or changes or repairs to drains. Particular subsoils may contain materials that colour the groundwater or produce stains. Your inspection would also include picking up smells and scents that could be indicative of a particular source of leakage or underground material.

A sketch of above- and below-ground drainage installations will help you to understand the drainage layout and the implications of defects or leakages to the drain itself, the property, the site and any adjoining property and site. Fluorescein dyes (green, red, orange, yellow or violet) can be used to confirm drain runs, detect water flow and also help in tracing leaks.

There are a variety of underground drainage systems that can be found:

• soil and waste discharge;
• rainwater discharge from roofs;
• surface water discharge; and
• a basement drainage system worked by a pump or by gravity.

Subsoil drainage by way of field drains or ‘french drains’ could also be present. Underground drainage of foul or waste effluent and surface water or rainwater can be separate or combined.

Checking inside an inspection chamber can tell you whether a drain is properly discharging, or whether there is evidence of a current or past blockage such as a build-up of soil or waste matter on benching. Blockages may also result from dislodged pointing, benching or internal renderings. Blockage of a drain run could well be the cause of leaks that could result in a dampness problem to the house. You could inspect the final section of a drain using torch and mirror, as faults can occur where the drain run meets the chamber.

Note the condition of deep inspection chambers, but they should only be entered for testing purposes by people with the necessary skills, experience and equipment because they present a considerable health and safety risk. Even just raising and replacing the cover on an old chamber presents a risk of dislodging upper corbelled masonry courses that may be loose and unstable, and some covers require considerable care in lifting due to their sheer weight and the fact they may not have been lifted for years. A trickle of water continuously running along the drain channel viewed in an inspection chamber could be a sign of a leaking water main, as leaks from mains sometimes find their way into drains.

Gullies are quite easy to test: simply fill up the gully, mark the initial water level, and observe that level over a few minutes to confirm gully integrity. If the water level in a gully appears low the gully may be leaking, and as gullies are often in close proximity to house walls, damp penetration could be occurring (see the Cornered by damp case study).

Drain faults and raised external ground There is potentially an important link between the raising of external ground levels and damp problems caused by drainage faults. Drains often run near to finished external yard levels where gullies and soil/waste stacks feed into the underground drain runs. For the traditional back inlet gully, the drain connection is only 1 or 2 inches below the level of the grating. Such pipework, only inches under the finished external ground level, is vulnerable to vibration and impact during any resurfacing of external yards, and particularly if any compacting of hardcore is involved.   A visual inspection of gullies prior to testing may reveal cracked gullies; and a gully out of level may also indicate problems. Grubbing up and replacing a damaged gully may be necessary to reduce damp penetration to the house wall or floor. Raising of external yard levels may also have been executed without consideration for easy discharge of surface water to gullies or away from the building.

Drains tests are not usually part of the pre-purchase survey unless specifically requested by the client, or unless deemed necessary by the surveyor following a reconnaissance visit to agree on scope of the survey and access arrangements. In general, drains tests and further inspection are recommended as ‘further investigation’ in a building survey or ‘ACTION – Further Investigation’ in a Homebuyer Survey and Valuation. Surveyors may specify the type of further investigation required, for example a CCTV survey, or a test for drain water-tightness.

If underground drains are blocked, recommend clearing the blockage by a drains specialist or competent builder. It will be necessary in any case to clear the blockage and flush out the drain in order for the drain to be tested.

In terms of a dampness investigation you will be concerned primarily with establishing whether or not leakage from the underground pipework may be causing dampness problems to the floors or walls of the house. Ideally, follow up further investigation by attending on site during the inspection, or help your client to make decisions on the basis of specialist report findings. An understanding of the strengths and limitations of the various tests and investigations will help you to advise your client more fully. Specialist drainage investigators are usually also drainage contractors so their view, no matter how honestly intended, may not be truly a balanced one.

A chemical analysis of samples may establish that underground drains are a possible origin of moisture found in building elements.

Example Chartered building surveyor Trevor Rushton told us of a drain problem that came to light when foul drains of a school building in Sydenham, South London, were blocked, and urgent investigation and remedy was needed.   Various lines of investigation were followed. The falls of the drain run were checked and found to be less than ideal, but probably not the cause of blockage. A CCTV survey found an obstruction near the building, caused by what appeared to be a metal rod piercing through the pipework. That section of drain was excavated and indeed a rod had pierced the pipework. It was actually a lightening conductor rod that had been initially driven into the ground safely to one side of the drain, but a large stone had caused its path to be deflected towards and eventually through the pipe. The obstruction would not have shown up by water test, as the rod was virtually sealing the 2 holes it had made. Without a CCTV inspection a much greater excavation may have been needed to find the location and nature of the problem. It may have been possible to find the offending obstruction using traditional drain rods, but this would have given no insight into the type of obstruction encountered.   Jetting through the run may have cleared the blockage but the drain would undoubtedly have blocked up again soon afterwards. Interestingly the drain had coped quite well in spite of the metal obstruction for 2 years, but Trevor discovered that the toilet paper used by the school had been changed to a more robust type that did not disperse readily and enabled blockages to build up, prompting the full investigation.   This is a typical example of the kind of approach needed to track down and solve a drain problem. The holed pipe section was easily replaced and the lightening conductor rod re-installed. Such blockages may well cause dampness problems where foul water escapes from the drainage system to soak into a house wall or floor.

