Why Getting the Specification Right Matters
The purpose of a timber treatment specification is to define a minimum performance standard that the contractor must meet, and to create a contractual mechanism for verifying compliance before treated members are concealed. A specification that is vague — "use treated timber" — achieves neither. A contractor reading "use treated timber" may reasonably interpret this as any surface-applied preservative, which provides a fraction of the protection of VPI treatment at similar cost.
A correctly written specification names the treatment method, the minimum retention level, the documentation required, and the compliance verification point. It eliminates ambiguity and gives the contract administrator the tools to enforce the standard.
The Four Elements of a Complete Treatment Specification
A complete timber treatment specification has four elements. First, the scope — which elements are covered. This should list every type of timber member that requires treatment, with no room for a contractor to argue that a specific element was not included. Second, the treatment method — which must specify VPI with Boron Borax, not surface application. Third, the minimum retention level — the amount of preservative that must be present in the wood, expressed in kg/m³. Fourth, the documentation requirements — what records must be submitted, by when, and to whom.
Scope: What Must Be Included
The minimum scope for a typical Sri Lankan residential building should include: all roof structural timber (rafters, purlins, ceiling joists, ridge boards, hip and valley boards, wall plates), all timber within 300mm of masonry (including wall plates, lintels, sill plates, and embedded beam ends), all door and window frames and sub-frames, and any timber in a sub-floor or enclosed, unventilated space.
Elements frequently omitted from specifications — and therefore frequently left untreated — include door and window sub-frames, architraves, and timber built into walls or floors. A specification that covers "structural timber" but not joinery is half a specification.
Sample Specification Clauses
The following clauses can be adapted for inclusion in a schedule of works or bill of quantities.
"All structural roof timber including rafters, purlins, ceiling joists, ridge boards, hip and valley boards, and wall plates shall be kiln-dried to 15–18% MC and Vacuum Pressure Impregnated with Boron Borax preservative to a minimum retention of 3.0 kg/m³, by an IPPC-registered treatment facility. Batch treatment records shall be submitted to the architect before ceiling installation proceeds."
"All door and window frames, sub-frames, and sill members shall be kiln-dried to 12–15% MC and VPI treated with Boron Borax to a minimum retention of 2.5 kg/m³. End grain exposed by site cutting shall be flood-coated with concentrated borate solution immediately after cutting. A damp-proof membrane shall be installed between all frame back faces and masonry before fixing."
Compliance Verification and Hold Points
The most important procedural element is the hold point — the stage in the construction programme at which treatment records must be submitted before subsequent work can proceed. Two hold points are essential: treatment records for structural roof timber before ceiling installation, and treatment records for framing and joinery before plastering.
At each hold point, the contract administrator should verify: the treatment facility name and IPPC registration number, the batch date and timber description, the species and cross-section dimensions, the VPI cycle parameters, and the achieved retention figure. Batches that do not meet the specified retention should not be accepted.
St. Xavier Timber provides standard-format treatment records that include all information required for compliance verification at the hold points described above. We are available to advise architects and quantity surveyors on appropriate specifications for specific applications. Contact us to discuss your project.