FUNGAL DEGRADATION OF WOOD IN BUILDINGS

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1 FUNGAL DEGRADATION OF WOOD IN BUILDINGS Lina Nunes, Helena Cruz Laboratório Nacional de Engenharia Civil, Portugal Abstract Timber has long played a role in buildings both as a load-bearing and as a non load-bearing material. Nevertheless, the specification, the design, the application and the maintenance of timber elements are seldom carried out with the care needed to guarantee their required performance and durability. This paper describes some conservation problems frequently observed in buildings, related to fungal degradation of timber elements and discusses their origin and some basic procedures to be followed in order to minimize these problems. 1. Introduction Strength and durability are essential properties required from timber in buildings and under ideal conditions timber can be in use for centuries without significant strength loss and biological deterioration. However, if conditions are adequate for their growth, fungi and insects (wood-boring beetles and termites) may be responsible for a quick and severe timber degradation in service. Although significant problems can be attributed to action of wood-boring beetles on dry wood, the most serious situations of timber degradation in buildings are generally related to the presence of moisture in buildings and the consequent attack on the wood by termites, when present in the country, and fungi. This paper deals only with the most common conservation problems frequently observed in buildings related to fungal degradation of timber elements. 2. Fungi Fungal attack in buildings starts with the spores of the fungus landing on timber surfaces. If the moisture content of the timber is adequate, these spores germinate and the degrading process is established. NUNES & CRUZ, Fungal degradation of wood, 1/9

Dickinson and Levy (1) identified the different stages of the successional colonization of timber: (1) Bacteria, that are able to colonize timber within a few hours of exposure, though the rate of

simpler than cellulose; (3) Wood-rotting fungi, from three main groups, one that produces the type of decay know as soft rot (ascomycetes and fungi imperfecti) and two groups of basidiomycetes, which

2 Dickinson and Levy (1) identified the different stages of the successional colonization of timber: (1) Bacteria, that are able to colonize timber within a few hours of exposure, though the rate of attack is very slow and does not produce in most cases significant degradation; (2) Molds and sapstain fungi, that penetrate wood through open pits and cell lumina and use sugars and carbon compounds simpler than cellulose; (3) Wood-rotting fungi, from three main groups, one that produces the type of decay know as soft rot (ascomycetes and fungi imperfecti) and two groups of basidiomycetes, which produce the types of decay known as white rot and brown rot; (4) Secondary molds, that use the breakdown products as a source of nutrients. In practical terms and taking into account the results of their action, two types of fungi can be distinguished: wood-destroying fungi and wood-disfiguring fungi. The wood-disfiguring fungi (sapstain and molds) normally have no significant effect on the mechanical resistance of timber as they mainly affect the aesthetic appearance of the material (figure 1), without destruction of the wood cell wall. They have, nevertheless, great importance not only when attacking freshly felled timber but also when causing extensive degradation of timber and decorative coatings, for instance in joinery and cladding. Figure 1: Sapstain in green timber Figure 2: Dry rot. On the other hand, wood destroying fungi that are enzymatically able to produce rot, significantly affect the mechanical resistance of timber. These fungi include the Basidiomycete wood rotting fungi responsible for brown or white rot and the soft rot fungi, which are grouped together on the basis of their ability to form cavities in the wood cell wall, leading to surface softening of the timber and eventually severe rot at depth. Brown rots cause the wood to become darker in colour, and to crack along and across the grain. White rots cause the wood to become lighter in colour and fibrous in texture without crosscracking (2). The most common wood-rotter found in buildings is the dry rot fungus Serpula lacrymans (figure 2), which is a particular species of brown rot that requires low moisture content of the timber (about 22%) to initiate the attack. NUNES & CRUZ, Fungal degradation of wood, 2/9

