BS-5268-5-1989.pdf

BRITISH STANDARD BS 5268-5: 1989 Structural use of timber Part 5: Code of practice for the preservative treatment of structural timber UDC 624.011.1:691.11:674.038.5:674.048 Licensed Copy: sheffieldun sheffieldun, na, Mon Dec 04 16:11:19 GMT+00:00 2006, Uncontrolled Copy, (c) BSI BS 5268-5:1989 This British Standard, having been prepared under the direction of the Civil Engineering and Building Structures Standards Policy Committee, was published under the authority of the Board of BSI and comes into effect on 29 September 1989 © BSI 01-1999 First published August 1984 First revision September 1989 The following BSI references relate to the work on this standard: Committee reference CSB/32 Draft for comment 87/11450 DC ISBN 0 580 17373 9 Committees responsible for this British Standard The preparation of this British Standard was entrusted by the Civil Engineering and Building Structures Standards Policy Committee (CSB/-) to Technical Committee CSB/32 upon which the following bodies were represented: British Woodworking Federation Chartered Institute of Building Department of the Environment Building Research Establishment Princes Risborough Laboratory Department of the Environment (Housing and Construction) Department of the Environment (Property Services Agency) Greater London Council Incorporated Association of Architects and Surveyors Institute of Clerks of Works of Great Britain Inc. Institute of Wood Science Institution of Civil Engineers Institution of Structural Engineers International Truss Plate Association National Federation of Building Trades Employers National House-Building Council Royal Institute of British Architects Royal Institution of Chartered Surveyors Timber Research and Development Association Timber Trade Federation Amendments issued since publication Amd. No.Date of issueComments Licensed Copy: sheffieldun sheffieldun, na, Mon Dec 04 16:11:19 GMT+00:00 2006, Uncontrolled Copy, (c) BSI BS 5268-5:1989 © BSI 01-1999i Contents Page Committees responsible Inside front cover Foreword ii 1Scope 1 2 Definitions 1 3 The need for preservative protection 1 4 New timber in old buildings 3 5 Risk assessment 3 6 Preservative treatment of timber in various structural situations 5 7 Preservative treatment 12 8 Properties of treated timber 15 9 Safety 16 10Checking preservative treatment16 Appendix A Decision process for using preservative treatment and examples of the use of Table 4 and Table 518 Table 1 Natural durability and treatability characteristics of selected timbers 2 Table 2 Classification of risk categories related to wood-destroying organisms 4 Table 3 Classification of risk categories related to safety and economic factors 5 Table 4 Timber in buildings 7 Table 5 Timber used for structural purposes other than buildings 11 Table 6 Preservative treatments and relevant specifications 13 Table 7 Pressure treatment 14 Table 8 Immersion treatment 14 Table 9 Double vacuum treatments 14 Table 10 Approximate conversions 15 Table 11 Anticipated minimum sapwood retention of CCA treated timber 17 Table 12 Decision process for using preservative treatment 19 Publications referred to Inside back cover Licensed Copy: sheffieldun sheffieldun, na, Mon Dec 04 16:11:19 GMT+00:00 2006, Uncontrolled Copy, (c) BSI BS 5268-5:1989 ii © BSI 01-1999 Foreword This revision of BS 5268-5 has been prepared under the direction of the Civil Engineering and Building Structures Standards Policy Committee and supersedes BS 5268-5:1977 which is withdrawn. Other Parts of BS 5268 are as follows: Part 1: Limit state design, materials and workmanship (for later publication); Part 2: Code of practice for permissible stress design, materials and workmanship; Part 3: Code of practice for trussed rafter roofs; Part 4: Fire resistance of timber structures; Section 4.1; Recommendations for calculating fire resistance of timber members; Section 4.2; Recommendations for calculating fire resistance of timber stud walls and joisted floor constructions1); Part 6: Code of practice for timber frame walls; Section 6.1; Dwellings not exceeding three storeys1); Part 7: Recommendations for the calculation basis for span tables1). Recommendations on preservation of structural timber were originally given in CP 112.100:1952 which was revised as CP 98:1964. CP 98 was then superseded by BS 5268-5:1977. The recommendations for preservative treatment or the use of durable timber given in Table 4 and Table 5 have been made after taking into consideration the degree of protection required for the end-use as described. They are made in association with a level of perceived risk that such protection will be required. Considerable alterations have been made in this revision to the presentation of risk assessment so that both the likelihood of decay and its consequences may be viewed separately. This allows the former to conform to the proposed European method of classification and is adopted in this code of practice in