Identifying And Eliminating Dry Rot
What is it?
Despite its name, dry rot, like most other property ‘ailments’, requires moisture to take hold. Dry rot is a fungus, called Serpula lacrymans, that infects the timbers of a building when they are damp enough for its spores to germinate. The fungus then grows, using and breaking down the timber it sits on as a source of food. Most alarmingly, and where the name ‘dry’ rot comes from, once it reaches a certain size, this fungus can send out strands that can cross dry, inert materials such as concrete to infect further timbers on the other side of it.
How to spot it
The characteristics of dry rot depend on its stage of development. The fungus flourishes in damp, unventilated conditions, so it will most often be found in areas that are rarely seen, on structural timbers such as joists and roof trusses, behind skirting boards and door frames and on the underside of wooden floors. Because it’s hard to keep an eye on these areas, dry rot is often quite advanced before it is detected.
Initially the fungus appears as white cushion of what looks like cotton wool, which may produce water droplets on the surface. Where the fungus is growing in less humid conditions or is exposed to light, it appears as a thin silky, grey skin that has yellow patches tinged with lilac on it.
These sheets produce strands that carry nutrients and water with them so they are capable of crossing adjacent inert, non-nourishing materials such as brick and concrete to reach new areas of damp timber, thereby allowing the fungus to spread over large and distinct areas, potentially causing extensive damage to the structural integrity of the property.
When the fungus is advanced and preparing to reproduce, it develops a fleshy but tough pancake-like fruiting body, often through plaster. This body holds spores on its surface which are ochre to red-brown in colour, giving it the alternative name of ’brown rot’.
Rotted timber becomes dark and dry as the fungus draws all the water from it, shrinking and splitting to form ‘cuboidal’ cracks that run with the grain and deep across the grain. This timber has almost no strength to it so it will crumble between the fingers, and can create highly hazardous living accommodation.
How to fix it
Having discovered dry rot in your property, before beginning any repair or treatment it is imperative that you conduct an extensive and thorough survey to identify the full extent of the rot spread. Investigate all woodwork including skirting boards, joists, floorboards and doorframes and examine masonry too. You can identify areas of decay by poking timbers with a screwdriver; if the screwdriver sinks into the wood, it is decayed.
Every single incidence of the rot must be isolated and eliminated to prevent future breakouts.
Dry rot elimination step 1
Because dry rot can only infect wood that is damp (with a moisture content of at least 20%), the primary course of action is to remove the source of water causing the damp, be it condensation, rainwater or plumbing, and dry the timber out.
There are several routes for water ingress in a property. Be sure to check the integrity of each of the following areas, and fix the problem as soon as possible:
- Guttering and drainpipes
- Roof tiles and roof flashing
- Cracks in brickwork or walls
- Damaged, bridged or missing damp-proof course
- Blocked air bricks, poor ventilation and persistent condensation
- Cracked or broken water pipes
- Persistent overflow from water tanks and cisterns
Once the source of water has been removed, encourage the area to dry out by ventilating the area as much as possible, using heaters where feasible.
Dry rot elimination step 2
Drying timbers out can take a long time, during which period the fungus will continue to spread. Dry rot spores can also sit dormant but viable for many years in dry conditions, so it is vital to remove any material that harbours the fungus and protect new or existing sound timbers from infection.
Remove all infected timber, with at least an additional 65cm (25½in) on either side of the last evidence of rot to ensure that all sources of fungus are eliminated.
- Take care to minimise the spread of spores when removing infected timbers, by handling carefully, and spraying any fruiting bodies with fungicide.
If this woodwork was structural (joist or roof truss) it should be replaced with new timbers that have been pre-treated with an effective preservative such as pressurisation, double vacuum methods or a 10-minute dip in an organic solvent preservative to satisfy the requirements of British Standard BS 5268. Remember to re-treat any cut-ends or joints. Wherever possible, replace joists using joist hangers or cover the ends in joinery wrap where they are built back into the brickwork to provide a physical barrier to any future spread of rot.
Liquid preservatives should be applied to any sound timbers that are allowed to remain in the locality of an outbreak to help this wood resist a new infection from any.
- To protect new and existing timbers, use a preservative that is based on boron, a natural mineral that is odourless and non-toxic.
- Select a preservative where the active ingredient is mixed with glycol, which does not rely on water to penetrate the wood, thereby allowing it to protect deep into new, dry timbers, not just on their surface.
Masonry and plaster that has been directly infected should be removed using a wire brush, and all masonry surrounding an outbreak should be cleaned down and painted with a chemical fungicide to present a barrier to any future spread of the rot. Fungicidal plaster should also be used to refinish the wall.
- Sterilise walls by spraying with a masonry biocide. When making good on walls, use paint and plaster that contains zinc oxychloride (ZOC), which is a fungicide.
If the dry rot has infected an area adjacent to the property’s subsoil, this too can harbour fungal spores. You should therefore spray the subsoil with fungicide, remove the top 75mm (3in) of soil from the surrounding area and respray the remaining subsoil.
Further information and useful links
- Find a Chartered Surveyor near you
- Find a Dry Rot Specialist near you
- Find a Water Proofing Specialist near you
- Find a Masonry Repairs Service near you
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