Wood does not degrade through spontaneous chemical ageing — a timber element kept dry can last centuries with no treatment at all, as shown by medieval trusses still in service in many Italian cathedrals. Its pathologies almost always have a biological origin (fungi, insects) governed by a single critical variable: moisture content. Fire, culturally perceived as the main risk, is actually the most predictable and calculable mechanism of the three.
Decay fungi: the 20% threshold and the case of dry rot
Wood-decay fungi fall into two broad families based on their attack mechanism: brown rot selectively degrades cellulose while leaving lignin, resulting in wood that cracks into dark, friable cubes; white rot also degrades lignin, leaving a lighter fibrous residue. In both cases, the necessary condition for attack is a wood moisture content above 20% — below this threshold, no wood-decay fungus can proliferate, regardless of wood species or exposure. This is why the design of constructional details (eaves, flashings, ventilation of cavities) matters far more than any surface chemical treatment for real prevention of fungal decay.
The most insidious case is dry rot (Serpula lacrymans), a fungus capable of behaviour that surprises anyone who encounters it for the first time: it develops thick mycelial strands (rhizomorphs) able to actively carry water for several metres, even through brick masonry or plaster, to reach and attack wood that appears dry and far from any visible moisture source. This is why dry rot, once established in a historic building, often requires remediation far more extensive than the visibly damaged area alone: the fungus may already have spread, invisibly, well beyond the original point of infiltration.
Wood-boring insects: from the common furniture beetle to the house longhorn beetle
The most common wood-boring insects in European timber structures are the common furniture beetle (Anobium punctatum), whose larvae bore galleries in the wood for years before emerging as adults through the typical 1-2 mm exit holes — often the first visible sign of an infestation already under way for some time; and the house longhorn beetle (Hylotrupes bajulus), whose larvae can remain active in softwood for 3-10 years, causing significant structural damage to roof timbers before being detected. Unlike fungi, wood-boring insects can also attack wood with relatively low moisture content, which makes moisture management alone insufficient as the sole preventive strategy: periodic monitoring (checking for emergence holes, boring noise, fresh bore dust) remains necessary even in well-ventilated, dry environments.
Fire: perception versus engineering calculation
Public perception associates wood with a higher fire risk than concrete and steel — an understandable but technically misleading intuition when talking about massive structural elements (not thin timber or lightweight cladding). Solid wood and engineered products like GLULAM and CLT burn at the surface at a known, constant rate — about 0.7 mm/min for spruce — forming a charred layer that, thanks to its low thermal conductivity, thermally insulates the still-intact core beneath. This behaviour is so predictable that it can be sized on paper: a section oversized by 40 mm relative to the structural minimum guarantees about one hour of fire resistance (REI 60) passively, with no additional treatment. An unprotected steel beam, by contrast, loses 50% of its strength already at 500°C and can collapse suddenly within minutes of direct flame exposure, without the "readable" warning of wood's surface charring.
This does not mean wood is free of fire risk — it means the risk must be assessed with specific engineering calculation (Eurocode 5, fire part EN 1995-1-2), not intuition. The most heated regulatory debate today concerns "encapsulation": whether and how much exposed timber surfaces in tall buildings must be clad in non-combustible materials, an issue addressed with growing caution after the Grenfell Tower fire in London (2017, not a timber structure but an episode that triggered a general review of British fire regulations) and the subject of specific tests on multi-storey CLT buildings in the UK and the United States in the following years.