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Pre-Purchase Building Inspections – Delignification of Roof Tile Battens and Structural Defects

Chemical Delignification of roof tile battens in WA is not uncommon and in rare cases, it may require some or all of the tile battens in a roof to be replaced. The question in relation to pre purchase building inspections is, will it ever amount to a major structural defect? The answer is likely to be no.

What is Chemical Delignification of Tile Battens?

Many people have not heard of the term “chemical delignification.” At one time, it used to be commonly referred to as defibrosis, and was also commonly known as “hairy wood.” Both of those terms described the appearance left behind by this chemical process.


Background: To understand this process, it is helpful to understand the nature of lignin, since it is this substance that chemical delignification assaults. Lignin is classified as an organic polymer that is found in the tissues of plants and certain varieties of algae. These polymers are essential for the formation of timber cellular walls, since it fills the spaces within a plant’s cell walls, providing needed rigidity. Some tree species contain more lignin than others, which is one of the reasons why different types of trees offer varying levels of rigidity.


You may have personally seen wood that has experienced delignification. The effects are noticeable even to the naked eye and include the appearance of hairiness on the exterior of the wood. This is the result of lignin being damaged, which in turn causes the timber fibres to separate from one another over a long period of time. That creates a frayed appearance that resembles hair on the timber surface. Timber that suffers from this deterioration loses its integrity over long periods of time, typically 50-100+ years, rarely less.


There are certain locations where this process is more likely to occur: around areas where salt water is common, and locations near industrial areas or heavy traffic. These areas offer everything that this chemical process needs to attack timber. Seaside regions are susceptible to delignification due to the way in which air carries salt to the timber. Those winds can carry this salt five or more kilometres into the interior of an area, where it settles down onto building tops, and eventually makes contact with tiles and roof battens. That salt can initiate the delignification process.While salt in the air might be a significant contributor, chemicals and other pollutants in the surrounding air also get carried by the wind only to eventually settle on rooftops. These chemicals can create the same type of delignification effect, causing damage to the timber.


Note: Without formal laboratory testing any loss of integrity cannot be confirmed. It is now being proposed that the chemical process can cease/stop, therefore limiting damage or deterioration. Research and testing is evolving with time. Further industry input required.


Tile Battens

Tile battens in roof frames run perpendicular to rafters. The sole purpose of the tile battens is an anchor point for clay, cement and slate roof tiles. They are classified as a secondary element (non-primary) and part of the roof cover in accordance with AS-4349 and REIWA Standards, applicable to pre-purchase building inspections in WA and are therefore deemed a non-structural component as they are not servicing the structure i.e. foundations, floors, walls and primary elements of the roof frame i.e. rafters and purlins etc. This is applicable to pre-purchase building inspections only.


- Secondary Elements means those parts of the building not providing load-bearing capacity to the Structure, or those non-essential elements which, in the main, perform a completion role around openings in Primary Elements and the building in general such as non load-bearing walls, partitions, wall linings, ceilings, chimneys, flashings, windows, glazing or doors -


Furthermore, the REIWA standards applicable to typical contracts of sale in WA also depicts roof battens to be part of the roof cover. When we combine both definitions outlined in AS-4349.1 and the REIWA standards we must conclude as pre-purchase inspectors that roof battens are not considered a structural element.


Roof tile battens are installed in sectional lengths with the provision of movement and deflection as the roof and building settles over time. No structural element is typically installed as a sacrificial non primary component. To add to this, the removal of battens does not jeopardise the integrity of the structural framing or the primary components of the building.


However, if the extent of chemical delignification is considered to be of significant magnitude, whereby it poses a safety hazard i.e. cannot walk on the roof, then the inspector will likely deem the defect a major non-structural defect and safety hazard. The ultimate question in most instances is, has the rate of delignification decreased the battens capacity to services the tiles? The answer is mostly always no. Is the extent of delignification consistent for the age, construction and location of the building? If the extent of delig. is considered to be minor (mostly surface only <0.2mm of the diameter of a batten) then the defect will likely not be considered a safety hazard or major defect, however we will always recommend further (invasive) inspection(s) to determine the true condition (when scraped) and adequacy of the battens.


After all, a standard pre-purchase building inspection is non-invasive. We will subjectively attempt to inspect and report on the overall condition of the said building elements within the limitations of the applicable standards and industry best practice.


For more information on this topic you can contact Rapid Solutions, Inspect WA (governing body for building inspectors in WA), or the Real Estate Institute of Western Australia (REIWA).