readiness for the intended harmonization of approvals for wood preservatives and preserved wood within the EEC. Timber is used for a wide range of structural purposes and the recommendations given provide guidance on the level of treatment considered necessary in various service situations. For the purpose of this Part of BS 5268 such timber usage has been divided into two main groups: a) timber in buildings; b) timber used for other structural purposes. Although this code of practice is concerned with the structural use of timber, there are certain non-structural components in buildings, e.g. windows, external doors and door frames, which are not listed in Table 4 under the heading of components, but for which preservative treatment needs to be considered. BS 5589 gives further guidance. In this revision the specification for treatment of the timber components is in the form of treatment schedules for copper/chromium/arsenic (CCA) and organic solvent (OS) preservatives, and minimum charge loadings for creosote. The OS schedules for resistant and extremely resistant timbers are interim recommendations which may need to be revised in the light of experience. All British Wood Preserving Association (BWPA) specifications referred to in this Part of BS 5268 are the 1986 editions. Copies have been deposited with the British Standards Institution. 1) In preparation Licensed Copy: sheffieldun sheffieldun, na, Mon Dec 04 16:11:19 GMT+00:00 2006, Uncontrolled Copy, (c) BSI BS 5268-5:1989 © BSI 01-1999 iii Product certification. Users of this British Standard are advised to consider the desirability of third party certification of product conformity with this code of practice based on testing and continuing surveillance, which may be coupled with assessment of a suppliers quality systems against the appropriate Part of BS 5750. Enquiries as to the availability of third party certification schemes will be forwarded by BSI to the Association of Certification Bodies. If a third party certification scheme does not already exist, users should consider approaching an appropriate body from the list of Association members. A British Standard does not purport to include all the necessary provisions of a contract. Users of British Standards are responsible for their correct application. Compliance with a British Standard does not of itself confer immunity from legal obligations. Summary of pages This document comprises a front cover, an inside front cover, pages i to iv, pages 1 to 20, an inside back cover and a back cover. This standard has been updated (see copyright date) and may have had amendments incorporated. This will be indicated in the amendment table on the inside front cover. Licensed Copy: sheffieldun sheffieldun, na, Mon Dec 04 16:11:19 GMT+00:00 2006, Uncontrolled Copy, (c) BSI iv blank Licensed Copy: sheffieldun sheffieldun, na, Mon Dec 04 16:11:19 GMT+00:00 2006, Uncontrolled Copy, (c) BSI BS 5268-5:1989 © BSI 01-19991 1 Scope This Part of BS 5268 gives recommendations for preservative treatment of timbers to be used for structural purposes in the United Kingdom to protect them from degradation by wood-destroying organisms. It does not cover the remedial treatment of existing timber in old buildings. It does not deal with the protection of timber against fire, nor with preserving the appearance of timber against weathering or staining. NOTE 1All wood preservatives used in the UK have to be approved under The Control of Pesticides Regulations (1986). The Health and Safety Executive is the approving body. NOTE 2The titles of the publications referred to in this standard are listed on the inside back cover. 2 Definitions For the purposes of this Part of BS 5268 the definitions given in BS 4261 and BS 6100-4 apply, except that the term “decay” is used in preference to “rot”. 3 The need for preservative protection 3.1 General Timber employed for structural purposes may be exposed to a wide range of environmental conditions. The risk of fungal decay or insect attack will differ for each service situation (see 3.2), as will the consequences of failure, e.g. damage to persons or property, cost of repair, ease of remedial action or replacement. This risk can sometimes be eliminated or reduced by good design (see 3.3). Where design is unlikely to provide adequate protection, a naturally durable timber (see 3.4) or the use of an appropriate preservative treatment (see clause 6) should be considered. 3.2 Risk of degradation The conditions conducive to the attack of timber by wood-destroying organisms are described in BS 1282. However, the following key facts should be noted. a) Timber is not at risk from decay if it is kept dry, i.e. at a moisture content below about 20 %. b) The natural durability (see Table 1), i.e. resistance to decay of heartwood, varies with different timber species. The sapwood of all species is classified as either perishable or non-durable. c) Attack of sound timber by wood-boring insects is usually confined to sapwood. It can occur in both dry and damp situations. d) Only a restricted number of timber species2) are naturally resistant to marine borer attack. 