LOSP (Light Organic Solvent Preservative)

LOSP stands for Light Organic Solvent Preservative. It is a timber-preservation system that delivers fungicide and insecticide actives in a white-spirits or other light hydrocarbon solvent carrier, rather than in water (as in CCA, ACQ, or copper azole systems). The solvent evaporates after treatment, leaving the actives in the wood without the swelling, raised grain, or surface staining that water-borne treatments can cause.

Where LOSP is used in residential construction:

H2 LOSP: above-ground, inside structures protected from weather. Treated pine framing for internal walls in non-termite-prone areas, ceiling battens, dry-side joinery.
H3 LOSP: above-ground, exposed to weather but not in ground contact. Eaves, soffit framing, weatherboard cladding, exposed pergola purlins, fascia and barge boards. H3 LOSP is the standard treatment for above-ground external pine framing in Australian residential construction.
Not used for H4, H5, H6 (ground-contact or in-water applications). Those use ACQ, copper azole, or CCA water-borne treatments.

Why builders prefer LOSP over water-borne treatments for some applications:

Cleaner appearance: no green or blue colour cast (CCA is green-tinted; ACQ varies). LOSP-treated pine looks closer to natural pine, important for visible structural timber and paint-grade joinery.
Dimensionally stable: no swelling-then-shrinkage cycle during treatment. The piece comes out the same dimension it went in. Critical for tight-tolerance joinery.
Paintable and gluable earlier: LOSP solvent evaporates within hours to a few days. Water-borne treatments need to be re-dried (kiln or air) to acceptable moisture content before paint or adhesive bonds reliably.

Limitations of LOSP:

Solvent VOCs at treatment plant: the treatment process releases volatile organic compounds; regulated by EPA in each state.
Flammability at the treatment plant during processing.
Penetration depth is lower than water-borne systems on dense or wet stock; LOSP is best suited to sapwood-dominant pine (Radiata, Slash, Caribbean).
No ground-contact ratings: LOSP cannot achieve H4/H5/H6 because the actives at LOSP loadings cannot resist soil-borne fungi and termites at those exposure classes.

Identifying LOSP-treated timber. The grade stamp on the timber identifies both the hazard class (H1-H6) and the treatment chemical: look for LOSP, TIMBOR LOSP, Tanalith ENVELOPE, PERMAPINE LOSP, or similar mill-specific markings. AS 1604.1 (or AS 1604.5 for laminated timber) sets the labelling rules.

For builders. Two practical points:

Match the treatment to the application. Internal LOSP H2 used in an external H3 application will rot within years. Read the stamp at delivery.
Smell test at unload. Freshly-treated LOSP timber has a distinct solvent smell. Off-gassing in a hot site shed is a fire and inhalation hazard; ventilate and let the stock breathe outside before bringing it indoors.

Also known as: Light Organic Solvent Preservative, solvent-based timber treatment.

Category: Materials / timber / treatment.

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