Premature decay of external softwood joinery in the UK reached serious proportions in ca. 1960; as a result, requirements for preservative pretreatment have been progressively introduced. To extend the useful service life of joinery installed either untreated or with minimum treatment, two methods of applying preservative in situ have been investigated. Organic solvent-based preservatives, applied under pressure, gave complete protection to the more vulnerable sapwood close to the joint. Treatment of heartwood was less effective. Distribution of boron from solid preservative plugs was dependent on moisture content but largely independent of heartwood, sapwood, or preservative plug type. At ca. 50% moisture content, decay was prevented in sapwood for ca. 90 mm along the grain but for only 40 mm at 30% moisture content under the test conditions. Greater protection was afforded to heartwood.