HI Technical Bulletin For registered members of the SAVA Certification Scheme
Issue 3 May 2008
This month: Cavity wall tie failure Effect of flood water on chemical injection moisture proof courses Interpretation of moisture meter readings – a cautionary tale. Do you agree with the Inspector? Monitoring cracks
Welcome to Issue 3 of SAVA’s Home Inspector Technical Bulletin The bulletin focuses on Home Condition Reports 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.
Cavity wall tie failure Cavity wall construction consisting of outer and inner leaves of stretcherbond brickwork with a 50mm cavity between tied together with steel wall ties has been in use since the mid-l9th century. It did not become common practice until the house building boom of the 1930s and since World War II it has become universal practice. Practitioners should, however, take care when looking at properties built before the 1930s. While it is true that nationwide cavity walls were not common before this time, they were not unknown. The concept of cavity walls was introduced in response to the Public Health Act of 1875 and the prevention of damp into buildings. It is therefore possible that much earlier examples of cavity walls will exist from the early 1900s and expensive negligence claims have arisen where older properties have been wrongly identified as solid wall. It is dangerous to assume that because a property was built during the first couple of decades of the century that it will automatically be of solid wall construction. Early ties, which were of the vertical twist type, were often protected with a coat of bituminous paint. From about 1930 the ties were galvanised which gave them better protection, but even this is considered substandard by today‘s standards. Any property built prior to 1981, when the standard was improved, could be subject to premature corrosion of wall ties. When steel corrodes it will expand up to ten times the thickness of the parent metal. The ends of the wall
ties embedded in the outer leaf will lift the brick course above causing a horizontal crack in the mortar bed joint. Where this affects the outer leaf for its full height, it can ‗grow‘ by up to 75mm, lifting the roof and the wall plate and causing internal cracking at the wall/ceiling junction. Some buildings are more at risk than others and the factors affecting the degree of corrosion are as follows: Use of inferior coatings, especially bitumen paint, to protect the steel. Use of substandard thickness of zinc galvanising on mild steel ties. The use of aggressive mortars, especially black ash mortars, which contain sulphates. The use of permeable mortars in the outer leaf. The proximity to salt laden winds from the sea. The degree of exposure. The age of the property. Commonly, it is the wall ties at high level and on the most exposed wall that are worse affected. The visible evidence of wall tie failure includes the following: Horizontal cracks at about 450mm (18‖) centres particularly at high level. These can be difficult to see from ground level,
HI Technical Bulletin but a close inspection of the first-floor window reveals will often reveal the problem.
public footpath when partial collapse of the outer leaf may occur due to wind suction in gale conditions (Photo 2).
Cracks at the junction of the ceiling with the first floor external walls where the expansion of the brickwork has lifted the wall plate.
If on examination it is found that a substantial proportion of the wall ties are corroded, consideration must be given to a replacement programme. This will normally consist of drilling holes through the outer leaf and into the inner leaf at specified centres and inserting stainless steel ties with either an expansion or a resin fixing depending on the construction of the wall.
Bulging of the outer leaf where the wall ties have corroded right through leaving the outer leaf detached. This can be detected again at the window reveals by gaps between the outer leaf and the window frame (Photo 1).
“Cavity wall tie failure in domestic properties is rarely an immediate threat to the
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For final corroborative evidence the cavity should be inspected by using a boroscope, which is an optical device for inspecting cavities, inserted through a 12mm diameter hole drilled in the mortar joint (Photos 3 and 4 next page). There are specialist firms who will carry out these surveys free of charge in the hope of obtaining a contract for the wall tie replacement. It must be stressed that wall tie failure in domestic properties is rarely an immediate threat to the safety of the occupants.
After the replacement ties have been fixed the original ties must be removed, bent back or isolated to prevent continuing rust expansion (Figure 1 below). R M HIGGINS BSc, C Eng, FlStructE, MConsE References: BRE Information Paper 1P28/79 ―Corrosion of wall ties : recognition, assessment and appropriate action‖ BRE Information Paper 1P13/90 ―Corrosion of steel wall ties - recognition and inspection‖ BRE Digest No 329 ―Installing wall ties in existing construction‖
However, there may be a hazard to pedestrians where complete failure of wall ties has occurred on gable ends adjacent to a
safety of the occupants.”
Photo 2: Detachment of outer leaf due to wind suction.
A cut away picture (right) showing the stages of remedial treatments. From left to right: A rusted tie. A tie cleaned for isolation A tie with an isolation sleeve. A new expanding tie installed.
