Technical Bulletin Issue 15 by Sava

SAVA Technical Bulletin For registered members of the SAVA Scheme

Issue 15 | December 2012 All content © National Energy Services, Ltd Welcome to the latest issue of the SAVA Technical Bulletin. The bulletin focuses on Home Condition Surveys and associated non-energy issues. We trust that you will find the bulletin useful for your day-to-day work and we welcome any feedback you have about what you would like to see covered in future editions. The contents of this technical bulletin may supersede certain scheme rules or requirements appearing in the Product Rules, Inspection and Reporting Requirements, training manuals or elsewhere. Members must therefore ensure that they have read and understood this document.

IN THIS ISSUE 

Rendered brick or piled earth?

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Private drainage

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Roof extensions

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Photo competition

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F l a t fe l t r o o f s

Rendered brick or piled earth? Those of us familiar with RDSAP will understand that the term cob describes earth walls. Despite our damp climate, there are thousands of earth buildings in the UK, some of which are over four hundred years old. They are made with a variety of earth materials including clay, chalk, and other aggregates bound with straw, and other vegetable matter or even animal hair and dung. Recognising that you are inspecting an earth building during a survey is not always easy and repair/maintenance can be a mine field.

These forms of earth wall construction can be roughly grouped into three basic methods of construction: 1. Layered earth methods 2. shuttered/rammed earth methods; and 3. unfired earth blocks.

1. Layered earth methods Cob: the thick cob walled houses in the West Country are probably the best known of all earth walled building types. (Photos 1and 2 over the page). Their thick walls are made by piling a mixture of subsoil and straw (about 600 mm thick) on the wall and paring the rough edges flush with the wall. The next layer is put on when the previous work has dried enough to bear the weight.

Each region of England and Wales (see map) tends to have its own form of construction dependent on the nature of the materials available locally. The principal forms (of which there are many variations) include cob, clay lump (or adobe), wattle & daub and pisé de terre. In Scotland, walls can be found which are made using grass and peat turf. The primary form of earth wall to which the term cob is attributed are walls where the earth is structural in nature, i.e. where the earth is load bearing.

This method of building extends east as far as Basingstoke where the subsoil used is mainly chalk. Cob or variations of it are also found in the East Midlands. The map shows the main locations (red areas) in England and Wales where earth wall construction has been found.

(Continued on page 2)

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SAVA Technical Bulletin Issue 15 | December 2012 | ÂŠNati onal Energy Services 3. Shuttered/ Rammed Earth Methods In addition to the techniques described above, there are numerous variations and hybrids, where the earth is used to fill between shuttering and then rammed until it is compacted and hard. PisĂŠ de terre, was a method introduced from continental Europe in the eighteenth century. Here shuttered earth is rammed in thin layers to form an extremely dense, hard material.

Photo 1: A building using the layered method of earth wall construction.

(Continued from page 1) To the north, on the Solway Plain, and on both sides of the border, walls are built with the continuous cob method in which the subsoil is placed in thin layers alternating with layers of straw. Because the layers are so thin, by the time one layer has been put right around the building the previous layer has dried sufficiently for the building work to continue. Whitchert: (also spelled wychert or witchit), meaning 'white mud' is local to the area between Oxford and Aylesbury. Here thin walls were constructed using the cob technique with a mix of earth and straw. In the Midlands the local earth is used to build in the same way. Clom: this is a local technique used in Wales where a mix of clay and aggregates are bound with chopped wheat straw (rushes, bracken moss or animal hair are used as alternatives to straw) in layers above a masonry base and are then trimmed to a battered finish.

Photo 2: The same building as in photo 1. Note the large overhang on the roof to prevent water from falling from the roof onto the earth wall surface. Note also the brick footings at the base of the walls.

