Choosing timber for outdoor work: species, treatment and detailing

Treated softwood: what the treatment actually does

Pressure treatment drives preservative into the outer zone of the timber. It is effective, but it is not through-and-through — every cut, notch and drilled hole exposes untreated core and needs re-treating with an end-grain preservative. Skipping that step is the single most common cause of early failure in fence posts and deck frames.

Hardwoods and durability classes

Naturally durable hardwoods resist decay without treatment, which is why they are used for gates and exposed structures. They are heavier, harder on tooling, need stainless or appropriate fixings to avoid staining, and will silver if left unfinished. Choose by durability class and intended use rather than by name alone.

The details that decide lifespan

Whatever species you use, these are the things that determine whether it lasts.

• Keep timber out of direct ground contact wherever possible.
• Shed water — weather any horizontal top surface so water runs off.
• Seal all end grain, including cuts made on site.
• Allow airflow behind and beneath; trapped damp is worse than rain.
• Use fixings compatible with the timber and the treatment.

Finishes: oil, stain or leave it

Oils and stains slow moisture cycling and hold colour, but they are a maintenance commitment. Leaving durable hardwood to silver is a legitimate choice; leaving treated softwood bare is a shorter-lived one.

Questions we're asked about this