Its Causes and Cure
The presence of dry rot in a building calls for immediate action, but if the trouble is recognized and prompt remedial measures taken much trouble and expense can be avoided. Here the characteristics of dry rot are described, how it is caused, and the steps which can be taken to combat the disease. Valuable information has been obtained from the leaflet No. 6. "Dry Rot in Buildings, Recognition and Cure," published by the Department of Scientific and Industrial Research, Forest Products Research Laboratory.
Dry Rot is caused by various fungi which live on the organic substances of the wood with the result that the timber loses its nature and strength, and becomes dry, friable, and powdery. It has much the appearance of charred wood, excepting that it is not blackened, and has deep cracks both with and across the grain. It tends to break away in cubes. Timber which has been attacked by dry rot can be broken away with the fingers and easily penetrated by a sharp instrument.
"Dry rot" is rather a misnomer, since the term describes the condition of the wood after the disease has attacked it. Actually, dampness and a favourable temperature are necessary for the disease to flourish. Although timber exposed to the weather is liable to rot owing to excessive wet conditions which encourage attack by fungi, this rot must not be confused with true dry rot, or Merulius lacrymans, or to Coniophora cerebella, the cellar fungus, to give their scientific names.
SYMPTOMS. If a musty fungus-like smell is present in a house dry rot can be suspected. If signs of dampness in a wall are found the adjacent skirting should be tapped, and if it gives a dead sound without the the ring of normal wood it is probably infected. Test also to see whether it is easily penetrated by a sharp instrument. Such a condition will warrent the removal of the skirting and the adjacent floorboards in order that a close examination of the floor timbers can be made.
APPEARANCE OF THE PEST. If the trouble is due to Merulius lacy mans it will be found that the timbers are covered in places by white fluffy cotton-wool-like masses spreading out like a silky white sheet. Where the wood is drier, the fungus spreads over the surface as a pearly-grey skin, showing here and there tinges of bright lemon-yellow and lilac shades of colour.
Branching from these areas are strings or strands which vary in diameter from the thickness of a thread to that of a lead pencil. These strings or strands conduct water and are the means by which the fungus seeks its food and spreads. In seeking food, the strings or strands conduct water to the drier parts of the timber thus creating conditions favoutable to its growth. This renders Merulius lacrymans especially destructive since, if not checked, it can spread throughout a house and even to adjoining houses.
On further examination, the fruit bodies will be found. These are shaped like plates or pancakes, having a whitish margin. The fruit bodies are of a soft fleshy nature and bear on the surface a number of folds or shallow pores covered with a rusty red powder, the actual spores of the fungus. These spores are microscopically small and light in weight. Consequently, they can be borne by air to spread infection elsewhere. The spores can lie dormant for years, awaiting suitable conditions for germination.
CELLAR FUNGUS. It is possible that the rot will be due to Coniophora cerebella, though this thrives only under very wet conditions. The strands of this fungus are never thicker than stout twine. These are at first yellowish-brown, becoming darker until they are almost black. Thick cushions or sheets of fungi will not be found as in the case of Merulius. At time a yellowish skin is produced on the surface of the wood. Timber attacked by this fungus tends to split longitudinally. The fruit body is irregularly shaped, and has a skin of olive brown covered with small lumps, but the fruit is not often seen except on felled timber. As this fungus cannot carry water to create conditions for life it can be checked by drying out the timber.
CAUSES. A damp situation for the house, or any conditions which give rise to dampness such as a leaky gutter, broken rain-water pipe, or the unchecked flow from the waste pipe of a flushing cistern creates circumstances favourable to the pest. Another likely cause of dry rot is the use of improperly seasoned timber. Conditions favourable to dry rot are set up if the free circulation of air under a floor is checked by the clogging of the air bricks. Unwittingly, earth may have been piled against a wall above the damp-course in order to form a rock garden, with the result that water percolates through the brickwork and reaches the timber. Thus conditions are favourable to dry rot, and the presence of a piece of infected firewood within the house is sufficient to start the infection, bearing in mind the nature of the spores.
COMBATING DRY ROT. If dry rot is suspected, a thorough examination should be made in order to trace the seat of origin. If a fungus is found, this should be definitely identified. Should it be Merulius lacrymans or Poria vaiblantii (vaporaria), a fungus which is akin to Merulius, drastic action must at once be taken. All the infected wood must be cut away, the cutting being done in the sound wood as far as 12 in. to 18 in. beyond the last visible signs of decay. The infected wood should at once be burnt and not sold to a firewood merchant. No vestige of fungus must remain. The walls and brickwork which have been in contact with the infected timber must be sterilized by the application of a blow-lamp flame and the remaining sound timber treated with a preservative such as creosote. Proprietary preparation, which are less pungent that creosote, can be obtained.
Fig. 2 shows the floor of a ground floor room with the infected timbers cut away, a temporary support being provided for the remaining sound portions of the joists. As there is insufficient room between the joists for the manipulation of a brace for the boring of the bolt holes, it will probably be necessary to remove as much of the sound timber as will allow the joists to be turned on their side for boring. If a new section has to be added to the wall plate, this should be scarfed, as shown in Fig. 3. It is advisable to treat all the new timber with a preservative.
Incidentally, dry rot is not associated with any human disease, but where a house is badly attacked the air is necessarily damp and for that reason it may be unhealthy.
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