Although adobe was reported in Perthshire before it was introduced in

Architectural Features Typically Associated With Earth Wall

England, none has been found in this area. Construction is similar to brickwork with regular bonded courses, but the dried blocks of adobe or clay-lump are much larger and are usually laid in a mortar of fresh earth or clay (Photo 3). In conservation areas etc. modern buildings and repaired older buildings can sometimes incorporate a fired form of clay lump that can provide a more resilient finish. However, firing is not always successful and the blocks can quickly degrade under frost action. Pugged chalk or chalk mud lump is a method of construction found in areas where chalk naturally occurs and is formed either in shuttered construction or as precast blocks. A mix of soft chalk and clay are used to form the walls.

Photo 4: Infill earth plasters such as wattle and daub are used between timber frames to create walls.

Construction Earth wall houses have distinct architectural features, the presence of which can be essential to prevent the deterioration of the earth walls. Most of the defects occurring in earth wall constructions (noted below) are dampness related. Most earth buildings have shallow foundations where the footings are constructed in brickwork or rubble masonry. This can vary from about 150 mm to 1200 mm in height (Photo 2). Rising-damp seldom crosses from the footing wall into an earth wall.

2. Unfired earth blocks Clay-lump or 'adobe': the universal mudbrick, adobe is found only in East Anglia in the area east of the A1 and south of the A47, where it was introduced, from abroad, around the end of the 18th century. For about one hundred years it was the principal walling material for every sort of building on the chalky boulder clays of Norfolk and Suffolk.

Shuttered clay is a variation on pisĂŠ de terre in which the plastic subsoil is placed between boards in a mix of chalk, clay and straw.

Photo 3: Unfired earth blocks.

Where earth walls do not have stone or brick masonry footings it is necessary to ensure good drainage around the building and to ensure that surface water from the roof and on the ground is quickly removed from around the building. (Continued on page 3)

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SAVA Technical Bulletin Issue 15 | December 2012 | ÂŠNati onal Energy Services (Continued from page 2) Many clay-lump houses have one or more elevations faced with brickwork which was either built with the house or put on as an improvement later. The brickwork was fixed to the clay-lump with bands of hoop-iron which were nailed to the clay-lump or were built into the mortar joints. In time these ties rust and fail. They can be replaced using remedial ties designed for cavity walls. Where the earth wall has not been protected by brick masonry it is typically protected by porous wall coverings of render/plaster. These are usually also based on clay and lime and they are typically finished with a lime wash decorative covering.

Six common defects and issues associated with load bearing earth wall construction 1. The strength of earth walls is proportional to their moisture content and some moisture is necessary to maintain that strength. Therefore common mistakes can occur by failing to recognise the construction form and undertaking inappropriate damp and timber treatment or the application of impervious external renders (such as cement based render). 2. The use of modern impervious internal decorative finishes such as modern paints can also cause problems similar to impermeable external renders.

Openings in walls are bridged with wooden lintels and often have blocks of wood built into the reveals for fixing doors or windows. In clay-lump buildings the windows and doors were built in. Because monolithic walls shrink as they dry, the openings were formed as the wall was built and the doors and windows were fitted afterwards.

3. Bridging of the stone-underpinning course by external ground levels can cause high levels of dampness in the walls.

The roofs of earth buildings are usually hipped, not gabled, because of the difficulty of providing the necessary restraint for a gable wall. Rafters are carried on wall plates positioned over the centre of the wall and the spaces between the rafter feet are filled with subsoil. The eaves of these roofs will also typically have a wide overhang also (Photo 2).

5. Poor protection of the roof can expose the walls to weathering and accelerated deterioration.

Chimneys are usually constructed of brickwork above the roof, although the flues below are often of clay-lump. Where there are no chimney pots, water is liable to get inside the chimney and erode the top of the clay-lump. Many brick flues were lined with subsoil and some had their brickwork laid in earth mortar.

4. Earth walls can be subject to slumping, especially at the base, where they are subjected to prolonged periods of dampness.