Smoke tests

Smoke tests of above-ground drainage installations can help to track down leaks that may have first been suspected from foul smells. It puts no pressure on pipework and does not create a loading problem as would water testing, and is therefore particularly appropriate for soil and waste stacks. The fault in the pipe will be traced by smell or sight of the smoke.

Air and water tests

Water and air tests tell you how watertight or airtight the drain is, but unless a staged testing strategy is used for below-ground drainage neither a water nor an air test can tell you initially where a drain is leaking along the run under investigation. Each test offers extra information on the condition and likely performance of the drainage installation.

Air testing is commonly used for new pipework, and is also suitable for testing existing above-ground drainage. The test does not create a loading problem as water testing would. However, air pressure is affected by change in temperature, and this must always be borne in mind: even the sun coming out during a test on new exposed underground pipework could cause air pressure to rise as the air in the pipe increases in temperature.

Air at relatively low pressure – 10 millibar (100mm on a standard air testing ‘U’-gauge) – will escape through the smallest of cracks, and such a test is considered ‘stringent’ for that reason. If a drain fails an air test, it may still pass a water test. Such a test for airtightness on existing underground pipework will not give information about the location of any leak.

Air testing is quicker than water testing, and there is no problem of disposing of water used in the test. In most cases failure of this test on underground drainage would be followed by other tests to establish the location of the fault. Failure of an air test for above-ground drainage may be evidenced by escape of air, traced audibly, or by the application of soapy liquid which will visibly bubble where leakage is occurring. Such tests are probably outside the scope of house surveyors, requiring specialist equipment and training.

In many ways a water test is the definitive test because drains are designed to carry water. During their normal operation drains will not fill up unless a blockage occurs and the whole of the pipe may be full of water. A drain may perform perfectly well and not leak, but if blocked and consequently able to fill up with discharged effluent, a pathway out of the drain may be found that ordinary usage did not allow.

Water tests basically involve blocking up pipework at the lower end using plugs or inflated bags, and filling up the pipework with water either by flushing appliances such as WCs, turning on taps, or by utilising hoses. After filling up pipework (and sometimes inspection chambers) the water level is observed over time, typically 30 minutes. During this time it is likely, particularly where drainage is made of absorbent materials, that water will need to be added (in measured quantities) to maintain the original marked or logged water level.

(a) air testing

b) bubble on pan connector

Figure 1: (a) The water gauge is pumped up to pressure; (b) soapy water is applied to a suspect WC connector, and a large bubble is produced, indicating a potential leak

For new drainage British Standard Code of Practice 8301 tabulates acceptable water loss for various pipe diameters, with a loss of 0.05 litres per metre run of pipework being acceptable for new pipework. Experience will help you appreciate whether you are observing a minor leakage or a significant leakage that could be causing problems.

Water tests are not carried out to existing drainage as often as they used to be, which is probably due to the increase in use and availability of CCTV equipment for drains inspection, and possibly the fear of damaging an old drain by exerting undue pressure on pipes and joints. The pressure put on pipes will depend on the head of water produced by the test. British Standard Code of Practice 8301 recommends a minimum head of 1.5m, usually produced by means of a 90° bend and standpipe. A 1.5m head may be too severe for an old drain, as water may be forced through faults in joints or pipes to cause damage to pipe bedding or adjacent structures. If not already cracked or defective, old pipes should be able to withstand such a pressure.

Standing water in a drainpipe is also quite a heavy load by virtue of the sheer weight of the water. But remember that the objective of the test is to try to establish how badly it leaks. The pipe may ‘self seal’ to a certain extent, where the flow of discharges creates a build-up of scale, etc. in joints. Water flowing within the pipe may not leak as long as there are no gaps or cracks to that part of the pipe over which the discharge flows. A crack at the top of a pipe, for example, may not cause the pipe to leak (unless subject to a blockage), although the pipe would fail a water test.

Melville and Gordon (The repair and maintenance of houses, London 1988) recommend a water test for old drains with a 610mm head of water (1lb/square inch), and describe procedures for water testing in considerable detail. They consider the water test ‘the only one of real practical value so far as the condition of drainage systems is concerned’. However, there are different perspectives on drains testing: some believe that no single test addresses all points.

Modern pipework properly installed should rarely fail a water test due to the reliability of the modern jointing methods.