3 The dry rot fungus is particularly significant in construction for its ability to spread through wall materials, which means that successful remedial treatments may require special and possibly expensive measures. 3. Natural Durability Natural durability of timber, defined as the inherent resistance of wood to attack by wooddestroying organisms (3), varies significantly from timber species to timber species and, within the same species, it is greater in the heartwood than in the sapwood (4). For practical purposes, sapwood is always regarded as having low natural durability against fungi and termites and most classifications available, namely the European Standard EN 350, Part 2 (5), refer only to the heartwood natural durability. The standard mentioned classifies the natural durability of a wood species to the various agents of biological attack within a five grade scale for fungal attack, a two grade scale for dry wood destroying beetles and a three grade scale for termites and marine organisms. The durability class should always be considered together with the treatability class of a given species, as a wood of a given durability which has low moisture absorbing characteristics will, because of reduced water uptake, generally last longer in out-of-ground contact situations. Table 1 gives some examples of the durability classification for fungal attack and treatability of a selected group of timber species. Table 1: Durability and treatability characteristics of selected wood species Timber species Durability classification for fungal attack Treatability heartwood sapwood Picea abies (norway spruce) (1) Pinus pinaster (maritime pine) Pinus sylvestris (scots pine) Cryptomeria japonica Afzelia bipidensis Castanea sativa (chestnut) Robinia pseudoacacia Quercus robur (european oak) Quercus rubra (american red oak) (1) There is no distinct differentiation between heartwood and sapwood. Durability class: 1=very durable; 2=durable; 3=moderately durable; 4=slightly durable; 5=not durable Treatability class: 1=easy to treat; 2=moderately easy to treat; 3=difficult to treat; 4=extremely difficult to treat. 4. Moisture content of timber Excess moisture, caused by rising and penetrating damp, condensation, leakage or spillage, is the most widespread and damaging cause of deterioration and decay affecting buildings in general and particularly old historic constructions (6). NUNES & CRUZ, Fungal degradation of wood, 3/9

4 Many factors, such as the nature of the substrate and the presence or absence of a preservative treatment, influence microbial invasion, colonization and consequent deterioration of wood. However, the main factor remains the requirement of micro-organisms for water (7). There is no precise moisture level at which fungi will attack timber, since the moisture threshold for decay can vary depending on the timber type and fungal species. There are, nevertheless, three points that should be considered when dealing with fungal decay and timber (8): Decay only occurs in damp timbers; Fungal decay is possible for measured moisture contents above 22% but rapid decay is likely only at 26% and above; For practical purposes, a safety margin of 2% should be given to account for possible inaccuracies in readings and inaccessibility of some wood components. Therefore, timber is considered safe from fungal decay if the measured moisture content is 20% or below. 5. Hazard classes As previously stated, the performance of a certain wood species will vary a lot depending on its natural durability and on the conditions of the timber in service, particularly the moisture content and the biological agents present. In Europe, a Standard (EN 335) was developed taking these factors into account. The first part of this Standard (9) defines five hazard classes, that include all possible in-service situations, the second and third parts of the Standard define the application of the system to solid wood (10) and wood-based panels (11). Table 2 presents the defined hazard classes and a summary of the service conditions and biological agents relevant for each class. Table 2: Solid wood. Hazard classes and general service situations (12). Hazard classes Moisture content of the General service Occurrence of biological agents in hazard classes (1) wood situations Fungi Beetles Termites Marine borers 1 20% Above ground, covered (dry) e.g.: pavements 2 Occasionally > 20% Above ground, covered (risk of wetting) Frequently >20% 4 Permanently >20% e.g.: timber structures Above ground, not covered e.g.: window joinery In contact with ground or fresh water e.g.: bridges Permanently >20% In salt water e.g.: marine piers (1) Referring to the situation in Portugal; Level of risk: +++ Very high; ++ High; + Low; - None NUNES & CRUZ, Fungal degradation of wood, 4/9

