Difficulties with Underpinning

From A Pre-Purchase Inspector’s Perspective

A paper presented to the Queensland Master Builders Association, Institute of Building Inspectors on 27 February 2001. (Abridged version).

Author - T D Betteridge

Brief

Provide a paper outlining areas of concern regarding building movement and potential for building movement to a group of people who are predominantly technically qualified and carry out pre-purchase building inspections.

Concentrate mainly on residential construction.

Disclaimer

Terry Betteridge & Co are licensed as General Builders, House Builders, Residential Building Designers and Completed Building Inspectors.

Terry Betteridge & Co do NOT carry out pre-purchase inspections. We are often requested to provide quotations to rectify defects including cracked brickwork and building movement identified in previous pre-purchase inspection reports.

We provide that service for a fee to prospective purchasers.

Outline

  • Reference documents.
  • Written material.
  • Overhead transparencies.
  • 35mm colour slides.
  • Oral discussion of the above.

History

The writer first experienced underpinning as an Apprentice Carpenter during approximately 1957.

Underpinning work was carried out to ensure stability of adjoining buildings during excavation for construction of new commercial buildings.

Underpinning at that time was just hard work that Building Apprentices were expected to carry out obviously under experienced supervision.

During 1978 Terry Betteridge & Co became involved in underpinning in Queensland.
Work was initially carried out on residential buildings through what was then the Builder's Registration Board and now the Queensland Building Services Authority.

Engineer's Reports under those circumstances at that time were voluminous, with high costs and often written with a view to potential legal action.

Work was usually carried out within the time limitations of the relevant legislation.
A large stock of housing exists in Queensland and elsewhere which because of the housing age is not within the time jurisdiction of the Queensland Building Services Authority.
Owners of those dwellings often observe building movement and require cost effective solutions.

Underpinning is only one of several possible alternatives.

Building movement is often first identified during a pre-purchase inspection to the surprise of the building owner and obvious concern of the prospective purchaser.

Many pre-purchase inspection reports identify cracks and building movement.

Some - hopefully a minority - make statements such as "cracks to the brickwork are the result of footing movement and will require underpinning".

Inspections - Tell Tale Signs

  • Cracks to brickwork also internal and external wall and ceiling linings.
  • Out of level or plumb.
  • Settlement of concrete floors.
  • Settlement of timber floors.
  • Previous repairs internal and external.
  • Differential movement between different building materials.
  • Lack of control joints.
  • Non-compliance with Codes i.e. Australian Standard AS2870.
    • Trees
    • Site Maintenance - drainage etc.
    • Leaking services, hot water systems etc.
  • Location of manholes.
  • Sewer, Stormwater and other service trenches.
  • General topography. i.e.
    • Cut and fill site
    • Low lying
    • Hollow or infilled gully/creek
    • Reactive clay sites.
  • Excavation below footings under hi-set homes.
  • Step back - Take a general overview of the property and topography.
  • Study and become familiar with areas of reactive clay.

Code Compliance

Does the property being inspected comply with relevant Australian Standard Codes?
Main code with reference to potential for building movement is Australian Code AS2870 which was first published during 1986.

CSIRO has published a companion document sheet number 10-91 revised August 1996 title Guide to Homeowners on Foundation Maintenance and Footing Performance (updated for AS2870/1996) this document is the current updated version of similar previous documents which have been available for many years.

Queensland Master Builders Association (QMBA) produced a similar document dated July 1987 with the title An Introduction to Foundation Footings and Slabs and Homeowner's Guide to Reactive Clay Sites.

Brick Development Research Institute Published Design Note 1C in August 1977 Permanent Expansion of Clay Bricks - A Chronological Summary.

Cement and Concrete Association of Australia produced a document titled Concrete Information Technical Note 61 Articulated Walling with the comment "this document replaces Articulated Walling (CN9) published in October 1984".

It is a reference document in AS2870.1 Residential Slabs and Footings/Part 1: Construction.

Most Engineers' site investigation reports include reference to some of the above documents.

Most Engineer's reports will also include comments regarding articulation, site drainage, distance of trees from proposed construction and the requirement to maintain a constant soil moisture content around the entire footprint of the dwelling.

All the above documentation is readily available, is written in plain English and has been available for many years.

