What a Complete Roof Timber Specification Covers
A roof timber specification is not just a list of sizes. A complete specification covers: the species, the minimum moisture content at installation, the treatment method and retention level, the structural grade, the documentation required for compliance, and any dimensional or processing requirements specific to the design. Leaving any of these elements unspecified passes the decision to the contractor — who will choose the option that minimises their cost, which may not be the option that minimises your client's risk.
The elements below cover what each part of the specification should include and why it matters.
Species Specification
Acceptable species for structural roof timber in Sri Lanka include pine (radiata or European softwood), mahogany (Swietenia or Khaya), and teak for premium applications. Rubberwood is not appropriate for large-span structural roof members — its moderate strength values and susceptibility to pests without treatment make it a second choice for structural applications where pine or mahogany can be specified.
If the design allows for any compliant species, specify a minimum density and grading standard rather than a specific species — this allows competitive procurement while still controlling the minimum structural performance. If a specific species is required (for appearance, for heritage reasons, or for a specific structural calculation), name it explicitly in the specification.
Moisture Content at Installation
All structural roof timber should be installed at 15–18% MC. Timber installed above 20% MC will dry in place — shrinking, moving, and loosening fixings as it does. Purlins that were tightly connected to rafters at installation may have significant gaps at the joints after one dry season. Wall plates that were a snug fit on the masonry may pull away from their bearing surface.
Specifying MC at installation, and requiring the supplier to provide MC measurement data as part of the delivery documentation, is the most effective way to prevent post-installation movement. Do not accept delivery of structural timber without verifying the MC at the point of receipt.
Treatment Specification
All structural roof timber — rafters, purlins, wall plates, ridge boards, hip and valley boards, ceiling joists, and any timber within 300mm of masonry — should be VPI treated with Boron Borax to a minimum retention of 3.0 kg/m³, from an IPPC-registered treatment facility. Treatment records (species, cross-section, pre-treatment MC, VPI cycle log, retention figure, date, facility registration) must be submitted before ceiling installation.
The specification should additionally require that any timber cut on site after delivery be immediately flood-coated with a concentrated borate solution on all exposed end grain surfaces. This partial protection does not substitute for factory VPI treatment but significantly reduces the vulnerability of site-cut ends.
- All rafters, purlins, ridge boards, hip and valley boards: VPI treated
- All wall plates and ceiling joists: VPI treated with DPC membrane under wall plates
- Any timber within 300mm of masonry: VPI treated
- Minimum retention: 3.0 kg/m³ for above-ground interior structural use
- Treatment records required before ceiling installation proceeds
- Site-cut end grain to be flood-coated with borate solution immediately after cutting
Structural Grade
For structural timber imported with a grading mark, specify the minimum structural grade required for each member based on the structural calculations — C16 or C24 for European softwoods, MGP10 or MGP12 for Australian/New Zealand pine. For locally sourced or ungraded timber, specify visual grading to a stated standard with limits on knot area ratio, slope of grain, and warp.
If no structural calculation has been prepared (for simple residential roofs within standard span limits), specify timber to minimum structural grade with a requirement that the contractor provide a structural engineer's confirmation that the member sizes are adequate for the span and loading.
Connector and Fixing Specification
Fixings in treated timber should be specified as hot-dip galvanised or stainless steel. Boron Borax treatment does not corrode mild steel fixings under dry interior conditions, but any moisture at the fixing interface can cause corrosion of unprotected steel, which stains the timber and weakens the connection over time. For coastal environments, stainless steel fixings are the correct specification.
Nail plates, joist hangers, and truss connectors should all be specified as hot-dip galvanised as a minimum for interior structural applications, and stainless steel for any exposed or coastal application.
St. Xavier Timber supplies kiln-dried, VPI-treated structural timber in pine, mahogany, and rubberwood, cut to your specified dimensions and marked with treatment batch information. Contact us to discuss your roof structure requirements and we will provide a quote with turnaround time.