Timber Degradation Preservatives

1. Timber Degradation & Preservatives Week 10

2. Durability of Timber Timber comes under attack from a number of sources: Biological Fungi Insects Fire

3. Longevity If protected from fire, insect & fungal attack, timber structures can survive for extremely long periods (in excess of 2000 years)

4. Natural Durability Good resistance to atmospheric exposure Virtually unaffected by rain, frost etc. Sunlight can degrade timber if exposure is prolonged � breaks down lignin adhesive � appears bleached & fibrous Mainly attacked for food � fungi & insects � enzymes �digest� the cellulose fibres and/or the lignin adhesive

5. Photochemical Degradation Exposure to sunlight causes a change in colouration - heartwood lightens (mahogany, oak) � some darken (teak) Exterior expose is the most severe & in a few months �weathering� will take place Light, rain & wind all contribute to the weathering process � silver grey appearance

6. Cont�d Produces loss of surface integrity due to the breakdown of lignin under the action of ultraviolet light Further exposure will cause the shortening of the chain length in the cellulose � erosion of the cell wall results Timber becomes brittle & resistance to load is reduced as the damaged lignin cannot fully transfer the stress

7. Cont�d The surface damage builds to protect & filters the UV light � slows the effects of weathering � slow process � 1mm/20 years The application of surface protection is recommended � weathered surface must be cleaned prior to treatment

8. Chemical Degradation Generally timber is highly resistant to various chemicals Timber is more resistant to mild acids than cast iron or mild steel Timber has lower resistance to alkalis � dissolves lignin & hemicelluloses

9. Cont�d Iron salts are acidic in the presence of moisture & leads to hydrolytic degradation of the timber � softening & discolouration in the area of iron fastenings Corrosion of metal fittings on boats causes chemical decay of the timber � �nail sickness� � electrochemical effect controlled by the availability of oxygen

10. Thermal Degradation Timber heated to 120�C for a period of about one month will experience a loss in strength of about 10% - small increases in temperature above this value will accelerate the process Browning of the timber takes place indicating the thermal damage & a caramel like odour can be detected � degradation of the hemicelluloses � continued exposure will affect the cellulose

11. Mechanical Degradation Timber is stressed under load for long periods � creep Duration of load, creep & the associated loss of strength with time � 50 years of loading � strength approximately 50% Designers apply time modification factors

12. Cont�d Compression failure � can occur naturally due to the formation of �kinks� in the cell walls under high compressive stress or as brittle heart due to growth stresses in the centre of the trunk Service conditions can induce over stressing of the cell walls due to longitudinal compression Results in reduced tensile strength & a major loss of toughness

13. Fungal Attack Directly linked to the natural durability of the timber The resistance can be explained by the make-up of the cell wall & the deposition of extractives The lignin offers some degree of protection against fungal attack Requires a moisture content of at least 20%

14. Cont�d Durability of heartwood varies according to species & is dependant on the type & quantity of extractives Sapwood of all timbers is susceptible to attack due to the absence of extractives & the presence of rays which store starch � this acts as food for the potential attacking fungus

15. Dry Rot Dry rot � Serpula lacrymans � a brown rot Leaves wood in a dry friable condition Affects areas without good ventilation Rust red spores come into contact with damp timber

16. Cont�d Fungus develops as branching white strands (hyphae) Form cotton-wool like patches (mycelium) Finally forms soft-fleshy spore producing fruiting bodies (sporophore)

17. Cont�d Grow rapidly once established Modify into vein like structures (rhizomorphs) 2 to 3mm in diameter Killed at temperatures above 40�C Can lie dormant at temperatures approaching freezing or if timber dries out Prolonged dry periods over 1 year may cause it to die off

18. Wet Rot Wet rot � Require higher moisture contents in order to develop Optimum value 50% - various types of wet rot � the following types are common in UK:

19. Cont�d Coniophora puteana � cellular fungus very damp situations � basements brown � cube formation large timber sections � skin of sound timber may be present � surface undulations Cut out & burn affected areas � remove affected plaster � use preservatives � rectify dampness

20. Cont�d Phellinus contiguus � common in window joinery white rot � wood breaks into soft strands � inadequate glue, poor design etc. water penetration � fungal growth � can occur in combination with insect attack (weevils) Cut out affected timber � replace with new timber � use of preservative � remove cause of dampness

21. Life Cycle of a Fungus