Curing and preventing wet and dry rot

Wet and dry rot can seriously weaken the structural stability of a house and may drastically reduce its value. This section on the subject deals with general causes, why they happen, and how you can go about sorting them out and curing them.

Even masonry-built houses contain substantial amounts of timber—in the roof, floors and so on. So it is vital to identify signs of decay and deal with them as quickly and thoroughly as possible. Early detection and cure becomes even more important with timber-framed houses.

Wet rot, which often attacks window and door frames as condensation drips down the glass, is relatively easy to spot and treat. But dry rot is more insidious: it may start undetected in the roof or beneath the floor then spread from here throughout the entire house.

By far the best way to deal with these attacks is to get some idea of the nature of the various types of rot. As with most things, once you know how they work, they become much easier to stop.

Dry rot

Dry rot is caused by a fungus. 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, and behind skirtings. In such conditions the growth can easily spread at an alarming rate. Whole houses have become infected throughout in as little as three 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 These are carried through the air spreading the decay if they land orj further damp wood. Even if the affected wood is removed, residua] fungal strands may continue growing and eventually infect sound timber.

The treatment for dry rot differs considerably from that for wet rot. U 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 between the fingers.

Wet rot

Wet rot is a general name for the damage caused by any of several fungi, the most common of which are Conio-phora cerebella and Fibroporia vail-lanti. These fungi require more moisture than dry rot fungus and develop where timber is in direct contact with wet or damp surroundings. Exterior joinery, where the paint film has cracked allowing water to penetrate, is a typical starting point for wet rot. The remaining paint film prevents any water drying out by evaporation, so the wood swells, joints open and wet rot spores enter the cracks.

To detect the first signs of wet rot, inspect any cracked paint on window sills or door frames. A thin veneer of surface wood may conceal a soft, dark mass of rotted wood dust beneath. The fungal strands of wet rot fungus are thinner than those of dry rot and are dark brown in colour. When growing over wood, these strands often develop a fern-like shape.

Treating 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 the UK 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 damproof courses broken or overflowing guttering and check for 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 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.

Treating walls and plaster

Where plaster is within 1m of a dam aged 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 before attempting to redecorate. When you come to re-plaster 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 10mm holes, 150mm deep and sloping downwards at an angle of 45 , at 225mm 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 fletton wall you should allow about 50 litres per m2. If the masonry is particularly badly affected, call in expert help.

Treating wet rot

Outbreaks of wet rot are much more frequent than those of dry rot but are seldom as difficult to treat. The fungus stays around the original site of damp wood in which it germinated, making it easier to isolate.

The treatment itself is less drastic than that required for dry rot. If the cause of dampness is rectified and the timber is allowed to dry out, there should be no further growth. Therefore, it is necessary only to cut out affected wood which has been seriously weakened by the rot.

However, it is essential to make sure that timbers around affected areas are treated with fungicide. As the timber dries out the moisture content falls until conditions are just right for dry rot to occur.

To check the extent of damage in decaying wood, prod all timbers in the affected area with a strong, pointed tool such as a bradawl. If the bradawl sinks right into the wood, then it is badly decayed.

Cut out and burn all timber that has suffered breakdown and remove all dust, dirt and debris. Select thoroughly dry, well-seasoned timber for replacement, cut it to size and treat it as for dry rot. Also treat the surfaces of adjacent timbers and brick, block and concrete areas with fungicide before installing the replacement wood section.

Expert help

Detecting rot—especially dry rot— is often a job best left to specialist firms, many of whom offer a free surveying service. Bear in mind too that if specialists carry out the work, they will give you a guarantee—a positive benefit when you come to sell your home.

In Canada, do-it-yourself treatment is not really possible, as hardly any fungicides are available over the counter. On the other hand, there is much greater use of already-treated timber.

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