What fire does to a structure
Steel loses a significant proportion of its strength at fire temperatures and can remain distorted or weakened even where it looks intact once cooled, making an untrained visual check unreliable. Masonry and concrete can suffer thermal shock cracking and spalling, where the surface breaks away and exposes reinforcement, reducing load-bearing capacity in ways not always visible from the ground.
Timber structures lose section through charring, and while charred timber can sometimes retain more strength than expected, connections, fixings and load paths are frequently compromised. Roofs are a particular risk after fire: collapsed or partially collapsed roof structures can be carrying debris loads well beyond their design capacity, and floors beneath a fire-affected roof can be similarly overloaded with fallen material and firefighting water.
Water damage from firefighting efforts often affects a larger area of the building than the fire itself, soaking into plaster, insulation, timber and electrics, and this needs factoring into both the make-safe assessment and the clearance scope.