6. Rats can burrow and weaken the walls.

Photo 5: wattle and daub

Like all the other forms of earth-based building materials the 'daub' is made of a subsoil which must contain a small proportion of clay mixed with animal or vegetable fibre. In this case the earth mixture is supported by an interwoven lattice of sticks and laths (the 'wattle'). A variation of this technique, known as mud and stud, can be found in Lincolnshire where there are a number of houses with mud and stud walls. In this case the mixture of straw and subsoil is placed around and between earth-fast posts. These houses are small and generally well recorded, well repaired and fiercely conserved. Another variation is the technique known as clam staff and daub found on the Lancashire plains uses clay based infill built up around thin timber studs set between the main timbers frame.

Earth used in timber frame buildings Earth is also used in the construction of timber framed buildings. The most common form of non-load-bearing earth construction is Wattle and Daub, which has been widely used to infill the panels of timber-framed buildings. (Photo 4 previous page and Photo 5 above).

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SAVA Technical Bulletin Issue 15 | December 2012 | ©Nati onal Energy Services

Roof extensions Government proposals, announced in September 2012 to relax planning regulation in respect of extensions are a timely reminder to us all to be on our guard. Extensions are always an area of potential for defects and issues. From differential movement caused by variation in foundation design to issues with cavity wall trays, we are all aware of the potential problems within the conjunction of the original and new structures. But the question of the build quality of the extension can sometimes be overlooked. We can perhaps defer too quickly to others and rely on the presence of building regulation approval and the belief that newer constructions are less likely to be defective in design or materials. Where building regulations and planning consent are not required for an extension we are rightly far more guarded: the conservatory with a floor area of less than 30 m2 is an obvious case in point, where the quality of the build is open to question. We have in previous articles drawn attention to the pitfalls of conservatory construction. However, in the case of the addition of a utility room, bathroom or more living space to a dwelling we still need to be on our guard. Even when there are no apparent defects upon inspection, deficiencies in the design can cause issues that could leave the surveyor exposed, if they are not highlighted in the Home Condition Survey (HCS). The extension to the Victorian terraced house shown in Photo 1 is a case in point. To all intent and purpose it is a perfectly adequate extension. Built, we learn from enquiry, during the last seven years and paid for by the current owner. The same conversation also confirms that the extension gained building control approval. The extension incorporates a wet room with shower, sink and toilet. Both the internal and external inspection confirm no apparent defects.

Photo 1: The single layer of felt is confirmed in this photo, taken when the ventilation tile was replaced after purchase.

The natural reaction is therefore to report this with no further comment and to simply suggest that legal advisers confirm that the extension did indeed gain building control approval—but stop and look again! The roof has a very low pitch. It is covered by artificial slates. The typical pitch of a slate roof is around 40o but admittedly it is usually recommended that slate can be used on pitched roofs as low as 25o. In the case of artificial slate, some of these have interlocking devices that allow for a pitch as low as 15o. In the case of artificial slate (as with other manufactured roof products), whether particular tiles are suitable to very low pitch roofs will depend on the manufacturer’s specifications. As a surveyor you are required to have a reasonable level of knowledge. Therefore, when inspecting a roof that appears to have a potentially unsuitable pitch for the covering, you need to understand the generally accepted minimum pitches. You will also need to identify where further scrutiny might be necessary (further research on your part or referral to other expertise) to verify the suitability of a roof covering. In making your inspection you can also check other elements of the covering to verify the suitability of the artificial slates for the pitch. The first consideration is the location of the roof. Low pitch roofs can be prone to greater risk of wind driven water

penetration: the greater the pitch the harder it is for water to be pushed up the slope to the top edge of slates and penetrate to the structure. Therefore, if the roof is in a location where it is protected from prevailing winds, the standard manufacturer’s recommendations on minimum pitch and associated fixing can be reasonably safely followed. However, where the exposure is greater to prevailing winds (even where the pitch is a more typical 40o) you should check for evidence more typically found in low pitch roofs (see below). Where artificial slates are to be used on lower pitches, manufacturers will recommend three main factors: 1. That the lapping of slates is increased; 2. that the size of the slate is increased; and 3. that consideration is given to double layers of the secondary weather barrier of roofing felt. Using the example of the common Marley Eternit slate, the manufacturer recommends 110 mm lapping of the 600 mm x 300 mm slates on a pitch as low as 23o in low exposure, or 26.5o in high exposure locations. For any pitch lower than these the manufacturer recommends that roofers take their further advice.

