Jump to: navigation, search

Dry rot is caused by a fungus (in Australia, the most widely found species is known as Serpula Lacrymans). These send out root-like threads called hyphae which can penetrate mortar, and pass over pipes and masonry to find timber. Here, the fungus lives on moisture within the structure of the grain, making the timber dry, brittle and structurally useless.

The ideal conditions for growth of the fungus are warm, damp, badly ventilated areas, including wooden floors, cellars and basements, under stairs and behind skirtings. In such conditions the growth can easily spread at .an alarming rate. Whole houses have been infected in months.

Because dry rot is extremely infectious, it is difficult to isolate. A mature dry rot fungus produces a flat pancake-shaped fruiting body, called a sporophore, which emits millions of tiny rust-red spores (see diagram at bottom of page). These are carried through the air, spreading the decay if they land on further damp wood. Even if the affected wood is removed, residual fungal strands may continue growing and eventually infect sound timber.

The treatment for dry rot differs considerably from that for wet rot. If dry rot is mistaken for, and treated as, wet rot, some of the affected area will be missed and the spores will be able to carry on their destructive work until detected at some future date. Eradication will then be a much more costly exercise.

With dry rot, an advanced attack produces a distinctive mushroom smell and the surface of badly affected timber, is covered by matted fungal strands, (the mycelium) in a thin, web-like sheet. This sheet, which grows rapidly in humid conditions, is greyish white with lilac tinges and resembles cotton wool. It develops bright yellow patches where it comes into contact with drier air or is exposed to light.

Unlike wet rot, which grows only on damp timber, dry rot carries its own water enabling it to travel unseen through the middle of timbers, through, behind and under brickwork, plaster and concrete. When the fungus is growing in particularly damp conditions, it forms globules of water like tear drops on its surface.

Deep cracks in the wood break the timber up into cubes. It becomes darker in colour, loses its characteristic resinous smell and eventually breaks into powder when rubbed.

[edit] How To Treat Dry Rot

Because of improvements in building techniques and the almost universal requirements that all new buildings should have a damp-proof course, dry rot is seldom seen in houses in Australia built after 1945. But because there is no absolute guarantee against dry rot infestation, it is essential to check all possible sources of damp regularly. If the house has been flooded, or timbers have been soaked by burst pipes, be sure to check that the wood has thoroughly dried out, especially in concealed corners and against walls.

If you are unlucky enough to find signs of dry rot in your home, the source of the damp causing the infestation must be corrected before the rot can be eradicated. Check for faulty plumbing, defective damp proof courses, broken or overflowing guttering and broken roof tiles, slates and flashings.

Bad ventilation will accelerate the progress of dry rot throughout the house so you should check airbricks as well, replacing any that are broken and clearing blocked ones.

Once the source of the damp has been located and rectified, make a systematic investigation inside the house. Look for signs of surface buckling in all wood and raise floorboards to check the condition of joists and wall plates. Also, look in cupboards and in the roof space, where rain may have been leaking in steadily for some time.

Wherever there is evidence of an attack, consider that point as being the centre of a circle with a radius of about 1m and make a close examination in every direction within this area. Wherever continued evidence of decay is found, extend the circle until the limits of the dry rot growth are established and encircled.

Treatment of affected timber must be drastic and thorough. Rotted wood should be cut out well beyond the last visible sign of decay and then burned to avoid spreading infection.

Cut away all timber in the affected area 1m beyond the last point of decay, but make sure that this will not weaken the building's structure. If the area of damage is large and there is a risk of weakening the structure by removing the required amount of wood, call in expert help. When all the affected timber has been removed, apply two liberal coats of fungicide to all wood within 1.5m of the area of decay.

When you replace wood, which has been removed, use only dry, well seasoned timber and give it two coats of fungicide on all surfaces. Steep any sawn ends into the fluid for five minutes to make sure the fungicide soaks well into the end grain. Also, treat any new joints or holes drilled in the timber. If you are replacing a rotten floor joist, give the new timber further protection by coating its ends with bituminous paint before resetting it into the wall.

[edit] Treating Walls and Plaster

Where plaster is within 1m of a damaged area, or where fungal strands are growing over plaster, hack away the plaster with a hammer and bolster and rub down all brickwork and surrounding timbers and pipes with a stiff wire brush. Collect the plaster dust with a vacuum cleaner, remove it from the house and spray it with a fungicide to prevent the spread of infection. Returning to the affected area, apply two coats of fungicide to all the surfaces you have brushed clean of dust.

Allow the wall to dry out thoroughly before making good the plaster or redecorating. When you come to replastering an area, apply a floating coat of zinc oxychloride plaster, about 6mm thick, to inhibit future fungal growth. Areas which only need repainting should be treated with two coats of zinc oxychloride paint before hand.

If masonry is affected, drill a series of 10 mm holes, 150 mm deep and sloping downwards at an angle of 45 , at 225 mm intervals across the contaminated area. Pour dry rot fluid through a funnel into the holes to irrigate the area. The amount of fluid you need varies according to the brickwork or stone, but for an ordinary common brick walls you should allow about 50 litres per m2. If the masonry is particularly badly affected, call in expert help.

[edit] Life cycle of the dry rot fungus (Serpula lacrymans)

A. Millions of tiny rust-red spores are emitted by the fruiting body of the developed fungus. These spores are transported through the air and some inevitably land on damp wood.

B. When the spores have settled, they begin to develop thin tubes called hyphae.

C. These hyphae spread over the surface of the wood, feeding on moisture.

D. The wood begins to dry out and break into deep, cube-shaped cracks.

E. Eventually the hyphae form themselves into a web-like sheet called a mycelium.

F. A flat pancake-shaped fruiting body, called a sporophore, grows on the surface of the mycelium and begins to emit fresh spores.