Preservation ~ treatment of timber to prevent damage from House
Longhorn beetle.In the areas specified (see previous page), all softwood used in roof
structures including ceiling joists and any other softwood fixings
should be treated with insecticide prior to installation. Specific
chemicals and processes have not been listed in the Building
Regulations since the 1976 issue. Timber treatment then was either:- Vacuum/pressure impregnation with a blend of copper,
chromium and arsenic (CCA), known commercially as `tanalising'. - Diffusion with sodium borate (boron salts).
- Steeping (min. 10 mins.) in organic solvent wood preservative.
- Steeping or soaking in tar oil (creosote). This has limitations
due to staining of adjacent surfaces.
The current edition of Approved Document A (Structure) to the
Building Regulations refers to guidance on preservative treatments
in the British Wood Preserving and Damp-Proofing Association's
Manual. Other guidance is provided in:BS 1282: Wood preservatives. Guidance on choices, use and
application.BS 5707: Specification for preparation for wood preservatives in
organic solvents.BS 8417: Preservation of timber. Recommendations.Insect treatment adds about 10% to the cost of timber and also
enhances its resistance to moisture. Other parts of the structure,
e.g. floors and partitions are less exposed to woodworm damage
as they are enclosed. Also, there is a suggestion that if these
areas received treated timber, the toxic fumes could be harmful to
the health of building occupants. Current requirements for through
ventilation in roofs has the added benefit of discouraging wood
boring insects, as they prefer draught-free damp areas.Note: EU directive CEN/TC 38 prohibits the use of CCA
preservatives for domestic applications and in places where the
public may be in contact with it.Timber Roofs---Preservation