(Continued on page 5)

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SAVA Technical Bulletin Issue 15 | December 2012 | ©Nati onal Energy Services (Continued from page 4) In situations of pitches less than those indicated on the previous page (and in the table on the right) you would expect to see more thought given to increased slate lapping, which can be observed at the gable end.

Size of slate (mm)

Typical laps (mm)

Moderate exposure rafter pitch

Severe exposure rafter pitch

600 x 300

110

23° and over

26.5° and over

600 x 300

100

23.5° and over

29.5° and over

500 x 250

100

23.5° and over

29.5° and over

You can also check the size of the slate and would expect to see larger slates in situations where the pitch of the roof is shallower.

Although not specified by artificial slate manufacturers, many building control officers will require (and many experienced roofers will recommend) improvement to the secondary weather barrier. In situations where the pitch of the roof is as low as 15o the use of two layers of bonded roofing felt can be found. This can be seen at the eaves where the felt should overlap and tuck into the back of the gutter. Also recommended is the use of plywood decking under the felt. This can be seen where the roof void is accessible. In the case of the extension featured (Photo 1 on the previous page) the roof is relatively sheltered. The roof pitch is 17.7 o and the slates are a standard flat artificial slate similar to Marley Eternit, size 600 mm x 300 mm. Inspection of the gable indicates that the slates have an overlap of 100 mm but they are also double-lapped (Photo 2). Inspection of the eaves indicates that the felt is a single layer (Photo 3). There is no roof void and so we could not easily confirm during a typical survey inspection if an additional layer of plywood decking was also used.

Text in section D2 of the HCS: The extension roof is sloping with a covering of manmade slates and with a secondary weather barrier of roofing felt. The pitch of the roof is very shallow and below the minimum pitch typically recommended for this type of covering unless additional measures are taken to improve the specification of the cover in order to prevent from the risk of wind driven rain and similar water penetration. I did not find any evidence of water penetration of the covering within the limits of my inspection but you should satisfy yourself that the roof was adequately specified and was granted Local Authority Building Control approval (see section C). In Section C we would also recommend that building control approval be verified.

In the HCS we can at least identify that the roof is low pitched and might therefore be prone to greater risk of water penetration, but none was seen.

Photo 3: The extension roof has a very shallow slope (17.7o) and is outside the typical tolerance anticipated for slate roofs (including artificial slates) that would normally be acceptable.

Photo 2: The slates have a lap of 100 mm but are also double-lapped.

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SAVA Technical Bulletin Issue 15 | December 2012 | ©Nati onal Energy Services

Flat felt roofs Small area flat roofs are commonly found at properties throughout the UK. They cover extensions, porches and other single-storey additions. Although when we inspect them we are not carrying out a detailed survey, we should be able to identify something of their construction and use this information to determine their condition, their need for maintenance and, if they have failed (or are likely to fail in the short term), what the scale of the remedy would be. When a roof is damaged and there is water penetration to the underside, it is relatively easy to condemn the roof covering as a Condition Rating (CR) 3 and call for re-covering. However, where there is no leak and a flat roof covering is worn, the risk of imminent failure can stick in the surveyors mind and deciding on whether it is a CR1, CR2 or CR3 can lead to us all erring on the cautious side. In most cases these flat roofs are covered with what we call ‘felt’. The term ‘roofing felt’ is now commonly used to refer to traditional weaker membrane products, which are sometimes used by jobbing builders to cover flat roof areas. Even these weaker membranes have a potential life beyond their anticipated life of 10–15 years (see Table 1 below). Felt roofs that are as old as this and more, can continue to perform well, especially if maintained and protected. However, it is poor maintenance, lack of additional treatments (such as gravel or solar reflective paint) or movement within the roof structure itself, that can lead to premature failure. There are now also a variety of reinforced bitumen membranes (RBMs) available that have very little in common with the original bitumen felts with their organic Hessian weave binding. There are basically three types of modern RBMs appropriate for each type of flat roof. “BS 8747: 2007, Reinforced bitumen membranes for roofing–Guide to selection and specification” provides guidance on which membrane is appropriate to each type of flat roof; they are summarised in Table 1 on the right.

