Dry rot is not nice.
It is the fungal equivalent of Eamonn Holmes after his latest fad diet ends (allegedly). It is as stealthy as one would imagine a mushroom ninja can be, and as fast as a tortoise (and if you've ever taken your eyes off a tortoise for more than a moment, you know what I mean). It can also eat your house. Seriously, I mean it.
To give a rather dry description, dry rot (serpula lacrymans) is a bracket fungus which infects timber and masonry, often creeping along behind plaster on walls and viciously attacking defenseless skirting boards. It will utterly desiccate timber, breaking down cell walls and turning healthy wood to so much sawdust. In order to do this, the average moisture content of the timber needs to be over 16%, but preferably over 20%. In order to spread, the fungus needs three things:
A moisture source - whether from outside, a leaking pipe or even the condensation forming on a metal surface
Food - normally and preferably timber, and the fast-growing, ever popular softwoods provide a great source of starch and fibre, as well as absorbing and retaining moisture better
Air - but not too much in the way of cold air. The fungus itself is surprisingly delicate and will not tolerate huge extremes of temperature. The British temperate climate suits it very well, thank you for asking.
Light is not a necessity, as it will often be found growing where there is very little or no light at all. Even the first requirement becomes less of a necessity as the fruiting body develops to maturity, as it seems to be able to extract enough moisture from the air to keep itself going. Indeed, this is where the second part of it's Latin name comes from - lacrymans meaning crying or weeping - as the surface of the fruiting body seems to be full of moisture.
The anatomy of dry rot breaks down roughly as follows:
The fruiting body is an irregular shaped fungal `bracket', white at the margins and deepening in colour to rust red and brown in the centre.
The mycelia are the white strands of fungus questing along and through timber and brickwork.
The sporophores are the spore-bearing part of the fruiting body.
The spores themselves, commonly seen as a brick-red dust, often the first sign that something is wrong.
Commonly, the first sign of dry rot is a cuboidal cracking appearing in timber (usually the skirting board or door architrave), or the appearance of an unsightly "mushroom". Care should be taken when attempting identification, however, as although dry rot is a "brown rot", not all brown rots are dry rot. With me so far?
My advice, if you notice anything like this in your house, is to call an expert. Try the website of the Property Care Association if you're in the UK, and search for a recognized, trained company who are members.
DO NOT ATTACK IT WITH A FLAMETHROWER!
Just thought I'd better mention that.
Treatment of dry rot usually involves removing of plaster, flooring and anything affected, sterilising the area with biocide, the filling of your house with hairy tradesmen, copious quantities of hot beverages and, occasionally and regrettably, swearing.
Make sure the company involved carries out a full exposure of the area first, so they know how far it has gone, and what it is infecting.
Get a fixed-price quotation.
Make sure there is a guarantee worth more than the paper it is printed on. Insurance-backed guarantees are available, so that if the company goes out of business the insurance company pay for any remedial work.
Leave it and assume it will be fine. Dry rot can stay latent for up to forty years and will eat windows, doors, floor joists and lintels (but fortunately not animals or small children).