Dry rot is a fungal attack on timber, causing it to crumble into a fine powder and become brittle. It can take hold very quickly yo devastating - and expensive - effect and needs to be treated straight away. It is aggravated by a combination of dampness, poor ventilation and heat.
Dry rot has a missleading name: it is caused by wetness. The humidity allows a fungus to develop, which feeds on the wood until it drains out all the moisture content of the timber, reducing it to powder. No cure once attacked: replace and rectify source of water.