RBM roof coverings are applied by four methods: pour-and-roll, torch-on, self adhesion and cold adhesion. Pour-and roll and torch-on are the most common application methods and rely on the use of hot bitumen to bond the RBM to the deck and to each consecutive layer. Self adhesion and cold adhesion are both proprietary products. It is important to establish when inspecting a flat roof how it has been constructed. Admittedly it is not always possible to see or inspect the flat roof decking. Many materials have been used over the years including: reinforced concrete (both pre-cast and in situ, prestressed), oriented strand board (OSB), plywood, timber boarding and wood wool slabs. With the exception of concrete decking most domestic roof decks are supported by timber joists, particularly where the spans are short. The deck can have important implications upon the often complex interaction of RBM roof components, including the coverings themselves, the thermal insulation, vapour barriers and how these components interact with surrounding details such as drainage outlets, flashing, gutter, parapet, abutment, roof lights surface paving, surface reflective treatments, etc. The majority of defects and issues with flat roofs are associated with one or more of the following: • physical failure; • constructional moisture;

Physical failure Physical failure of the materials is typically associated with the quality of the materials or the quality of the installation. Traditional glass-fibre or rag-based felts are usually affected by degradation due to solar radiation or oxidation which ultimately leads to their failure. The more modern RBMs with polyester reinforcements and polymer-modified bitumen are far more durable, and hence less susceptible to the same forms of degradation. All these products fail prematurely due to inadequate detailing in design and construction. This leads to blistering or cracking at joints, and other details or to rupture, caused by differential movement between roof coverings and the structure beneath. In larger roof areas thermal movement stress has been a major cause of roof covering breakdown in the past. The more modern polyester-reinforced RBMs are far more capable of accommodating such movement, hence their greater longevity. However, typically splitting is most likely to occur within the early years of a roof’s life, as a consequence of the inadequate detailing of the roof covering (when fitted) to accommodate any anticipated movement in the underlying structure. If splitting does occur on older RBM coverings it is most likely to be caused by weakness of the waterproofing materials themselves.

• condensation; and • water penetration from sources other than the roof itself.

(Continued on page 7)

Anticipated life expectancy assuming professional installation Traditional bitumen roofing membrane felts (Type 3 or earlier)

10-15 years

Polyester-reinforced bitumen membranes (Type 5)

15-30 years

Elastomeric polyester RBM systems

Up to 50 years

Table 1: Classification of modern RBMs

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SAVA Technical Bulletin Issue 15 | December 2012 | ÂŠNati onal Energy Services (Continued from page 6)

Constructional moisture

The presence of constructional moisture is most typically associated with poured concrete slabs or wet screeds laid to falls. However, moisture can sometimes cause problems from the drying out of walls associated with the flat roof. Water can become trapped by the waterproofing layers of the roof and lead to considerable problems within the building. The problem of entrapped water or water vapour can affect the inside of the building and be mistaken for roofing failure; alternatively it can affect the roof covering by causing blistering in; and both water vapour build up and condensation can lead to the premature failure of roof decking or coverings. You should also be aware that sometimes vents are provided to assist with the drying of concrete or similar roof decks during the original construction. However where they provide ventilation to voids their effectiveness and any cross flow ventilation should be considered as part of your analysis of the causes of roof failure.

