Why Specification Matters
A timber treatment specification in a schedule of works does two things: it defines the minimum standard the contractor must meet, and it creates a contractual record that can be used to verify compliance before concealing finishes are applied. Without a specification, contractors choose the cheapest option — which is often no treatment at all, or a surface application that provides no meaningful protection.
The cost of writing a correct timber treatment specification is essentially zero — a few lines in the schedule of works. The cost of omitting it and then discovering termite damage after plastering and painting are complete is measured in tens of thousands of rupees minimum, and in structural risk in the worst cases.
What a Complete Timber Treatment Specification Covers
A complete specification for structural timber treatment should cover: the elements to be treated (with no ambiguity about what is included and what is excluded), the treatment method (VPI with Boron Borax — not surface application), the minimum retention level required for the application and exposure class, the requirement for kiln drying to a specified MC before VPI treatment, the requirement for treatment by an IPPC-registered facility, and the documentation required for compliance verification.
Elements that are frequently omitted and should always be included: door and window sub-frames (not just the visible frame), timber embedded in or within 300mm of masonry, any timber at or near floor level in the roof space, and any structural timber in contact with concrete.
- Define every element to be treated — frames, sub-frames, structural members, embedded timber
- Specify VPI with Boron Borax — not surface treatment
- State the minimum retention level (typically 2.5–4.0 kg/m³ for above-ground interior use)
- Require kiln drying to 15–18% MC before VPI
- Require treatment by an IPPC-registered facility
- Require batch treatment records submitted before plastering or concealment
Documentation to Require Before Concealment
The most important procedural requirement is to withhold approval for plastering, ceiling board installation, and any other finishes that conceal structural timber until treatment records have been submitted and verified. Once the timber is hidden, it cannot be inspected without destructive investigation — and if the treatment was not done correctly, there is no practical remedy short of opening up the finishes.
The treatment record should confirm: the name and IPPC registration number of the treatment facility, the date of treatment, the timber species and cross-section dimensions, the pre-treatment moisture content, the VPI cycle parameters (vacuum level and duration, pressure level and duration), the preservative concentration, and the calculated retention figure. A reputable facility will provide all of this as a standard document.
Common Specification Mistakes
The most common mistake is specifying "treated timber" without defining what treatment means. This leaves contractors free to substitute a surface-applied preservative for VPI, which appears to meet the specification letter while providing a fraction of the protection.
The second mistake is specifying treatment without requiring documentation. A contractor who knows that no records are required has no incentive to use a certified facility or to confirm that treatment was actually carried out. Treatment records should be a contractual requirement with a defined submission deadline before relevant works proceed.
The third mistake is not specifying kiln drying as a prerequisite for VPI. VPI treatment on green timber gives substantially lower retention than treatment on kiln-dried timber. If the specification requires VPI treatment without specifying pre-drying, the contractor can have timber treated at a lower effective retention level and still claim compliance.
Sample Specification Clause
The following clause can be adapted for inclusion in a schedule of works: "All structural timber — including roof rafters, purlins, ceiling joists, wall plates, floor joists, door and window frames and sub-frames, and all timber within 300mm of masonry — 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, carried out by an IPPC-registered treatment facility. Treatment batch records confirming species, cross-section, pre-treatment moisture content, VPI cycle parameters, and achieved retention shall be submitted to the architect or project manager before any concealing finishes are applied."
St. Xavier Timber provides VPI treatment with Boron Borax and issues full batch records for every order. We work with architects, quantity surveyors, and main contractors across Sri Lanka and can turn most orders around in 3–5 working days. Contact us to discuss your project timeline and treatment requirements.