In relation to delignification of root tile battens and after inspecting thousands of roofs throughout Perth, generally the affected layer of timber is usually very thin, typically equal to or less than 0.2mm of a 20-40mm thick unseasoned batten, overall appearing much worse than it probably is. It is important to note, that as the battens are typically installed unseasoned (also known as 'green') they actually gain strength over time as the timber cures (sometimes rated as high as F17 following testing after 40 years in service).


While tile battens are critical in holding tiles firmly in place, the battens are not considered a structural element of the roof frame or the building in accordance with AS-4349.1, the only standard pre-purchase building inspectors are bound by in Western Australia.


Structural Defects

AS-4349.1 which defines a range of criteria for a major defect. Appendix A refers specifically to structural defects, and most importantly defines what needs to be excluded when considering structural defects. In particular the Standard notes:


(a) Any non-structural element, e.g., roof plumbing and roof covering,

In essence the entire roof cover is excluded from the definition of a major structural defect and by association the wooden tile battens which the tiles are attached would also fall outside of the definition of a structural element subject to structural defects. The roof batten is part of the roof cover. Roof battens are not a primary element or component in accordance with the relevant standards.


But, substantial long-term chemical delignification can still represent a significant issue to the property owner. In its very worst condition, chemical delignification of tile battens can render the batten ineffective in holding the tile correctly in position and hence a major non-structural defect. Further, significant chemical delignification of tile battens can represent a safety hazard.


From experience, this process takes >100 years to occur (after inspecting over 2000 homes throughout metropolitan Perth), sometimes less with changes in external variables.


Chemical Delignification and Major Defects

A building inspector may consider significant amounts of chemical delignification to be a major non-structural defect as defined by AS-4349.1 but not a major structural defect as the building element is not considered a structural element in accordance with AS4349.1 and the REIWA Standards.


So What Does All of This Mean for Pre-Purchase Building Inspection Reports?

It is possible that during a pre-purchase building inspection, chemical delignification of tile battens may be identified. In very extreme cases chemical delignification may need to be remediated. In some cases, excess chemical delignification might be classified as a major non-structural defect indicating a degree of urgency to remediate the issue, otherwise noted as minor defects. To gain major defect status the failure must be imminent or there is a loss of utility. The inspector will consider if the roof can be walked on or battens loaded and if the tiles are reasonably solid. All pre-purchase building inspections are completed by comparing the property to those of similar age and construction (not the house next door) and are completed subjectively in the opinion of the inspector.


Where a property is being purchased in accordance with the WA REIWA pre purchase building clause which is based on AS-4349.1 major structural defect will generally relate to the foundations/sub floor frame or concrete slab, load bearing walls and the primary elements of roof frames, also including any attached structures or external framing. The balance of the building elements are likely to be secondary non-structural elements and this includes roof tile battens regardless of personal opinion. We are simply following the applicable standard, backed by industry best practice.


(readers and home buyers are encouraged to read the Public Release for Position Paper 01.2019 by Inspect WA regarding roof tile batten delignification and defect status).


Conclusion - Where chemical delignification is identified in a residential home during a pre purchase building inspection:


For Sellers If chemical delignification in tile battens is identified in a property being sold it is unlikely to be a major structural defect requiring remediation by the seller, if the REIWA pre purchase building inspection clause has been utilised as the tile battens are not a structural element.


For Buyers If chemical delignification is identified in tile battens in a property being purchased, it is unlikely to be a major structural defect requiring remediation by the seller, if the REIWA pre purchase building inspection clause has been utilised. The buyer may choose to remediate the issue post settlement as the issue can develop into a more significant problem if not addressed long term depending on the extent of delignification, however this is only rarely encountered. Significant chemical delignification in tile battens may be classed as a major non-structural defect or a safety hazard, but either of these classifications may be insufficient to trigger the pre-purchase building inspection clause.


If you are buying an older home with a tiled roof, we recommend buyers add a special condition to their contract to protect their interests as the (2019) Standard REIWA Building Inspection Condition excludes the roof cover as structural and the relevant Standard for Pre-Purchase Building Inspections AS-4349.1 does not consider roof cover or battens to be a primary or structural element.

Video above: Depicts chemical delignification on the surface of 120-year-old roof rafters (not battens) - for general guide only. Please watch.


Note: Pre-purchase building inspections in Western Australia are not completed in accordance with the National Construction Code (NCC). Any reference to the NCC is for pictorial purposes only or in error. The governing standards is AS-4349.1 only. We do not deny that the National Construction Code (NCC) applicable to new buildings in Australia may define roof battens and their fixings contrary to the above, particularly in metal roofs. That is a seperate topic and requires industry input going forward. There is no (current) requirement in WA to complete pre-purchase building inspections in accordance with the NCC.


Perth (Western Australia)

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