3.3 Design In some instances design can reduce the risk of degradation by wood-destroying organisms. However, where reliance is placed entirely on design it is particularly important that the associated workmanship is of a high standard. Penetration of liquid water from outside the building must be prevented at all times by ensuring that external components shed water satisfactorily. Sources of moisture inside a building include damp originating from the ground, and water vapour generated through occupancy. In buildings with a high internal relative humidity, precautions have to be taken to prevent structural timber and wood based components becoming wet through condensation. This may occur in cold external walls and roofs unless suitable precautions are taken in the form of thermal insulation, vapour barriers and/or ventilation appropriate to the construction concerned (see BS 5250). In some cases it is not possible to eliminate the risk of degradation by good design and in these cases naturally durable timbers and/or preservative treatment should be used. However the treatment of timber and other wood products with preservatives should not be regarded as a substitute for appropriate design features which minimize the risk of degradation by wood-destroying organisms. Rather, it is supplementary and the guidance contained in this code of practice with regard to the advisability of treatment and the processes to be used assumes that good design, workmanship and maintenance prevail. 3.4 Natural durability If a timber component has sufficient natural resistance to decay by virtue of the natural durability of its heartwood it may be used without treatment even where a decay hazard exists. Sapwood should never be used in a hazardous location without preservative treatment. The degree of natural durability required for a range of situations is indicated in clause 6. The natural durability of a selection of commonly encountered commercial timbers is given in Table 1. Where it is not possible to use timbers which have sufficient natural durability, preservative treatment should be used. 2) See PRL Technical Note Number 59 Marine borers and method of preserving timber against their attack. Licensed Copy: sheffieldun sheffieldun, na, Mon Dec 04 16:11:19 GMT+00:00 2006, Uncontrolled Copy, (c) BSI BS 5268-5:1989 2 © BSI 01-1999 Table 1 Natural durability and treatability characteristics of selected timbersa Common namebor commercial designationc Natural durability (heartwood only)d Treatabilitye HeartwoodSapwood Balau (not including red balau)D/VDfn/an/a Beech, EuropeanPPP Cedar, western red (imported)DRR Douglas fir-larch (USA and Canada)MDR/ERn/a Elm, English and DutchNDMRP EkkiVDERMR Fir, Douglas (UK)MDRMR GreenheartVDERn/a Hem-fir (USA and Canada)NDRn/a IrokoVDERn/a JarrahVDERn/a Kapur, MalaysianVDERP KarriDERn/a KempasDRn/a Keruing, MalaysianMDRMR Larch, Dunkeld (UK)MDRMR Larch, European (UK)MDRMR Larch, Japanese (UK)MDRMR MerbauDfn/an/a Oak, American whiteDERMR Oak, EuropeanDERP OpepeVDMRn/a Pine, Caribbean pitchDMRP Pine, CorsicanNDMRP Pine, ParanaNDMRP Pine, ScotsNDMRP Radiata pine (New Zealand)NDRfP Redwood, EuropeanNDMRP Southern pine (USA)NDMRP Licensed Copy: sheffieldun sheffieldun, na, Mon Dec 04 16:11:19 GMT+00:00 2006, Uncontrolled Copy, (c) BSI BS 5268-5:1989 © BSI 01-19993 Table 1 Natural durability and treatability characteristics of selected timbersa 4 New timber in old buildings 4.1 General This Part of BS 5268 is concerned with pre-treatment of timber and does not cover the remedial treatment of existing timber in old buildings. However, timber is often employed in improvements and renovations to old premises that were built to standards different from those prevailing today. Timber to be used in previously or potentially damp situations should be treated with preservative. Provided certain safeguards that attend modern construction, e.g. damp-proof membranes, external cavity walls and adequate ventilation, have been included, the provisions of clause 6 should be sufficient. However if these features are missing, or other factors are present that are likely to lead to persistently damp conditions, then an assessment of the risk (see clause 6) may require treatment appropriate to a higher risk category to be adopted. 4.2 Bats The Wildlife and Countryside Act 1981 gives special protection to bats because of their particular requirements for roosting. Where restoration or reinstatement work is planned in buildings carrying evidence of bat occupation, the Nature Conservancy Council should be notified of this intent before work is started, especially where preservatives are used. 5 Risk assessment 5.1 General Timbers used for structural purposes should remain sound and serviceable for the duration of the intended life of the