5 6. Case studies As stated above, biological attack is a major cause of premature failure of wood structures, especially associated to high moisture content levels. Remedial actions are always required, in order to prevent damage progression or propagation to contiguous timber elements and to return the building component to its intended performance and durability. It should be born in mind that favourable conditions for wood to decay generally result from inadequate choice (specification) of timber species or timber treatment, poor design, or incorrect application. In these cases, the whole problem has to be re-addressed in order to have a better adjustment between the material durability and the risk of biological attack one or the other should be changed. It may also happen that the actual service conditions for the timber and the corresponding hazard class are different than expected for some period during the building life, due to lack of maintenance, or accidental conditions. Frequently wood decay is just another sign of a sick building where excessive moisture is able to build up. A decrease of moisture in the building materials should therefore be attained (by identifying and solving the origin of water ingress) prior to any other action. Replacing defective water pipe systems, improving the roof and its water drainage, repairing exterior walls and windows and increasing ventilation of all timber members should be amongst the required corrective measures. A different problem is the evaluation of biological attack effects on timber. Particularly in the case of fungal attack, the modification of the wood cell structure may be deep and heavy and a significant decrease in the mechanical properties may have already occurred even with little loss of mass. Fungal attack on timber frequently causes local but severe deterioration. For this reason, partial replacement or strengthening are generally necessary. Timbers which are damp but show little or no decay, and which are still fulfilling their structural function, may be retained, provided that they can be dried out sufficiently rapidly to prevent further decay (8). If the drying process is slow, then the application of a wood preservative is recommended. Wood rot may generally be identified from the following symptoms (2). Loss in strength, softening or disintegration of the wood (probing with a sharp tool essential), splinters break off very short when raised by levering with probe; A hollow sound or similar change in note from larger members when struck with a hammer; Discoloration of the wood, often in patches or streaks; Fungal mycelium, strands or fruit-bodies; Mushroom -like smell; Presence of certain wood-boring insects that only colonise decayed wood. The following cases exemplify typical situations of wood rot in relation to common hazard classes for buildings. NUNES & CRUZ, Fungal degradation of wood, 5/9

6.1. Ground floor pavement structure (Hazard class 2) Non-durable untreated timber beams are frequently used in floors.

attain moisture contents in excess of 20

Roof timber structures (Hazard class 2) Roof structures are often hidden and their maintenance neglected until rain water gets through and reaches living spaces below (figure 4).

6 6.1. Ground floor pavement structure (Hazard class 2) Non-durable untreated timber beams are frequently used in floors. However, in the case of ground floors with permanent or seasonal high water level, where suitable water-proof barriers and ventilation of the inner space were not built in, timber will occasionally attain moisture contents in excess of 20% and extensive damage can occur, for instance by dry rot, as shown in figure 3. Figure 3: Severely damaged floor beams 6.2. Roof timber structures (Hazard class 2) Roof structures are often hidden and their maintenance neglected until rain water gets through and reaches living spaces below (figure 4). Some frequent defects include missing, broken or displaced tiles (figure 5) as a consequence of ageing, storms or human access for repairs; in addition, water-proofing treatment of roof singularities, such as chimneys and plane intersections, is difficult to accomplish, and the ageing of the materials used reduce its effectiveness. A common origin of problems lies in the roof rain water drainage system, which may be inadequate, obstructed with tree leaves or bird remains, causing water to overflow and infiltrate. Figure 4: Water leakage through defective roof Figure 5: Missing roof tiles In order to improve the thermal insulation in winter, ventilation is often reduced and leaking water stays in contact with the timber, causing moisture content to build up. In extreme situations, lack of maintenance may lead the timber to be directly exposed to rain (figure 5). If the situation persists, the timber will reach a higher hazard class that the one considered for its specification. NUNES & CRUZ, Fungal degradation of wood, 6/9

A similar situation of the actual hazard class becoming higher than the one considered at the time of building construction may result from inadequate refurbishment.

In spite of its now being considered to be in a higher hazard class than before, additional protection was not considered and fungal attack occurred.

