Identifying Dry Rot, Woodworm and Death Watch Beetle and Treatments

Provided woodwork is coated with paint or any other wood preservative and is not subject to undue adverse weather or atmospheric conditions it should require very little attention beyond repainting, revarnishing, or restaining, etc. Woodwork that is not sufficiently protected may be subject to diseases affecting timber such as dry rot and woodworm.

Dry Rot

The term ‘Dry Rot’ is something of a misnomer, because the condition is actually caused by dampness which encourages the growth of a fungus. Although dampness is necessary to commence the growth of the fungus, once it has developed, even if the cause of dampness is dealt with, growth may still continue and the rapid development of the fungus is such that it quickly spreads across brick walls or metal girders until it reaches wooden beams, floor-boards and other timbers. The fungus, which spreads to perfectly dry timber after being incubated in damp conditions, absorbs sufficient moisture from the atmosphere to continue its life and will spread to attack more areas of dry timber. In the final stages of the disease the affected timber will be found very dry, light and brittle and it is this feature of the disease which gives the condition its name — dry rot. This, of course, should not be confused with wet rot, which may be set up in house timbers that come into contact with water, such as areas under sinks and around washbasins, etc.


Dry rot spreads with great rapidity and it will attack any part of a dwelling-house from cellar to roof timbers. Dry rot is a very contagious disease and the fungus spores may be carried from place to place in the same house, or to other houses, on workmen’s tools. Therefore, when treating the disease it is very necessary to disinfect all the tools used before putting them into use on another job. It has been explained that dry rot is caused by dampness and there are many reasons why this malignant disease attacks timbers of a house. The most usual place of origin is under floor-boards or in a cellar and the condition may be set up by the use of wood that has not been properly seasoned, which is used in the construction of a new house or in the repair of an old house. The condition may also be set up by the omission of a layer of concrete over building sites, but this layer of oversite concrete is now compulsory in most places.

In many cases the disease has been known to start with small shavings and chips of wood dropped on to the oversite layer of concrete when the house is built. These small pieces of wood decay in damp conditions and provide breeding grounds for the fungi. The most usual cause of dry rot is lack of ventilation under flooring and this may be aggravated by faults in the damp-course which permit ground moisture to enter the timber fabrication of the house. If this is understood, the importance of maintaining adequate ventilation and the condition of the damp- course is obvious. To obstruct the passage of air by covering one of the gratings or metal air bricks near the foundation of dwelling-house walls is simply asking for trouble. This will almost certainly lead in time to an outbreak of dry rot. The condition may also be encouraged accidentally, besides blocking gratings, by short circuiting the damp-course, and the most common example of this is the erection of a rock garden or raised flowerbed against the side of a house.

Water may also enter the structure and attack the timbers, particularly the ends of joists on which floor-boards rest, if the edges of the roof overhanging the cavity wall have any gaps such as missing or broken slates or tiles. Provided sensible maintenance is carried out at regular periods the incubatory conditions favourable to dry rot should not be set up, but if, despite precautions, this disease — and it must always be considered a dangerous and malignant disease — should affect house timbers it is not difficult to detect.

The most easily recognizable symptom of dry rot is a persistent musty smell in rooms containing the affected timbers. If the condition is advanced a series of squared cracks will be seen on the face of the skirting-board or floor-board under which the fungus is growing. Wood affected by dry rot gives off a dull heavy sound instead of a sharp ringing tone when tapped with a metal tool. A test for dry rot in suspected timbers may be carried out by piercing the woodwork with a sharp blade of a penknife. If the blade meets firm resistance the timber is sound. If, however, it sinks easily and deeply into the timber and the blade is stained by a reddish brown powder when withdrawn, this indicates the presence of dry rot. The fungus is rarely visible on the surface of boards but in most cases it can be clearly seen on the underside of the affected timbers.


There is only one effective cure for dry rot — this should never be regarded as a patching-up job — and that is to completely remove the affected timbers and replace them with new wood. All the diseased timber removed and any sawdust chips or rubbish under the floor-boards should be burned at once — the disease may be spread if the damaged timbers are stored, perhaps for use as firewood. A description of dry-rot fungus is given on Home Inspection Nightmares. After removing the affected timbers the timber and wall surfaces for a considerable area around the affected parts must be treated with a preservative. Before applying the preservative it will be found advisable to cauterize the areas of brickwork and timber surrounding the affected timbers, also the surface of the concrete or earth under the floor, with a blow-lamp. Obviously reasonable care should be taken when using the blowlamp not to char the timbers, or this might result in a fire breaking out, perhaps some hours after the job has been completed.

