What causes decay?
What are these conditions?
How can decay be prevented?
What are wood preservatives?
What kind of preservative should be used?
How can preservatives be applied?
What role does paint play in wood protection?
What finish should be used on the interior of a fish room?

What causes decay?

Wood rot, or decay of timber, in fish rooms is caused by fungi, which are tiny living plants consisting of a mat of very fine hairs or hyphae. These start to grow on the surface of damp wood and later penetrate more deeply, breaking down and feeding on the substance of the wood, and spreading to neighbouring pieces of timber. Occasionally, like any other plant, the fungus produces fruiting bodies, which may be seen on the surface of the wood. These vary in appearance, depending on the particular type of fungus producing them. Each of these bodies produces millions of tiny seeds, or spores, which are so light in weight that they can easily be dispersed by the slightest draught of air, and can infect other pieces of timber, even some distance away, under certain conditions.

What are these conditions?

Decay normally develops in the presence of moisture, and in the absence of good ventilation since this helps in the formation of an atmosphere rich in moisture. It often occurs when wet or unseasoned timber is used, particularly where there is poor ventilation, as is often the case behind the linings of fish rooms.

Other more obvious sources of moisture are bilge water, ice melt water, condensation due to warm air meeting a cold surface, and leaks. These sources of water are particularly important when fresh water is involved. The salts present in salt water tend to inhibit the growth of most moulds and fungi, hence the old practice of packing rock salt between the hull and lining so that any water leaking in formed a brine solution which discouraged fungal growth on the wetted wood.

Temperature also affects fungal activity, accelerating the rate of growth at higher summer temperatures, while almost stopping it at temperatures near freezing point. Relatively high temperatures, such as they employed in steaming timbers before bending, kill any fungus or spores present on the wood, but the timber is not, however, rendered immune to later attack by fungus.

How can decay be prevented?

Where timber is to be used in permanently or intermittently damp conditions, or where it is not readily accessible for frequent inspection, as behind fish room linings, it should be either of a naturally durable species, or it should be treated under pressure with a wood preservative. The durability of the different timbers can be obtained from Forest Products Research Record No. 30, 2nd ed. 1959, "The Natural Durability of Timber" and only those timbers listed as "durable" or "very durable" should be used in the above conditions. It is only the heartwood of these timbers that is durable; the sapwood is normally very susceptible to decay and should always be excluded.

Table based on average life of untreated 2 × 2 in. stake in ground

Perishable 5 years or less	Non-durable 5-10 years	Moderately durable 10-15 years	Durable 15-25 years	Very durable Over 25 years
SOFTWOODS (HEARTWOOD)	Fir, (home-grown) Scots Pine Redwood Spruce Whitewood	Douglas Fir Larch Pine 'Western red Cedar' (Home-grown)	'Western red Cedar' Yew Pitch Pine (Honduras)
HARDWOODS (HEARTWOOD)
Ash	Elm	African Mahogany Sapele	Agba American Mahogany European Oak	Afromosia Greenheart Iroko Purpleheart Teak

Nearly all the naturally durable timbers are hardwoods and in general this type of material is more costly and often rather less convenient for this type of work than softwoods. None of the commonly used softwoods however, such as Baltic redwood, spruce or Douglas fir, is sufficiently durable to use without thorough preservation, but if treated under pressure with a suitable preservative they would give long service without trouble from decay. Of these timbers, Baltic redwood is to be preferred since it accepts preservative treatment more readily than the others.

In addition to natural durability, particular attention should be paid to the following points: -

1. Good ventilation. The fish room should be so constructed that there is a good supply of air directed round and through any wooden structure, with no pockets of dead air. A good through draught of dry air helps in the removal of the moisture necessary for the growth of fungi, but obviously it cannot deal with large quantities of water such as that produced by melting ice.

2. Reduction of moisture. The need for this has already been mentioned, but in addition it should be remembered that, where wood is in contact with water, this is absorbed, much more readily by the end grain where this is exposed at butt-ends, bolt-holes and joins. These regions should therefore be carefully protected, as described below.

3. Use of wood preservatives.

What are wood preservatives?

These are chemicals which, when properly applied, prevent the growth of decay fungi and they should be used to treat the less durable types of timber employed in construction. Generally, preservatives fall into three main groups; tar oil types, organic solvent types and water-borne types. The tar oil types, such as creosote, have a strong tainting effect and also interfere with subsequent painting; water-borne types may probably be more valuable in new construction as they generally necessitate application in special tanks; the organic solvent types are probably the most suitable for general use in fish rooms. Essentially, the last class are solutions of fungicides, such as copper naphthenate or chlorinated phenols in white spirit or other volatile solvent, which evaporates, leaving the fungicide in the wood. This permits subsequent painting of the wood where this is desired.

What kind of preservative should be used?

There are many brands of preservatives on the market, in which the active fungicide present may be borax or other boron compounds, copper compounds, organic-mercurial or chlorinated phenol products, etc. The final choice of preservative must rest with the user, preferably after consultation with the manufacturer, bearing in mind the location of the wood to be preserved. Where wood is likely to come into direct contact with fish, it is essential that the preservative used does not taint foodstuff's and is non-toxic to humans.

How can preservatives be applied?

The effectiveness of any wood preservative depends on the way in which it is applied. In general, impregnation under pressure is by far the most effective method, particularly where a copper-chrome-arsenate compound is used. The softer and more porous timbers can be given permanent protection against decay by these means. The timber should be cut to size, drilled and shaped before treatment; if any cutting to length or drilling afterwards is unavoidable, the exposed surfaces must be further treated with preservative.

Soaking or dipping, particularly of small items, where there is much end grain, is effective in getting the fungicide to vulnerable regions. Surface application is generally not a very reliable method for long-term protection where there is a high decaying hazard, unless the treatment can be renewed periodically, say every two years or less.

Some whitewood is now obtainable in this country that is impregnated with borates as part of the seasoning processes, and this timber might be useful in situations where serious leaching-out of the preservative would not take place.

What role does paint play in wood protection?

Paint, well applied, prevents direct absorption of water in liquid form. It reduces the rate of water vapour uptake or loss, but does not prevent a large uptake of water over a long period. Thus, in some cases, paint helps to prevent decay, but in other cases it may encourage it. For example, heavy white lead primers are useful as end grain sealers at joints, bolt holes, etc., and whole external surfaces may be painted to prevent direct absorption of water. But where the coating of paint is old and cracked, and does not completely cover the whole surface, moisture may seep in and be trapped behind the paint, so setting up pockets of decay. On internal paintwork in a confined space, paint may reduce the beneficial effects of ventilation and here it may be better to apply wood preservatives, rather than paint. Where paint is necessary, it should be applied over suitable wood preservatives, as the ingredients of ordinary paint are only mildly fungicidal.

What finish should be used on the interior of a fish room?

Here the question is perhaps more one of hygiene and its effect on the quality of fish, than of active prevention of wood rot. This requires a coating which should be:

1. Smooth with a hard surface, easily scrubbed.

2. Impervious to water, slime, blood, bacteria and fungi.

3. Odourless, non-toxic to humans and non-contaminating to fish.

4. Resistant to mechanical abrasion and to chemical action produced by sea-water, cleaning agents, weak ammonia and amines from fish.

5. Non-brittle and reasonably non-skid.

6. Durable, to keep down maintenance costs.