Dry rot – the scariest mushroom of them all

I've just seen one of the scariest outbreaks of dry rot of my career. What had seemed to be a bit of mould in a tenanted early Victorian house quickly became a full blown dry rot infestation creeping into attics, behind kitchen units and reaching out for the entire house in just a few short months. Environmental health swung by at the request of the tenant, took one look at his pregnant girlfriend and insisted the landlord rehouse them. So the house owner now has one house that is virtually unsaleable, and a legal obligation to pay rent on another, but still doesn’t have anywhere to live. How dark do you like your nightmares?

All my old textbooks warn that once the fungal infestation that is dry rot has a grip, chemical disinfestation is only the cherry to be placed on the completed cake. First you must remove all affected joinery, plus a further meter of sound timber, being a gentle as possible and with all room entrances sealed to prevent the further spread of spores. You then bag the debris and burn it well away from buildings. Back at the house you remove all plaster and mortar to a meter back from the last trace of mycelium (the thin tentacles by which the fungus grows) and remove this together with all dust from the affected areas to a secure burial site. And then you wheel out the chemical poisons, not just because you might have missed some spores, but because we know the spores are everywhere – they’ve even been found floating miles up in the air, which might make you nervous about taking a ride in an old Tiger Moth.

In other words dry rot is viewed as closer to black magic than other building defects. About the only consolation a textbook will offer you is that because it hates extremes of temperature dry rot will never be found within a meter of a south facing external wall. Except that’s exactly where it started in this instance, and in about every house in Bath I’ve ever surveyed. So the message is clear; if you have dry rot your life a proud home owner is over and life in a steel drum is all you have to look forward to.

This is clearly, well, rot. Dry rot is a life form and like any other can be killed by drought or famine, although I grant you it is brilliant at hibernating which is its evolutionary party piece. Dry rot probably originated in the Himalayas, and appeared in this country in the Eighteenth and Nineteenth Centuries as we started to import stuff back from the Empire, together with softwood for building having deforested our own countryside of native hardwoods. In the Himalayas stone riverbeds lay dry for most of the year, and sometimes for more than a year. Come Spring the ice and snow melt water rushes down these riverbeds in a brief and glorious flume, bringing down trees on the way before drying out for at least another year. Dry rot clung onto life here because it could doze away in old branches and deadwood despite the lack of moisture, yet burst into life as the flood waters and warmth of Spring arrived, completing its lifecycle in just days before the riverbeds ran dry again.

So here’s the truth about dry rot; yes, it’s everywhere and it seems to love the lime which is so prevalent in period houses, from plaster to mortar to stone window sills. Yes, once it gets going it speads like wildfire. But it is still just a big headed mushroom and needs food, usually decaying softwood, although paper and cardboard will do, and once it really gets going it would be a brave man who guaranteed your oak beams were safe. But most of all it needs water. The outbreak I’m talking about can probably be traced back to an overflowing gutter spilling onto outside ground levels above the inside floor – which was concrete. The rot starts weakly, barely surviving on a bit of skirting board, spreading through plaster made damp by the downstairs bathroom. And then it reaches the attic, where those wormy old lathes and a small leak in a flashing let it tear away, aided and abetted by our wet summer.

And I’ve seen this so many times; a small leak in a flat roof, a split in a flashing, a rusty gutter and, quite literally, the rot starts. The mycelium spreads behind plaster, any signs of trouble still small; perhaps a small damp stain and black tracery that looks like a dirty spiders web. And then the fungus find something meaty – say rotting floor joists or purlins buried away from view in a damp wall – and it’s away, eating the very fabric of your home.

Don’t let it happen. Keep your house dry, investigate leaks, clear out gutters and keep outside ground levels where they should be. Because once dry rot gets started you’ll find you can run out of options – and cash – very quickly indeed. You might even find yourself thinking about introducing nasty chemicals into your home, when the reasons you shouldn’t would justify an article in itself, ideally by someone with medical training. And that really shouldn’t be necessary when making sure dry rot never gets started is just basic good housekeeping, not Witchcraft.