Figure 1: Installation of replacement wall ties
Photo 1: Detachment of outer leaf
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Cavity wall tie failure cont...
Photos 3 and 4: Use of a boroscope to inspect the cavity
“Damp proof
The Effect of Flood Water on Chemical Injection Damp Proof Courses Following last years floods which affected many parts of the country, it is likely that many residential surveyors will come across properties with flood damage. It is important to understand how to recognise and understand flood damage as opposed to DPC failure. Chemical injection dpcs (damp proof courses) have been available since 1980 and many of the older properties with solid walls have had this type of damp proof course, often as a requirement of the building society when purchasing the property. The process consists of drilling the external walls on both sides at approximately 100mm (4‖) centres and injecting a water repellent liquid (Photo right). The liquid used is silicon or aluminium stearate based. They are either injected under pressure or, more often, transfused under gravity. The presence of plugged holes will indicate that an injected DPC has been installed (Photo right). Because the wall affected by rising damp will contain hygroscopic salts which attract moisture from the atmosphere, the original plaster is removed to a point above the highest level that the damp has risen to and is replaced with a water resistant render which will act as a barrier against any residual salts and moisture in the wall. The flood water should have little effect on this type of dpc and any existing guarantees should remain valid. However, this should be checked with the company who installed it originally if known.
flood water could contain hygroscopic salts leached from the ground and these could be deposited and possibly infused into the render and plaster skim coat. Samples of both should be sent to a public analyst for analysis to determine the presence of such hygroscopic salts. If found to be present, both the plaster and render should be stripped off up to 750mm above the tide and this wall rerendered after drying out. If the wall is saturated right through it could take up to two years for it to fully dry out. Obviously policy holders are reluctant to wait that length of time before they can re-decorate so a value judgement has to be made to allow the walls to be re-rendered once the surface is sufficiently dry. The moisture then is allowed to evaporate to the outside. As a precautionary measure it is normal to specify pressure treated timber for skirtings etc that will be in direct contact with the wall to eliminate the risk of dry rot breaking out. R.M. HIGGINS, BSc, CEng, FIStructE References www.environment-agency.gov.uk www.floodprotetcionassociation.org www.ciria.org.uk/flooding/ www.defra.gov.uk/environ/fcd/ default.htm
courses are inserted by drilling the external walls on both sides and injecting a water repellent liquid.”
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Interpretation of Moisture Meter Readings – a Cautionary Tale Why should Inspectors be so concerned about the presence in walls of excess levels of damp? A damp environment is not particularly healthy, particularly for those who suffer from bronchial complaints, and gypsum type finishes will react adversely to excess damp creating aesthetic problems.
The absence of any circular mortar marks underneath the windows and elsewhere told me that post construction insulation had not been introduced, so this ruled out any possibility that cavity insulation might be responsible for cross wall penetration.
In addition, moisture is needed to support ‗life‘ and combined with the food source inherent in timber (plant material) can create the right conditions for insect and fungal attack. Over time such attacks can be very costly to remedy. This case study is provided by a practising surveyor and home inspector
“A moisture meter is an essential item in an inspector’s tool case.”
I was asked to confirm the findings of another surveyor who had found ―extensive damp‖ in a property. They had reported that high moisture readings had been obtained at varying levels on both external and internal dividing walls in a 1960‘s semi-detached bungalow, part of an estate on the fringes of a former coalfield town in the Midlands. Bearing in mind my local knowledge of house construction in this area, I was perplexed as to why this particular dwelling appeared to be so badly affected.
Protimeter_MMS_capacitance mode - Moisture Meter
No two properties will ever provide you with identical readings mainly because the hygroscopic qualities of wall finishes may vary considerably. Room use and the lifestyle of the occupants are further factors that can influence meter readings obtained, so I find it essential to start with a base line or ambient reading to which all subsequent readings may be compared. I started my inspection in the living room by taking a control reading in capacitance mode (I was anxious not to leave resistance mode pin marks on an otherwise pristine wall surface) on the internal, dividing wall about 2 metres above floor level using a Protimeter MMS. When I tapped the wall, the dull thud told me that this appeared to be a solid wall with an applied plaster finish. The meter reading of 178 was accompanied by slow bleeps and the message ―At risk‖. Ever mindful that I might, by chance, be scanning over a section of concealed metal conduit, I moved my meter some 300mm diagonally across the wall, but the reading was repeated. When this action was repeated at skirting level and then subsequently elsewhere on other walls all the readings obtained were between 168 and 175.