Case study: hydrogen gas pressure testing A case of modern plastic drainage pipes used to serve a large ornate fountain were routed underground between the building containing the pumps, water storage tanks,  the outside fountain and water feature. The system is a sealed circuit and therefore relies on the same water level re-circulating from the holding tanks to the fountain and back to the holding tanks via the return drainage pipes. However, there was found to be water loss over time.   The base of the fountain was sealed off and checked for any drop in water levels, none were found. The tanks were completely drained and hydrogen gas was introduced into the sealed circuit of pipe work. A hydrogen gas detector was used to scan the ground above the location of the underground section of flow and return pipework, where it was discovered that there was a leakage of nitrogen at one location and when the ground was subsequently excavated the location of the rogue pipe joint was found and repaired.   Figure 2: water tanks servicing the fountain and water feature were drained and the pipework sealed off, and a hydrogen gas supply connected (see figure 3 below) Figure 3: hydrogen gas was connected to the drained sealed pipe circuit and water tanks and the gas pressure turned on to over 2 bar pressure Figure 4: an external hydrogen gas detector located the escape of gas, which found the rogue section of pipe underground resulting in only a localised excavation to effect repairs

Permission to test may be needed legally, and must certainly be sought to reduce the likelihood of any disagreements, and particularly if the client is not the owner of the property, or if testing would affect an adjoining property. Advice must be sought on the likely effect of a water test on the integrity of the drain, bearing in mind that the drains most likely to suffer damage from such a test are those in poor condition and would be subjected in any case to similar potentially damaging pressures when blocked.

It is possible for house surveyors to conduct simple water tests to underground drains using metal plugs or inflatable test bags, if site conditions allow. Drains are usually tested from head to base, so the same water can be used for the next section of drainage down. Air locks need to be removed. Drain runs or inspection chambers are held under test for 30 minutes. It may be possible for leaks to be traced using dyes, providing that there is no risk of causing staining from their use.

If traps are built into gullies or sanitary fittings at the head of a drain are to be tested, allow the trapped air to escape as the pipework fills up by using a short length of tubing. Water testing of drain runs from inspection chamber to a ground floor WC may involve temporarily removing the WC pan. If the WC pan is cemented or concreted into position, it will be difficult to test the drain run alone, and any test from chamber to pan could be problematic if a patent coupling is used at the back of the pan, because this may not hold up under water test.

The emphasis should be on testing runs, because faults in underground pipework may require very expensive remedial work that could be a significant financial outlay. Old traditional masonry inspection chambers are not likely to hold water for long as they are usually in need of basic maintenance.

Once you have found a drain run that does not hold water satisfactorily, and is therefore probably leaking, further investigation is often required to find out exactly where the leak is, as any drainage works require considerable excavation and consequential disturbance and expense that ought to be kept to a minimum. A CCTV survey may be the next investigation, although many argue that, due to the likely failure of an old drain under water test, a CCTV inspection is more useful as the first investigation anyway. However, a water test will certainly show up clearly a major leak and for shallow chambers requires little specialist equipment.

If the drain appears to fail a water test there are implications not only for potential dampness problems but also the possibility that the property is subject to ground movement that has disturbed drains, or damage by tree roots (see figure 2 below) that may be linked to a subsidence problem. Faulty drains could affect a neighbouring property. There are also obvious environmental health issues if a drain is found to be leaking.

Figure 5: Tree root penetration into a drainage system

If a blockage is found it is traditional practice to excavate in order to establish condition, although a CCTV survey may now be a practical alternative. In the interests of the client, follow up and ideally attend all further inspections by drains specialists, subject to the client agreeing your attendance fee, because there may be a tendency for over specification of remedial work by a specialist who is also a contractor. Any works to drainage probably require notification to the drainage authority and any parties that share the drain run.

CCTV inspection

Closed-circuit TV surveying (CCTV) gives you the opportunity of a visual inspection of drain runs, and you can keep a record of the inspection on film. But a CCTV inspection does not tell you whether a drain is watertight or leaking, although where obvious damage to pipework is noted, it is likely that leakage will be occurring where the discharge flows across cracks or holes, etc.

The advantage of CCTV is that, where a fault is found, you will know exactly where the fault is located without the need to excavate. The skill with CCTV lies in the interpretation of the film footage. You need to know, for example, whether faults observed are significant in terms of the drain's performance. If pipe lengths are slightly out of alignment, for example, the drain may still be able to perform satisfactorily as long as there is not any lipping against the direction of flow, which could cause a build-up of solid matter to cause a blockage. Some pipework joints may appear open on CCTV film footage when they are in fact perfectly sound patent connections.

You will also need to assess the findings in relation to the age of the drain. If there is not a history of drain blockages you might accept 1 or 2 imperfections and recommend that the client contact you for advice should any problems arise in the future.

A CCTV inspection report usually includes a plan of the underground drainage, with all gullies, chambers and drain runs marked. There will be detailed reporting of the condition of each drain run, with faults located very accurately along each pipe run. There is not always very close reporting on the condition of inspection chambers, gullies and so on. Such a survey may well cost more than £200 for a standard 3-bedroomed house underground drain system.

CCTV is not often used for above-ground drainage as soil and waste pipework is usually quite visible and any leaking joints would show themselves by seepage or staining to the pipes or adjacent surfaces. However, CCTV can sometimes come into its own when internal foul or rainwater runs run inside concealed ducts or are encased in concrete. Providing bends are not too sharp the camera can offer you a most useful insight into pipe condition.

The CCTV survey is usually undertaken by specialist investigators who may also be drainage works contractors, and their findings must therefore be closely scrutinised. There are established training programmes run for CCTV investigation specialists by the water training industry and surveyors are well advised to seek out CCTV investigation teams with the best training credentials.