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Sheet S-131
PPDPDD CE

Woodconcept

Wood treatment and preservatives

One-line orientation

Preservatives fall into three families — oil-borne, waterborne copper, and borate — each with a distinct limitation; the copper-corrodes-fasteners rule and “borate is above-grade only” are the tested points.

Key points

  • Oil-borne (can’t be painted):
    • Creosote — historic; utility poles, railroad ties, marine; long-lasting but toxic, smelly, being phased out; oily, so can’t be painted.
    • Pentachlorophenol — chlorinated pesticide/disinfectant; effective but restricted/phased out for health/environmental risk; oil-based, can’t be painted.
  • Waterborne copper (corrosive to other metals):
    • CCA (chromated copper arsenate) — excellent protection but contains arsenic; phased out (residential, ~2004).
    • ACQ (alkaline copper quaternary) — arsenic-free; more corrosive to aluminum and steel fasteners than CCA → requires special hardware.
    • CBA / copper azole — similar to ACQ, arsenic-free; also corrosive to aluminum/steel.
    • Micronized copper and carbon-based PTI — newer, less corrosive alternatives.
    • Rule: most copper preservatives are corrosive to other metals → use stainless steel or hot-dip galvanized fasteners/connectors.
  • Borate (above-ground, not for exterior):
    • DOT (disodium octaborate tetrahydrate) and SBX (sodium borate) — low toxicity, broad pest protection, but water-soluble → leach when wet. Limited to above-ground; not for exterior or ground contact unless sealed.
  • Where pressure treatment is required:
    • Wood joists < 18″ above grade.
    • Beams/girders < 12″ above grade.
    • Plates, sills, or sleepers in direct contact with masonry/concrete on the earth.
    • Framing or sheathing < 8″ from soil.

Wood preservatives: finish? · wet or ground?

Screen by two questions: can the surface take a finish, and can the treatment tolerate wet or ground contact?

View diagram Hide diagram 3 panels

3 panels side by side — swipe to compare

Wood-preservative families by paintability and ground-contact suitability Three equal preservative-family cards answer the same two questions on shared row baselines: can the surface take paint or finish, and can the treatment tolerate wet or ground exposure? Each card is crested by a drawing of what the family is. Oil-borne creosote or pentachlorophenol, crested by a solid oil drop, is not paintable but suits industrial exposure. Waterborne copper ACQ or CBA, crested by a clear water drop carrying copper particles, passes both screens but corrodes aluminum and standard steel, so use stainless or hot-dip galvanized fasteners. Borate DOT or SBX, crested by a mineral salt crystal cluster, is paintable but limited to protected above-grade use because it leaches when wet. Oil-bornecreosote, pentapaint / finishwet / groundindustrial exposuresurface cannot be painted Waterborne copperACQ, CBApaint / finishwet / groundcorrodes aluminum + steeluse stainless / hot-dip galvanized BorateDOT, SBXpaint / finishwet / groundprotected above gradeleaches when wet

The corrosion rule is qualified, not flat: micronized copper and PTI formulations are less corrosive to fasteners than the older waterborne coppers.

Confusions / comparison

FamilyExamplesKey limitationWhere used
Oil-borneCreosote, pentachlorophenolCan’t be painted; toxicIndustrial — poles, ties, marine
Waterborne copperACQ, CBA, (CCA phased out)Corrodes Al & standard steelExterior / ground contact w/ right hardware
BorateDOT, SBXLeaches if wetInterior / above-grade only

→ Wood fasteners and connectors (this topic): why treated wood demands hot-dip galvanized/stainless hardware · Wood moisture and seasoning (this topic): the decay/moisture risk treatment addresses · Wood structure and shrinkage (this topic): naturally decay- and termite-resistant species as an alternative.

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