How to deal with dry rot and wet rot

How to deal with dry rot and wet rot

One of the biggest threats to structural timber is fungal decay. Both wet rot and dry rot, united in their love of damp, can cause serious structural damage to your home. In this article we look at the differences between dry rot and wet rot, their causes, symptoms and treatment.

Scroll down for advice on how to deal with rot or jump to the following sections:

What is dry rot?

Dry rot to wood is like kryptonite to Superman. Serpula lacrymans is the technical term for this fungus that can reduce stalwart beams, joists and timbers to crumbling, hazardous structures. In spite of its name dry rot needs moisture to thrive, in fact it needs wood with a moisture content of at least 20%. It loves damp, warm, unventilated conditions so it’s often found in areas that aren’t easily visible such as roof trusses, the underside of wooden floors, beneath stairs and behind skirting boards.

The nightmare scenario with dry rot is that its spores travel quickly, always on the lookout for more timber to feed on. And not just timber – masonry and plaster can be affected too. Within a matter of months the entire structural integrity of a building can be compromised so it’s vital to act quickly if you suspect dry rot.


What are the symptoms of dry rot?

Be on the lookout for:

  • matted whitish growth tinged with yellow and/or lilac patches
  • a growth resembling a pancake with a russet colour at the centre. If you see this you’ll know the fungus has really taken hold. That russet colouration is in fact millions of spores of dry rot – approach with caution!
  • a distinct and quite acrid mushroom-y smell
  • splitting, shrinking or crumbly wood. Try poking timbers with a screwdriver; if it sinks into the wood you’ll know it’s decaying.


How is dry rot treated?

Treating dry rot is a two-step process:

The first step is eliminating the source of water causing the damp. Be particularly vigilant for:

  • leaky plumbing, guttering or pipework including downspouts and overflows from cisterns
  • condensation
  • rainwater coming through gaps in doors and windows
  • damaged or ‘bridged’ DPC (damp proof course)
  • missing roof tiles, flashing or cladding
  • blocked air bricks

For more advice read Dealing with damp problems. Once the damp has been identified you’ll need to thoroughly ventilate the area using dehumidifiers and keeping the heat on low. This will take a while and remember, during this period the fungus can continue to spread.

The second step is to get rid of all infected timber. It’s at this point you may wish to call in professional help. If you do so make sure the company is a member of the Property Care Association (PCA) or alternatively contract a local building surveyor via the Royal Institution of Chartered Surveyors. If you’re doing the work yourself you’ll need to remove all affected areas of timber including a 60cm (24in) allowance either side of infection. Try to minimise the disruption of russet-coloured spores when removing timber and spray liberally with fungicide.

Structural woodwork such as joists should be replaced with new timbers treated in accordance with Eurocode 5, ie preservatives applied by pressurisation, double vacuum impregnation or dipped in organic solvent. Cut ends should be re-treated and wrapped to physically prevent the spread of rot.

Healthy timbers near a site of infection should be treated with a preservative containing boron, a safe and odourless mineral, and glycol, for deep penetration. Affected plaster should be removed and replaced with fungicidal plaster. Masonry should be cleaned with a chemical fungicide and sprayed with a masonry biocide.


What is wet rot?

More common than dry rot, wet rot is caused by a fungus called Coniophora puteana, aka the ‘cellar fungus’. This type of fungus is only attracted to very damp wood or plaster and unlike dry rot, remains confined to the wet area only. It’s generally deemed less destructive than dry rot but serious cases can prove hazardous to a building’s structure.


What are the symptoms of wet rot?

Wood with wet rot has a typically soft and spongy feel and often looks darker than surrounding healthy wood. If the wood is painted it can mask the decay and appear quite healthy; it’s only by poking with a screwdriver you’ll discover if it’s affected. On walls the fungus manifests with brown/black strands in a fern-like pattern.


How is wet rot treated?

Wet rot, as its name implies, is present when there’s direct and sustained contact with water, so the first step is eliminating the source of ingress as above.

The second step is removing and replacing affected timbers. If only a small area has been affected the rotten wood should be removed back to sound timber and the surrounding area treated with an epoxy wood hardener and preservative. If timbers are structural Eurocode 5 again comes into play so new timbers must be pre-treated as above with cut-ends and joints re-treated. On brickwork or plaster wet rot can easily be removed using a wire brush.

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