7 A similar situation of the actual hazard class becoming higher than the one considered at the time of building construction may result from inadequate refurbishment. In the case of figure 6, a lintel previously encased in the wall masonry had its mortar cover removed for aesthetic reasons, thus becoming fully exposed to the weather. In spite of its now being considered to be in a higher hazard class than before, additional protection was not considered and fungal attack occurred. Figure 6: Lintel fully exposed to weather conditions 6.3. Joinery (Hazard class 3) Window frames are to be considered typical examples of hazard class 3, although well designed and correctly maintained, window frames may very seldom reach such high moisture content levels. Most frequent reasons for fungal degradation of wood windows are their poor design and bad fabrication leading to water retention, as well as inadequate choice of coating products or incorrect painting operations (forgetting to paint the non-visible surfaces - the ones that generally stay in contact with and absorb the encased water), associated with the choice of nondurable timber species. Figure 7: Decayed window frame. NUNES & CRUZ, Fungal degradation of wood, 7/9

8 7. Conclusions Most serious situations of timber degradation in buildings are generally related to the presence of moisture and the consequent attack on the wood by termites (in infested regions) and fungi. Therefore adequate performance and durability of wood in buildings can only be achieved if a proper choice of timber species or timber treatment is made, together with sound design and its correct application. The overall behaviour of the building and the construction procedure/process must be understood in order to avoid inaccurate biological risk assessment along its life. Besides, it should be born in mind that timber comprehends an enormous variety of mechanical properties and natural durability and its choice and commercialisation should be taken seriously by everyone involved. Moreover, building owners and users must learn that nothing is forever, especially a building left to its own. Maintenance is essential: it should be planned beforehand and carried out frequently and any sign of defective behaviour should be promptly attended. Although timber will be amongst the first building materials to show the results of neglecting, wood decay should be regarded as just another sign of a sick building where excessive moisture was able to build up. 8. Acknowledgements The authors acknowledge the financial support of the FCT Fundação para a Ciência e Tecnologia within the project Metodologias para a Mitigação do Risco Associado à Degradação das Construções. 9. References 1. Dickinson, D J and Levy, J F 'Timber and Forest Products', Encyclopedia of Microbiology, (4) (1992) Bravery, A. F., Berry, R. W., Carey, J. K. and Cooper, D. E., 'Recognising wood rot and insect damage in buildings' (BRE, Garston, Watford, 1992). 3. EN350-1 'Durability of wood and wood-based products. Natural durability of solid wood. Part 1: Guide to the principles of testing and classification of the natural durability of wood' (European Committee for Standardization, Brussels, 1994). 4. Nunes, L. and Sousa, P. P. de, 'Durability-Preservative treatment'. in 'Timber Engineering. STEP 1. Lecture A15' (STEP/EUROFORTECH, Centrum Hout, The Netherlands, 1995) 8pp. 5. EN350-2 'Durability of wood and wood-based products. Natural durability of solid wood. Part 2: Guide to natural durability and treatability of selected wood species of importance in Europe' (European Committee for Standardization, Brussels, 1994). 6. Watt, D. and Swallow, P. 'Surveying historic buildings' (Donhead Publishing Ltd. Donhead St. Mary, Shaftesbury, Dorset, 1996). 7. Eaton, R A and Hale, M. D. C., 'Wood. Decay, pests and protection' (Chapman & Hall, London, 1993). 8. Berry, R W 'Remedial treatment of wood rot and insect attack in buildings' (Building Research Establishment, Garston, Watford, 1994). NUNES & CRUZ, Fungal degradation of wood, 8/9

9 9. EN335-1 'Wood and wood-based products. Definition of hazard classes of biological attack. Part 1: General' (European Committee for Standardization, Brussels, 1992). 10. EN335-2 'Wood and wood-based products. Definition of hazard classes of biological attack. Part 2: Application to solid wood' (European Committee for Standardization,, Brussels, 1992). 11. EN335-3 'Wood and wood-based products. Definition of hazard classes of biological attack. Part 3: Application to wood-based panels' (European Committee for Standardization, Brussels, 1995). 12. Machado, J S; Cruz, H; Nunes, L and Monteiro, G 'Especificação de madeiras para estruturas', 'Madeira para Construção M1' (LNEC, Lisboa, 1997). NUNES & CRUZ, Fungal degradation of wood, 9/9

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