Protimeter_MMS_Kit17405 - Moisture Meter
An initial external inspection of the property provided some essential evidence. The stretcher brick bonding clearly indicated an external cavity wall which included a bitumen-felt dpc.
Now common sense, a most essential quality for any inspector whatever their level of experience, should tell you that neither rising nor penetrating damp would provide such a consistent pattern of readings. Furthermore, I could see that there was no evidence of any salt stains or efflorescence on the wall finishes, and there was no smell
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Interpretation of Moisture Meter Readings of damp anywhere in the property. Therefore I decided to test the softwood skirting using the pins, which revealed a reading of 15% wood moisture equivalent, comfortably below the level at which softwood is at risk from fungal attack. The timber was ―dry‖ whereas just 50mm above the skirting the meter readings were telling me that the wall was ―wet‖. The applied plaster finish on the wall ruled out any possibility that my meter was being ―led astray‖ by the presence of metal in the form of foil-backed plasterboard. Taking into account the lack of any salt stains or efflorescence on the wall finishes, the absence of damp smells and reflecting on my previous experience of properties in this area, by now I was fairly sure of the actual levels of damp present. So what was responsible for the high readings which had convinced the previous surveyor that ―extensive damp‖ was present in the walls?
Now we all know, or should know, that we should not attempt to undertake any survey without such an essential aid as a moisture meter in our work case. But notice that I use the word ‗aid‘, since it is only the foolhardy that would consider it to be anything more than that. Interpretation of the readings, considered in conjunction with other evidence, is essential to determine the presence or otherwise of a defect. Author: Keith Layton After considering the article, Ralph F. Burkinshaw, who is a SAVA trainer, gave us the following feedback: ―An interesting case. I'd like to know whether all skirtings in the property registered 15% using an MMS in pin probe mode - as such a reading is not usually indicative of a 'dry' skirting! So there could be an underlying damp problem. Taking a short length of skirting off would progress the investigation - although I appreciate this might need to be undertaken as part of a 'further investigation'. I doubt if the capacitance meter of an MMS could pick up carbon in block work if solid plastered. What is the plaster mix? This is where you need to double check with the pins. If you can't use your damp meter pins on walls - either walk away from the investigation or make sure it is flagged up in the report as well as the implications. An interesting article and hopefully it will fuel debate!‖
Protimeter_MMS_with probes - Moisture Meter
As I returned to my car the explanation was found in a wheel-barrow. Alteration works were being carried out to the adjoining property and a barrow load of dark, slate grey coloured blocks with plaster coatings was being deposited in the builder‘s skip. Confirmation of my suspicions was possible and after a quick word with the barrow man the pins of my meter were firmly pressed into a block – the rapid bleeps from the meter and the a reading of 34.5% wme told me that the blocks contained a coal based clinker aggregate. It was the high carbon content in the clinker that the meter was detecting, and the fact that all the readings were similar reflected the uniformity of the wall construction. The moral of this story is ―Don‘t believe every thing that you read‖!
Home Inspectors wishing to increase their knowledge of damp related matters will find ―Diagnosing Damp‖ by Ralph F. Burkinshaw of considerable assistance. Ralph is an independent chartered surveyor specialising in damp diagnosis. rf.burkinshaw2@ukonline.co.uk We aim to make this bulletin relevant to your work as HI. Have you come across any issues that you would like us to feature as a case study in one of our forthcoming issues? Please get in touch with your suggestions and ideas at bulletins@nesltd.co.uk.
―Interpretation of the readings, considered in conjunction with other evidence, is essential to determine the presence or otherwise of a defect.”
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Do you agree with the Inspector? This is a new series which will be focussing on the assignment of Condition Ratings for the HCR Our first case study was submitted by Steve Pierpoint, DipHI Do you agree? - We need your feedback! Please contact the Editor with your views and opinions at bulletins@nesltd.co.uk. Dangerous/broken switch to Cupboard of front
My condition rating takes the following
bedroom 1:
path, using the SAVA protocol:
There was a damaged switch to the cupboard of the
A— A hazard is present i.e. there is a
larger of the front bedrooms (bedroom 1).
risk of electrocution B— The scale of remedy is low i.e. replacing switch. C—The level of risk is high i.e. the hole is big enough for fingers to enter and possible electrocution to occur so a CR3 from a risk point of view. J—A repair is required i.e. replacement of switch. L—The repairs are minor issue.
“Do you agree
M— This path delivers a CR1 rating from a repairs point of view.
with the
So a CR3 is reported and because there
Inspector? -
were no other defects found to the elec-
Send us our
trical system this is separated out in the
views and
final report.
opinions.”
