Most of Grantham's pre-war housing stands on suspended timber ground floors over a shallow void, with timber joists, rafters and purlins overhead. Keep that timber dry and it lasts centuries. Let damp reach it, through a blocked airbrick, a leaking gutter or a failed damp proof course, and rot and beetle follow.
The terraced streets built around the town's Victorian ironworks era, and the interwar semis that followed on the Harrowby and Earlesfield estates, mostly have suspended timber ground floors ventilated by airbricks just above ground level. On the valley floor, where clay-rich ground drains slowly and the water table sits close to the surface near the Witham, the air in those sub-floor voids is humid for much of the year. The design copes, provided air keeps moving. Problems start when airbricks are blocked by raised paths, rendered over, or buried under decking and flower beds, and the void turns still and damp.
Roof timbers have their own risks: slipped slates, defective flashings around chimneys, and condensation in lofts that have been insulated without maintaining ventilation.
The most common finding. Wet rot fungi attack timber that stays persistently wet, typically above about 30 per cent moisture content: joist ends built into damp solid walls, floorboards over a waterlogged void, sole plates behind leaking downpipes. The timber darkens, softens and cracks along the grain. Treatment means fixing the moisture source, replacing timber that has lost strength and treating what remains.
Rarer but far more serious. Dry rot (Serpula lacrymans) can spread from a single damp spot through masonry and behind plaster, consuming sound timber as it goes, and thrives in exactly the still, humid, unventilated spaces a neglected sub-floor void provides. Signs include a deep cuboidal cracking pattern in timber, matted grey growth, a rusty-red spore dust and a distinctive mushroom smell. Suspected dry rot needs professional investigation quickly, because the cost of dealing with it rises with every metre it spreads.
Woodworm is the general name for wood-boring beetle larvae, most often the common furniture beetle in this area. Fresh flight holes about 1 to 2 millimetres across with clean, pale dust around them suggest an active infestation, and beetles favour timber that damp has already softened. Treatment is usually a targeted insecticidal application to affected timbers once moisture levels are brought down.
Because rot is always a symptom of a moisture problem, timber work is quoted alongside the damp repair that caused it, whether that is a failed damp proof course or rain penetration. Replacing joists without fixing the water gets you the same rot again in a few years.