In designing roof trusses employing metal plate fasteners it is generally assumed that the roof will remain dry in service. Whilst this is generally true, damp conditions do arise under some circumstances, although it is not possible to quantify the extent of the risk. It is only possible, therefore, to give general recommendations which must be interpreted in the light of local experience of service conditions and site practice. Under severe conditions in the presence of moisture and salts, zinc coatings may not afford long-term protection against the electrochemical processes described above, and suitable precautions must be taken to minimise degradation. Moisture control is the most effective way of avoiding corrosion and if this can be achieved at all stages of manufacture, storage, and use, no diffculties should be encountered. If CCA treatments are employed, time must be allowed for the 'fixation' reactions to be completed before the plates are pressed into the timber. In cases where the assembly may become wet organic solvent preservative treatments are to be preferred in conjunction with metal fasteners. Those formulations which contain water-repellents will give added protection. Salt based flame retardant treatments should not be used in conjunction with fasteners, as the potential corrosion risk is too high. Connector plates manufactured from stainless steel may be considered for use in high-hazard situations. Finally, it has been shown at PRL that the use of nonconducting plastic coatings on the plates confers very considerable durability on these products, which may then be used successfully over prolonged periods under very adverse conditions. Such an approach may be well worth consideration in certain situations and further information on this method can be obtained from Princes Risborough Laboratory. This paper has been produced with the co-operation of the Agrement board, the International Truss Plate Association and the British Wood Preserving Association.

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