Timber maintained at a moisture content below 20% is unlikely to be attacked by timber decaying fungi. However, the same does not apply to insect attack. Building legislation does not at present require structural timbers to be preservative treated (except those in flat roof constructions), with the exception of an area of England affected by the house long horn beetle. This area is defined in the Building Regulations. NHBC require external cladding to be treated with preservatives; this includes fascias, barge boards and timber soffits.
The various types of preservative and their application processes are set out in BS 5268: Part 5. It is not intended to enter into the detail of timber treatments. Suffice it to say that two basic types are used for constructional timbers in this country, the first being a waterborne preservative system, the second being organic solvent-borne. Their application can be by total immersion or vacuum and/or pressure depending on the type used. Brushing of these preservatives is generally not adequate to afford a high degree of protection for structural work.
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