Condensation Moisture producing activities take place in most buildings. Extensions with flat roofs which are protecting areas of severe or extreme humidity levels such as kitchens and bathrooms have commonly been constructed. Less common, but becoming more so, are areas of extreme humidity such as increasingly popular wet rooms, swimming pools or saunas. When the dew point is reached within a building the risk of interstitial condensation is present. This can cause more problems in a roof structure than a roof covering failure. Particularly where the roof and building lack adequate ventilation. To comply with Part L of the Building Regulations, cold-decked roofs have been superseded by warm deck roofs as the better practical option to achieve full ventilation of each and every joist void, coupled with a vapour-tight ceiling.

Warm-decked flat roofs by design provide reassurance and protection against condensation. Even so, cold deck roofs are still to be encountered and are particularly prone to defects and issues.

Water penetration from other sources In many cases leaks attributed to a roof covering can be traced back to a failure in other components of the building that are connected to the roof. Parapets, copings, damp proof courses, roof lights, gutters, service pipes in the roof space or void and condensate pipes all have the potential to fail or cause failure in the roof around them.

To repair or recover? A flat roofâ&#x20AC;&#x2122;s condition is difficult to assess when determining the condition rating. As stated above the leaking roof is perhaps the obvious candidate for recovering worksâ&#x20AC;&#x201C;but not always. A small leak that has not damaged the decking and will dry out once a patch repair has been undertaken could be either a CR2 or CR3; this depends on the immediacy of the need to repair as well as the complexity of achieving the repair. A pin prick leak that has not caused severe water penetration and is traceable might therefore be an obvious candidate for the CR2. It is also the case that the surveyor undertaking a HCS is not expected to be flat roof experts, but a basic knowledge of the type of defects common to flat roofs and how they can be repaired helps to inform our condition rating decisions. Some repairs to flat roofs are obviously not urgent and might be considered by roofers as part of a regular maintenance programme intended to ensure the anticipated life of a flat roof membrane, these include: Lack of solar reflective chippings This can occur as a consequence of failure to provide the chippings when the roof was covered initially, or because the chippings are insecurely bedded and have washed away. Chippings can easily be replaced, making sure that application to gulley areas are avoided (which should have a mineral surfaced membrane instead).

Failure of solar reflective paint Some solar reflective paints have a life span of less than four years, but failure can also occur early because it might have been applied during unsuitable weather conditions or the roof has a pitch of less than 5o. Again, this is a repair that can be undertaken as a matter of routine and unless the felt has seriously deteriorated would be assessed as a CR2 only. The more serious defects require greater thought as to whether recover or patch repairs can be undertaken. One guiding rule is embodied in Part L of the Building Regulations, which requires that the renovation of more than 25% of any roof area will require the roof to be converted to a warm decked build up. Therefore, if patch repairs are likely to be extensive to more than 25% then we can assume that recovering is the only option in many cases. Splits, tears and cracks in the main roof area therefore can be assessed using the Part L principle and even where there is no water penetration (Photo 2) the recovering might not be urgent but it is certainly a serious undertaking and justifies the CR3.

(Continued on page 8)

Photo 1: The surface top layer of felt has completely disintegrated on this sun lounge roof, but the roof is not leaking.

Photo 2: The ceiling of the sun lounge shows no evidence of water penetration.

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SAVA Technical Bulletin Issue 15 | December 2012 | ÂŠNati onal Energy Services

Private drainage

(Continued from page 7) Blisters both below the covering and between the layers can also be assessed readily (Photo 3). Water vapour pressure below the full roof covering can sometimes be cured by introducing vapour vents but blisters can crack and damage occur to other areas of the roof leading to the need to recover anyway. In any case, badly blistered roofs usually have inadequate solar reflective treatment. Physical damage such as dents punctures and small rips can be capable of patch repair as can pulling and lifting of lap joints (caused by thermal movement or poor bonding). Similar repairs can also be made to skirting up-stands that fall away from the junction of parapet walls and abutments. With all these issues a CR3 is the most likely conclusion but if there has been no water ingress then a CR2 could be argued in some cases. Of more a certain CR3 is water ingress around internal rainwater pipes etc. However, softening of the surface can be a localised defectâ&#x20AC;&#x201D;caused often by contamination of the BRM by mineral oil, organic compounds or, as in the case shown in Photo 4, by weak acid from a condensate pipe. If the source of the contamination is removed, the localised area of damage can be cut out and patch repaired. The question of CR2 or CR3 will usually be determined by the extent of the damage and if water has been allowed to penetrate the structure as a result of the contamination.