There are several good proprietary brands of preservatives for treating timbers near a dry-rot area. Any of these may be used or a very effective preservative can be made by dissolving 3/4 oz. of sodium fluoride in a pint of water. Whatever type of preservative is used it should be liberally applied to the timbers round the area of infection, also any new timbers, replacing diseased timbers, should be liberally coated with the preservative. The preservative coating must be done very thoroughly and even the ends of new joining timbers should be coated before they arc fixed in position. It is essential during the period of repair that all the tools used for the job be kept on the site and everything, including the soles and welts of shoes, disinfected.

After dealing with the condition and renewing the timber the under-floor area should be inspected at intervals of about two months to ensure that a complete cure has been effected. The best way to facilitate inspection of under-floor timbers is to screw the new floor-boards to the joists instead of nailing them. In this way it becomes a simple matter to remove two or more boards so that the whole area can be inspected.


Woodworm is not a disease: it is the result of an attack on timbers by a small grub, and it mainly affects timbers in exactly the opposite conditions of those attacked by dry rot. The parts of the house subject to woodworm are usually those which are dry and warm. The most common cause of woodworm in used timbers is from pieces of wooden furniture that already have been attacked when they are brought into the house. Another possible cause is the use of unseasoned timber. Woodworm is a condition that can be effectively dealt with if the treatment of affected timbers is methodical. The best time of the year to apply anti-woodworm treatment is the Summer months of June, July, and August, as it is during this period that the woodworm is most susceptible to treatment.

Recognition of the woodworm is dealt with on the post about Home Inspection Nightmares. It is a popular fallacy that the fine dust and small holes indicated by the presence of woodworm are caused by the small grubs eating into the wood, but this is incorrect. The visible conditions, fine dust and small holes, are set up by the adult woodworm as it emerges. The female woodworm lays its eggs, which are very small indeed, in scratches and cracks in the wood and the hatching period is about four weeks. The grubs which hatch from the woodworm eggs are the main cause of damage to the timbers. They have a life of several years’ duration. They live inside the timber and they bore a series of holes and tunnels in the wood.


When their destructive life is over the grown woodworms emerge from the wood, leaving a tell-tale pile of fine dust, and they die about five to seven days from emergence, usually after laying fresh eggs which may lodge in the joints and surface cracks of other woodwork.

There are two ways of dealing with woodworm. If the condition is very bad and the timbers — particularly those supporting roofs and floors of houses — have been badly attacked, they must be replaced. If the attack is not extensive the woodwork may be treated with a preservative. There are several types of preservative liquids obtainable from local suppliers. Application may vary between different brands. Some are brushed on to the wood and repeated coatings are necessary to ensure that the preservative penetrates deeply into the timber. When purchasing a woodwork preservative ensure that it is of a type that will not damage the polished surfaces of wood. The use of a preservative must be thorough; it is pointless merely to seal off the surface of the timber as in all probability there are several generations of woodworm grubs alive and active deep in the wood. In addition to coating the surface, the preservative should be injected into the small holes which tunnel into the woodwork colony, and the preservative may be injected into the timber by means of a small syringe, a high-pressure fine spray, or with a special injector supplied by the manufacturers of the preservative.

Dealing with woodworm is a long job that requires a great deal of patience, and thoroughness is essential to cure the condition completely.

Death Watch Beetle

This most destructive timber insect seldom attacks the healthy timbers of a house that is well maintained, but it may attack the woodwork of homes that have been neglected, especially where the timbers have been attacked by wet or dry rot. The presence of Death Watch beetle is easily detected by large holes in the timber. The treatment of timbers affected by Death Watch beetle is similar to that of woodworm, where a suitable preservative is injected into holes and is coated on the face of the woodwork.

07. July 2011 by admin
Categories: Problems & Insects, Woodworking | Tags: , , , | Comments Off on Identifying Dry Rot, Woodworm and Death Watch Beetle and Treatments


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