Pressure-Treated Wood

Pressure-treated wood is lumber that has been chemically preserved by forcing a wood preservative deep into the fibers under high pressure in an industrial treatment plant. The treatment protects the wood from rot, fungal decay, insect damage, and moisture degradation, making it suitable for outdoor applications such as decks, fence posts, retaining walls, and ground-contact structures where untreated wood would quickly deteriorate.

In Finland, the most common treatment class for residential outdoor use is UC3 (above ground, exposed to weather) and UC4 (ground contact or freshwater contact). Higher treatment classes contain more preservative and provide longer protection. When selecting pressure-treated wood for a project, it is important to choose the correct treatment class for the intended application — deck boards (UC3) have different requirements than fence posts set in the ground (UC4).

Despite the chemical treatment, pressure-treated wood still requires regular surface maintenance. The preservative protects the interior of the wood from biological attack, but the surface is still vulnerable to UV degradation, weathering, and cracking. Without protective oil or stain, the surface grays rapidly, develops surface cracks (checking), and becomes rough. Annual or biannual treatment with deck oil extends the wood's appearance and structural life significantly.

When working with pressure-treated wood, use stainless steel or hot-dip galvanized fasteners, as the preservative chemicals can cause accelerated corrosion of standard steel hardware. Cut ends should be treated with end-grain preservative to maintain protection. Always wear gloves and a dust mask when cutting pressure-treated wood, and never burn offcuts, as the combustion products are toxic.