We are constantly seeking ways to improve the quality and substance of the HCS. The HCS is provided with a small number of additional factsheets and we will soon be adding another one advising clients on the types of private drainage and the issues commonly encountered. Most properties in the UK are connected to a public mains sewer which collects effluent from the foul drains and conveys it to a central sewage treatment works for processing. However, particularly in rural areas, some properties have private drainage installations because the public main sewer is not available nearby. There are two main types of private drainage, which may be shared with adjacent properties, and may sometimes be sited on an adjoining site. These are septic tanks and cesspools (the latter are often referred to as cesspits). Much less common, but becoming increasingly more so, are selfcontained treatment plants. Each of these installations must be located such that there is no risk of contamination of watercourses or wells nearby.

Septic tanks Septic tanks are of two basic types: Photo 3: The slight discolouration in the centre of the photo to the roof surface is difficult to see here and is the only indication (aside a slight blistering of the damp felt) that this roof has failed. The roof is only around 3 m2 and around a metre of coverage would need to be repaired so a patch repair is uneconomic.

ď&#x201A;¨ Traditional septic tanks Traditionally the installation consists of two or more large underground settlement chambers built of either brickwork or concrete (Photos 1 and 2). Effluent is piped into the first chamber, where heavier solids sink to the bottom and lighter materials such as fat and grease float on the surface. Pipes within the chambers ensure that the surface layer (known as a crust) is not disturbed. This is important in order to ensure that the contents of the chamber are not in contact with the air.

(Continued on page 9) Photo 4: This condensate pipe protruding from the gable wall is incorrectly discharging onto the mineral felt roof. The weak acid of the condensate will soften the surface and lead to a defective roof cover.

Photos 1: old two chamber septic tank.

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SAVA Technical Bulletin Issue 15 | December 2012 | ÂŠNati onal Energy Services Maintenance of septic tanks Most septic tank installations work efficiently, and give little trouble, but regular maintenance is required.

Photos 2: old septic tank.

(Continued from page 8) Anaerobic bacteria which cannot survive in contact with the air break down the materials within the chamber causing a dense sludge to form at the bottom of the chamber. A system of pipes allows excess effluent to flow into the second chamber, where further settlement of finer solids and further treatment by bacteria occurs. Finally, the remaining effluent, which by this time is relatively clear in appearance, is drained to a series of underground perforated pipes which act as a soakaway. The effluent then seeps into the ground. Each settlement chamber requires a vent to the open air to allow sewer gases to dissipate. ď&#x201A;¨ Modern pre-formed septic tanks In recent years it has been common practice to use a pre-formed unit which is installed below ground level and connected to inlet and outlet drains as for the traditional type of septic tank. The processes which occur within the unit are exactly the same, since the chambers are formed within the unit. This type of septic tank has the advantage that it is quicker and easier to install. A typical installation can be completed within a day.

Whichever type of septic tank is in use, it is important to ensure that the system operates efficiently. If the system becomes over-loaded by an excess volume of effluent, this will pass through too quickly and will not be fully broken down. This may cause pollution of the surrounding sub-soil, and will probably result in solids being washed into the soakaway pipes, causing an obstruction to build up. It is therefore necessary to ensure that the system is of sufficient capacity to cope with the expected volume of effluent from the property. Storm water must not be discharged to the foul drains since the effluent will be diluted and greatly increased in volume such that the treatment process will be impaired. The use of bleaches and other household cleaners which will be discharged with the effluent will tend to harm the bacteria which break down the effluent, and this will also result in solids passing through to the soakaway pipes. Special detergents are available which are suitable for use with septic tank installations. Over a period of time, which will vary according to the volume of effluent treated, the sludge in the settlement chambers will build up to the point where the effective volume of the installation is significantly reduced. At this stage the chambers must be pumped out by a local contractor who specialises in such work. The sludge is taken away for disposal elsewhere.