Monitoring Cracks In a previous article on settlement and subsidence (see HI Technical Bulletin No 1), reference was made to monitoring cracks.
A history of the movement can often be determined in this way. A close inspection and investigation can reveal more evidence.
Before monitoring is implemented, much evidence can be determined just by carrying out a detailed inspection and using your experience as an Inspector.
Internal cracks, walls papered
Questioning the owner/occupier may elicit some useful information, e.g. for internal cracks, when was the room last decorated? When was the crack first noticed? With external cracks, has any re-pointing been carried out? Was the crack there when the house was purchased? Was it mentioned in a previous surveyor‘s report? Has it increased more recently?
The paper may not be torn but slightly wrinkled or evidence of a crack may be noted by running one‘s finger across the suspect area to determine any irregularities in the plaster below (Photo 1). With the owners permission take a sharp penknife, make two cuts across the crack and peel back a thin strip of wall paper to reveal the plastered wall. Is there evidence of previous filling of an old crack and subsequent opening up? Has the owner papered over the crack?
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is more (Photo 3)
likely
to
be
longstanding?
The visual inspection may not give all the information required or it may need confirmation to determine whether the movement is active and continuing or whether it is historic and static. There are a number of methods which can assist in determining this depending on the accuracy and time scale involved. Although Home Inspectors would not normally be involved in crack monitoring, it is useful to be aware of the methods used in the context of insurance claims for structural movement, etc. Photo 1: crack has torn wallpaper
Is the minor tear in the paper revealing a more serious condition in the wall? Replace the wall paper using a glue stick, which you should always keep handy in your pocket.
“The visual
Internal walls, unpapered and emulsion painted
inspection may
Is there a clean break in the plaster surface? Or is there evidence of paint within the crack indicating it was last painted after the crack had appeared? (A hand magnifying glass is another useful piece of pocket equipment.) Is there any evidence that the crack had been filled prior to the last coat of paint with subsequent reopening? (Photo 2)
not always determine whether Photo 3: historic crack following bedjoints perpends
active and
The simplest and quickest method is to fill a short length of the crack with some proprietary crack filler and return after an appropriate time to check whether the filler has opened up. Where a more accurate method is required, telltales comprising small steel discs with a dimple in the centre can be used. The distance between them can be measured with calipers, and the full extent of the movement can then be recorded over the period of investigation. Alternatively, plastic telltales can be purchased from Avongard, and again calipers used to record any movement (Photo 4).
Photo 2: crack filled and re-cracked. Note dimpled disk telltales
External walls in brick or block Does the crack follow the mortar joints or are bricks or blocks cracked? Is there any evidence of recent re-pointing and subsequent reopening? Is it a clean-edged relatively recent crack, or does it have eroded edges or is it filled with dirt/dust, indicating it
movement is
Photo 4: telltales
Continues next page...
continuing or old and static.�
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Monitoring Cracks cont... If, as is often the case, the crack is in the corner of a room, Avongard also supply plastic telltales at right angles (Photo 5). These monitoring are often recommended by loss adjusters and are also normally acceptable to insurance companies if the owner intends to make an insurance claim. R.M Higgins, BSc, Ceng, FIStructE.
Photo 5: plastic telltales at right angles
References: Cracking in Buildings K B Bonshor and L Bonshor BRE. Building Failures, Diagnosis and Avoidance. W H Ransom. E & N Spon.
We were recently saddened to hear of the death of Rodney Higgins, regular contributor to the Home Inspector Technical Bulletin. Rodney‘s articles were written and refined over the course of a long and distinguished career, incorporating structural engineering, building defect analysis and a full range of energy and condition surveys. We are grateful for his help in getting the HI Technical Bulletin up and running in such a smooth manner. Our thoughts are with Rodney‘s family at this time.
We hope you found this technical bulletin useful. We would like to get SAVA The National Energy Centre Davy Avenue Milton Keynes MK5 8NA Email: bulletins@nesltd.co.uk Web: www.sava.org.uk
your feedback on this edition—good or bad. Are there any areas you would like to see covered in future editions. Are there any particular problems that keep coming up? Drop us a line at: bulletins@nesltd.co.uk Registration Services & Compliance 01908 540605 info@sava-cs.org.uk Technical Support Helpdesk 01908 442105 support@nesltd.co.uk Candidate Advisors 01908 442240 Training enquiries 01908 442254 NES One Credit Top up line: 01908 547941 To download electronic copies of the bulletins, login to NES one and go to ‗Useful Documents‘ All original content © 2008 National Energy Services Ltd