Vintage View: How to deal with wet and dry rot in your home

Whether you're a DIY beginner or expert, Irish Examiner columnist Kya deLongchamps has the answer to your questions

Typical wet rot shows up where there is a failure in the plumbing or ingress of water into timber as here with this soft old window frame. Once dried out and treated, the rot will only return if the surface gets wet again (the spores are already present).

When we were kids, our mother would find the furriest boreen in whatever county we happened to be roaming around, and buy whatever hoary old building was at the end of it. She would fly into an instantaneous passionate panic — offering the full asking price if the seller would take it off the market. 

At least, that seemed to be her property buying mechanism.

Along with charming old houses came a host of hilarious adventures, including various forms of creeping dilapidation crouching behind bead-boards and porridgy plaster. The addition of Portland cement in the place of breathing lime plaster was really taking hold, and without any way for a building to breathe as it had in former centuries, damp was an absolute certainty after many well mean mid-century Irish “renovations”.

Wet rot and dry rot were a regular topic around our tea table. Mum (any worries shored up by climbing roses over her rose-tinted Aviators regarding Charm Lodge) would declare airily: “Sure wet rot is....much better than dry rot — and c’mon, those timbers are soaking!”

Dad would pop his head and shoulders into a musky attic occasionally: pause, stiffen, report nothing, and put the ladder away smartly.

All that we noticed in our infant innocence was that this or that ceiling was sagging, or a familiar hole in the floorboards was fringing prettily at the edges and inviting in mice.

We managed to grow a fungus at the base of an unused guest-room wall large enough to hold a little ornament.

Damp spots were chased away with glutinous sealants, and Mum would wonder at their inevitable cumulus return under fresh paint, muttering that my father should have “temporary” tattooed across his forehead.

On reflection; he worked hard to realise her ancient-pile fantasies, once tying himself with a rope around the waist to a chimney stack to reroof a three-storey Georgian money-pit in his dress shoes. Old homes are a difficult mistress. Well, someone was.

Rotten timbers in any house engender terror. It’s there even in that sharp bark of a word: rot, defined as “to decay by the action of bacteria and fungi; decompose” (Oxford Dictionary).

There’s a huge amount of wood in a stone house: the floors, studwork, ceilings and the roof depend on wood members being structurally sound and dry.

Being biodegradable, Nature of course wants that dead material to come home to Mama, and once the moisture content hits 20%, a discreet little horror story unfurls.

The two types of fungal spores that lead to dry and wet rot are already inside the timbers, especially old timbers that were put in green rather than being kiln-dried’ they just need the addition of damp conditions to flourish. Their scientific names of these spores are Serpula lacrymans (dry rot) and Coniophora puteana (wet rot).

A typical situation for wet rot would have a lot more water involved, resulting in moisture content in the affected wood of 50% or more. It might be a cunning leak from old lead piping, or a drizzle of water sneaking down a crack from a broken gutter, through a gap in the render into woodwork on the north side of the house.

The reason surveyors don’t get their clipboards in a clatter over wet rot (and it turns out Mum was completely right) is that once the source of the wet is taken away the Coniophora puteana spores go dormant. It stops growing as the wood dries out.

Wet rot spores also don’t move across dry areas as dry rot spores can.

Yes, you might have to replace parts of the board, plank, rafter whatever if it has collapsed into a spongy mess but in general, it’s easier to put wet rot right. Still, with imperfect, cavalier treatment as the timber dries out, but stays wet enough, dry rot can take up where wet rot left off.

Dry rot has a more sinister reputation as it can be caused by pretty boring, everyday problems like damp and condensation caused by typical poor home ventilation.

You notice misting windows, but what you don’t see is the decaying timber under the floor or buried in the walls. There’s no signalling event as there often is with wet rot, and it can run through a house once the wood is up to 28% moisture content. It feeds on the water, hemicelluloses and cellulose in the timber, leaving it brittle and rotten.

Sometimes dry rot is suspected as the floor seems oddly bouncy (because it’s unsupported). Checking an attic, unheated room or behind stud-work, homeowners might notice a little, no account sugaring of brown material. It seems like nothing.

It typically takes a long time for dry rot to actually start flowering on interior walls in full view. 

However, once uncovered, dry rot looks pretty hideous (which doesn’t help the nerves) with fluffy white branches, orange and rusty red, spore-rich dust, dry strands of mycelium and, worst of all, ghastly mushroom-like fungal growths (fruiting bodies) you expect to see on deciduous trees trunks in a haunted forest.

As the fungus cheerfully digests parts of your house, it burps out gas, and it’s that earthy mousy smell that most pros can identify on stepping through the front door.

A survey for wet or dry rot will cost in the low hundreds of Euros, depending on what’s involved to get eyes on the problem area.

The extent of the rot will need to be fully traced and that can mean tearing back into the fabric of the building or using dry rot detection dowels inserted in drilled out holes. Treating dry rot will depend on the severity, but it’s always a good idea to have a surveyor or structural engineer take a look if you don’t have the experience, to avoid underestimating the extent of the ingress.

Dry rot is attacked by cutting out crumbling, ruined wood to beyond 500mm of the last sign of fungal growth. A professional firm will then scrub down the surroundings. 

After cleaning a specially formulated micro-emulsion fungicide is delivered in a spray or injection to treat the area long-term. Rot can move through masonry and brickwork (the mycelium just journeys through, it doesn’t damage this material) but nearby stonework will need to be treated too.

Kya deLongchamps.

There are DIY remedies and recipes involving boron salts, vinegar and bleach but keep in mind if you don’t get the job right, the rot will be entombed back into the building and the spores could live again. 

Getting to and addressing the source of any damp issue and improving ventilation throughout the building will be crucial. Dry rot tends to signal other, whole-house issues, so chances are you are facing a wider renovation.

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