Curing Woodworm and Dry Rot

Wood under attack is always a cause for alarm. Wet and dry rot, and woodworm, may cause fundamental damage. The signs of attack, once spotted, should be dealt with immediately, eradicating affected areas, replacing, as necessary, with new wood and treating to prevent further unseen encroachment.


The term, woodworm, refers to the larvae of several species of wood-boring beetles which are able to digest the substance of wood. The adult beetle lays eggs on the rough surface of unpolished wood and the grubs which hatch out bore into the timber. These leave no sign of entry, and tunnel inside the wood, for as long as up to ten years.

When ready to pupate, the larvae make a pupal chamber just below the surface of the wood. The adult beetles then bite their way out, leaving tell-tale ‘flight holes’.

Piles of white wood dust, or ‘frass’, on horizontal surfaces, will indicate where the grubs have been active above, and close examination will reveal the flight holes, which vary from 2mm to 4mm across.

Sapwood, which is used in a high proportion of modern building, is particularly susceptible to attacks, and because of their generally small dimensions, modern rafters and joists may not readily withstand severe attack.


Treatment of structural timbers in a house can be carried out by one of the specialist firms, which guarantees work for 20 years and offers a free survey and estimate. If, however, you decide to treat an attack yourself, remember it is no use just treating the area where you see woodworm holes; other larvae may be active but unseen in the adjacent timbers.

Thoroughness is the keynote to success in all timber treatment. For woodworm attacks in rafters, joists and flooring, apply woodworm fluid with a coarse spray using 5 litres to 1850m2 of surface area.

To estimate the area of timber to be treated in a roof where the rafters have been boarded in under the tiles, find the area of each slope of the roof. Add the sums together and to this amount add twice the depth of a rafter, multiplied by its length and by the number of rafters.

If, however, the roof is not boarded, simply add the thickness of a rafter to twice its depth, then multiply by the rafter’s length and by the number of rafters.

Use a similar procedure for the joists and purlins.

Allow a little more, say 90m2, for purlins and gable-end rafters. At a coverage rate of 18-50m2 per 5 litres, a minimum of 38-litres of fluid is needed, but if the timber is very dry, it may soak up more fluid.

Treatment may be carried out at any time of year and modern woodworm fluids, such as Rentokil, will destroy all stages of the woodworm’s life cycle and prevent future attack, provided all timber surfaces are treated.

Before commencing treatment, all timbers must be thoroughly cleaned down to allow penetration of the fluids, and water cisterns should be covered throughout the entire treatment process.

Cistern lagging and roof insulation should be removed or protected from the fluid, and any exposed rubber-covered wiring cables should either be covered or coated with a polyurethane varnish before you start spraying.

Make certain any electrical wiring in the area to be sprayed is sound and well insulated. Never smoke during spraying and wear a pair of old leather or rubber gloves.

Eyes should be protected with suitable goggles and a light fume mask should be worn, to avoid the inhaling of vapour which builds up in the confined roof space.


The selection of a sprayer is important but the majority of garden sprayers are suitable provided they will maintain good pressure. Ideally, the unit should hold at least 5 litres and have a fairly coarse nozzle which will produce a ‘fan’ spray pattern. Suitable sprays may also be hired.

Too coarse a nozzle may result in excess fluid staining the ceiling area; on the other hand, a very fine nozzle will tend to vaporize the spray, making the work unpleasant and reducing the amount of fluid penetration into the timber. A 610mm long extension line will also be required to reach into the roof apex, eaves and any other less-accessible areas.

If you are treating a floor against woodworm, take up every fourth or fifth floorboard, so that you can treat the joists beneath and the undersides of the boards. Replace the boards and then thoroughly treat the upper surface.

It is then necessary to cover the floor area with a large sheet of polythene if you wish to re-lay floor coverings immediately. Alternatively, you may wait seven to 14 days for the surface of the timber to dry out.

After treatment, floorboards will take at least six months to dry out completely, if they have been fully impregnated, and an impermeable floor-covering, such as vinyl tiles or sheet material, will be spoiled if laid directly on them. A temporary floor-covering should be used wherever possible.

Finally, if by accident during treatment you stain plaster with fluid, leave it for a few weeks to dry and if it still remains, apply aluminium primer and then redecorate.

10. November 2011 by admin
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