Timber preservation (natural solid timber)

3.3.2Durability

Timber component groups and preservative treatment required are shown in Table 1 below (based on BS 8417), which provides information to establish the appropriate type of treatment according to the particular element and conditions of use. Table 2 provides information on the timber species and durability.

Table 6: Timber component groups and preservative treatment

Notes
1.Preservative treatment of timber should be in accordance with the recommendations of BS 8417, Table 4.
2.Preservative treatment of timber should be in accordance with the recommendations of BS 8417, Table 5.
3.Almost always, packs of timber contain sapwood. It should be assumed that timber is sapwood and preservative treated accordingly unless the timber has been specifically selected as heartwood only.
4.Natural durability classes are given in Table 2.
5.Sole plates should be positioned above DPC. Preservatives used should be resistant to leaching or, for boron, treatment should be to full cross-section retention standard. Treatment should be carried out in accordance with BS 8417.
6.The hardwoods known as Meranti, Seraya or Lauan should be treated in the same way as European redwood / Scots Pine when used for joinery.
7.The pressure treatment process used for these types of preservative will cause timber to swell, so these treatments are generally not used for window or door frames and other uses where dimensional precision is required.
8.Decking that is more than 600mm in height should have a desired service life of 60 years. Reference should be made to Chapters 7.1 ‘Flat roofs and balconies’ and 10.2 ‘Drives, paths and landscaping’.
9.Where timber structures more than 600mm high are used for retaining ground in boundary situations, they should be designed with a desired service life of 60 years. Reference should be made to Chapter 10.2 ‘Drives, paths and landscaping’.

Table 2: Natural durability of building timbers (heartwood only)