Information contained in the above and similar documentation is so readily available as to be considered common knowledge certainly within the Building Industry.

Most homeowners have no knowledge of the contents of the above documents.

Most express surprise, some express disbelief and refuse to except they must remove the beautiful trees which they sometimes state is the main reason they bought the property.

Installation of cutoff walls to isolate the dwelling from the effects of tree roots is an expensive process with obviously detrimental effects to existing landscaping.

Building Movement - Cracks

Significant building movement can occur very quickly.

Resulting cracks and building damage can be severe.

We have received enquiries from homeowners who are adamant that cracks have occurred very quickly, sometimes overnight. A number of people have stated they heard a loud bang during the night causing doors and windows to rattle.

Subsequent inspection by the owners has located cracks which in some cases are 20 to 25mm in width.

We have many instances where people advise their dwellings previously had no sign of movement or cracks with significant cracks developing from 0 to 20 to 25mm in less than two weeks.

Excluding obvious extraneous events such as burst water mains, impact damage, inundation by water and similar events the main cause of sudden movement is usually shrinkage of reactive clay soils. Foundation soil shrinks forming an increasing void below one or more parts of a concrete footing.

Concrete footings cantilever over that void and can no longer support that cantilever resulting in sudden failure.

Concrete footings often move downwards more slowly with shrinkage of reactive clay foundation soil.

Many homes were constructed before the existence of AS2870.

Concrete footings and slabs of those dwellings will in most cases not comply with the design and construction requirements of AS2870.

Site maintenance, articulation and moisture control all remain relevant.

Repair Options

Repair options are many and varied. Consideration of all aspects of any particular project must be given due consideration.

Building owners will ultimately make the decision on the repair option which best suits their requirements and budget.

Owners of existing buildings suffering foundation movement may often be perfectly satisfied with a repair option which requires some on going maintenance and moisture control.

A prospective purchaser may have an entirely different opinion. Due care and skill is needed to ensure repair options and the consequences of those repairs are clearly understood by the person who will be paying for those repairs.

Water Penetration

This item does not fall under the general heading of this paper, however given the level of enquiry and concern on this item I decided to make some reference to the matter.

Domestic dwellings in broad terms can have areas of habitable construction (lounge room, bedrooms etc) which must be waterproof and non-habitable construction (store rooms, garages etc) which is not required to be waterproof.

Many building owners and potential purchases are not aware that non-habitable areas are not required to be waterproof.

A common problem relates to a typical highset brick veneer construction on a sloping site. Lower level is non-habitable type construction.

Single skin external brick walls.

Non-structural concrete floor.

Concrete floor to underside of floor joists generally less than 2400mm.

Many homes containing non-habitable construction to the lower level are utilised as storerooms, bedrooms, studies, living rooms etc which is all quite normal.

Single skin brickwork is not waterproof. Internal floor below external ground level will in most cases result in water penetration to the lower level during or shortly after heavy rain.

Endeavouring to waterproof lower level non-habitable construction is difficult and expensive.

Inconvenience and expense rapidly escalates when the internal floor is below the external ground.

The question usually is not IF water will penetrate but WHEN.

We regularly receive enquiries from homeowners who have recently purchased a dwelling and are concerned with water penetration into areas of non-habitable construction and especially so when those areas are being used as habitable rooms. Many believe the Building Inspector who carried out their pre-purchase inspection should have told them of that fact, i.e. non-habitable construction will allow water penetration.

Conclusion

Be clear, be aware, ensure your written report conveys your intended message in language which can be clearly understood by your client.

Do not assume your client has any building knowledge or any prior understanding of the matters raised in this paper.

Be careful when making recommendations for remedial work in relation to building movement.

Ensure you can substantiate any remedial recommendations you may make.
Should you have any uncertainty or concerns, recommend that your client should engage the services of someone with appropriate experience and qualifications sufficient to advise your client of potential remedies and costs.

Keep your early warning system well tuned.

Use all your technical knowledge and experience all the time.

Take nothing for granted.

Should you be uncertain about any item you discover during a building inspection be prepared to say so in writing, in your report.

Do not provide verbal comment which is different to or not substantiated in your written report.

Should you wish to discuss any aspect of the material or information covered by this seminar, please feel free to contact me:

Office 07 3376 2633
Mobile 0408 747 871

Terry Betteridge

     
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