engaged to empty the chambers can usually arrange for the soakaway pipes to be cleaned with high pressure water jets. If the effluent is seen to be below the normal level this might indicate that there is a leak from the chamber. This is potentially very serious since significant pollution of the surrounding ground may occur. Leaks are most likely in the case of a very old installation. If the pipes within the chambers are broken, the surface of the effluent will be disturbed, and this will hinder the normal treatment process. This will lead to the problems described above. Any required repairs should be carried out by suitably qualified contractors.

Cesspools In some cases there is insufficient space for soakaway pipes adjoining a septic tank, or the sub-soil is not sufficiently porous. In such cases a cesspool can be used. A cesspool (also commonly referred to as a cesspit) is simply a large underground chamber which collects effluent discharged by the foul drains (Photo 3). Older chambers may be built of brick or concrete, and more modern installations are usually pre-formed. When the chamber is full it must be emptied by a specialist contractor. The contents are taken away for disposal. In many cases the cesspool will need to be emptied at intervals of around 14 days, but this will vary with the volume of the chamber and the quantity of effluent discharged from the property. If the cesspool becomes too full there is a risk of pollution of the surrounding area.

It is prudent to inspect the installation at regular intervals in order to identify a need for attention at an early stage. If the level of effluent within the chambers is above the normal level this may be an indication that the soakaway pipes are not draining effluent away at an adequate rate.

(Continued on page 10)

The pipes may be obstructed, or the local water table (the level of water within the sub-soil) may be raised such that effluent cannot be readily absorbed into the ground. The contractors Photo 3 â&#x20AC;&#x201C; A cess pit

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(Continued from page 9)

Treatment plants A further alternative is a self-contained treatment plant. This form of private drainage is becoming increasingly common because the effluent is recognised as being cleaner than that discharged from a septic tank. When the site is tight and/or there is more than one house involved they have become the preferred private drainage solution.

If a private drainage system is located on an adjoining site, or is shared with one or more other properties the extent of liability for repairs and maintenance will need to be established. Your client’s Legal Advisers will be able to assist in this regard. Original information sheet prepared by Terry Wallace, Chartered Surveyor, with additional contributions to the article from Alasdair Beal of Thomasons, Civil Engineer, Structural Engineer &Expert Witness.

Photo 4 – A modern package treatment plant.

In simple terms a motorised mechanism assists in the treatment of the effluent, which is discharged to a soakaway drain at the end of the process. The Klargester Biodisc and the Conder Clereflo ASP are both typical examples of this type of installation. This type of treatment plant requires regular maintenance. Further information and guidance can be obtained from the manufacturers or by searching for their maintenance manuals on line.

Health and safety It is important to ensure that only authorised persons are able to gain access to private drainage installations. Ideally the access covers should be secured in order to exclude adventurous children. Damaged covers must be replaced without delay to minimise the risk of persons falling into the chambers.

Limitations of inspections? One of your fellow surveyors sent in this photo. The Triffid attacking this house had all but obscured any chance of inspection. Have you been confronted with a similar limitation during one of your inspections or other challenging situation while conducting a survey? We’d like to see your photos. For your chance to win a credit toward one of our training courses enter our Photo Competition. The best entry will be selected by our panel of judges. Email your photo to: bulletins@nesltd.co.uk.

Send your feedback to bulletins@nesltd.co.uk; back copies of all bulletins and an Index are available in the NES one Useful Documents section. Registration Services: 01908 442 277 registration@nesltd.co.uk Compliance: 01908 442288 compliance@nesltd.co.uk SAVA, The National Energy Centre Davy Avenue, Milton Keynes, MK5 8NA Web: www.nesltd.co.uk

Technical Support Helpdesk: 01908 442105 support@nesltd.co.uk Training enquiries: 01908 442254 training@nesltd.co.uk NES one Credit Top up line: 01908 442299

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