Dry rot (Serpula lacrymans) is a primitive form of vegetable life that derives its nourishment from timber by attacking the cellulose & hemicellulose, thereby depriving the wood of all its natural strength. Once established, it grows rapidly by pushing out fungal strands (hyphae) even through thick masonry, in its search for more timber on which to feed.
Dry rot will grow sheets of a substance known as mycelium, from this, fruiting bodies (sporophores) may eventually form producing millions of airborne spores which carry the infection elsewhere, not only through the building but to other buildings where conditions for germination occur now or in the future. There is a musty-mouldy odour present when active.
Caused often by a combination of dampness and insufficient ventilation, resultant from such causes as roof faults, defective gutters, blocked under floor air vents, bad pointing and rendering, the lack of an effective damp proof course or the sealing of any space into which moisture has access.
With Dry Rot, a fungal decay, it is important to bear in mind that it is a malignant destroyer that can remove all the strength from buildings.