Your Questions Answered Dual Purpose Timber Treatment Products - Are they worth it?
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Dual Purpose Timber Treatment Products - Are they worth it?
Dual Purpose Timber Treatment Products - Are they worth it?
‘Dual Purpose’ in timber preservation terms means a treatment formulation that is designed to be effective against wood boring beetle infestation as well as wood rotting fungi - but is there any benefit to be gained by the application of such material?
For the control of wood boring beetle a standard dual purpose formulation should certainly be effective since most contain the same level of active insecticide as a normal insecticidal preservative. But is the fungicidal content sufficient to control wood rotting fungi such as wet rot and dry rot and would a surface application alone prevent such an outbreak in the future?
When a timber preservative is applied in an existing building the method of application is usually spray but it matters little how it is applied. Most standard surface applied dual purpose timber treatment formulations only penetrate into timber between 4-6 mm which is sufficient for the control of wood boring beetles but is this enough to control wood rotting fungi? If the preservative only penetrates 4-6 mm then the bulk of the timber has no preservative protection at all. Furthermore access to all surfaces of timbers already built into a structure is limited. In terms of floor construction embedded joist ends, wall plates and bearing surfaces of sleeper wall plates will receive no treatment whatsoever yet it is these vulnerable points where fungal decay is most likely to start if favourable conditions are present. Once fungal decay has developed at these vulnerable points it then has unrestricted access into the mass of timber within the ‘envelope’ of the preservative applied; it is not bothered by what may have been applied onto the surface. Wood rotting fungi in an existing building is therefore unlikely to be controlled or prevented by the surface application of a standard ‘dual purpose’ preservative alone.
Wood with a moisture content that does not exceed 20% will not rot since there is insufficient moisture present to initiate an attack by wood rotting fungi. Timber with a moisture content in excess of 20% is considered ‘at risk’ to fungal decay. The single most important point when controlling wood rotting fungi is to keep timber ‘dry’ - that is below 20% moisture content but 10 -12% is much better.
If timber is wet then whatever is causing this to happen should be rectified and any damp timber dried out under well ventilated conditions. To achieve this at ground floor level it is very often necessary to improve sub floor ventilation level by the introduction of additional air vents as well as increasing the size of any existing 9” x 3” vents to 9”x 6”. Terra cotta type vents should be avoided as they are only 20% efficient and have the ability to bridge a damp proof course but alas they are the easiest to fit hence they are popular The best type of air vent to use is square pattern cast aluminium or plastic louvre type as these are much more efficient.
So is it worth bothering to apply a standard dual purpose formulation? Almost certainly not. Furthermore to use such a material without adequate justification would be the unnecessary use of a biocide and be in contravention of the COSHH regulations.
It is still a source of amazement and disappointment that many contractors’ still use phrases like ‘We will apply a dual purpose insecticidal and fungicidal preservative’ even if they are just treating woodworm in a roof! Perhaps in ignorance the contractor believes that it ‘looks more impressive’ to a potential client but irresponsible use of such material by a contractor also throws their guarantee net far wider than they need or perhaps even realise.
‘Dual Purpose’ in timber preservation terms means a treatment formulation that is designed to be effective against wood boring beetle infestation as well as wood rotting fungi - but is there any benefit to be gained by the application of such material?
For the control of wood boring beetle a standard dual purpose formulation should certainly be effective since most contain the same level of active insecticide as a normal insecticidal preservative. But is the fungicidal content sufficient to control wood rotting fungi such as wet rot and dry rot and would a surface application alone prevent such an outbreak in the future?
When a timber preservative is applied in an existing building the method of application is usually spray but it matters little how it is applied. Most standard surface applied dual purpose timber treatment formulations only penetrate into timber between 4-6 mm which is sufficient for the control of wood boring beetles but is this enough to control wood rotting fungi? If the preservative only penetrates 4-6 mm then the bulk of the timber has no preservative protection at all. Furthermore access to all surfaces of timbers already built into a structure is limited. In terms of floor construction embedded joist ends, wall plates and bearing surfaces of sleeper wall plates will receive no treatment whatsoever yet it is these vulnerable points where fungal decay is most likely to start if favourable conditions are present. Once fungal decay has developed at these vulnerable points it then has unrestricted access into the mass of timber within the ‘envelope’ of the preservative applied; it is not bothered by what may have been applied onto the surface. Wood rotting fungi in an existing building is therefore unlikely to be controlled or prevented by the surface application of a standard ‘dual purpose’ preservative alone.
Wood with a moisture content that does not exceed 20% will not rot since there is insufficient moisture present to initiate an attack by wood rotting fungi. Timber with a moisture content in excess of 20% is considered ‘at risk’ to fungal decay. The single most important point when controlling wood rotting fungi is to keep timber ‘dry’ - that is below 20% moisture content but 10 -12% is much better.
If timber is wet then whatever is causing this to happen should be rectified and any damp timber dried out under well ventilated conditions. To achieve this at ground floor level it is very often necessary to improve sub floor ventilation level by the introduction of additional air vents as well as increasing the size of any existing 9” x 3” vents to 9”x 6”. Terra cotta type vents should be avoided as they are only 20% efficient and have the ability to bridge a damp proof course but alas they are the easiest to fit hence they are popular The best type of air vent to use is square pattern cast aluminium or plastic louvre type as these are much more efficient.
So is it worth bothering to apply a standard dual purpose formulation? Almost certainly not. Furthermore to use such a material without adequate justification would be the unnecessary use of a biocide and be in contravention of the COSHH regulations.
It is still a source of amazement and disappointment that many contractors’ still use phrases like ‘We will apply a dual purpose insecticidal and fungicidal preservative’ even if they are just treating woodworm in a roof! Perhaps in ignorance the contractor believes that it ‘looks more impressive’ to a potential client but irresponsible use of such material by a contractor also throws their guarantee net far wider than they need or perhaps even realise.
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