Caring for Wooden Greenhouses
The last ten years have seen rapid transition from wood to alloy as constructional material for greenhouses. Aluminium alloy has many advantages. It is strong (where extruded), light and extremely resistant to corrosion without any painting or other treatment. Superior or properly pressure-treated wood cannot, however, be discounted and is still viewed with much favour, since a tightly glazed wooden greenhouse is considerably ‘warmer’ than an alloy one, which means in effect that wooden houses cost less to heat. There are usually fewer condensation problems in a wooden greenhouse.
Main timbers available
The four timbers mainly used are Baltic redwood (Pinus sylvestris), British Columbian pine (Pseudotsuga menzieii taxifolia), Western red cedar (Thuja plicata), and Norway spruce (Picea abies/excelsa). Hardwoods such as oak and teak are only used to a limited extent on account of their far higher cost.
Strength/bulk ratio is good, being 497kg/m3 (31 lb/cu ft) and it does not split badly when being nailed. It is readily impregnated with preservative.
British Columbian pine
Similar in many ways to Baltic redwood, it can be obtained in long lengths. It is difficult to impregnate with preservative. It has a strength/bulk ratio of 529kg/m3 (33 lb/cu ft) and is therefore broadly similar to Baltic redwood.
Western red cedar
This is an excellent wood for amateur greenhouse work when it is not used in long lengths, as it has a poor strength/bulk ratio of 384kg/m3 (24 lb/cu ft); for this reason larger-sized members may have to be used, particularly for astragals and ridge boards in exposed areas or where heavy snowfalls are frequent. Its chief virtues are inherent resistance to decay and excellent appearance, which makes it highly acceptable aesthetically for porches or conservatories adjoining the home. Western red cedar lends itself to treatment with oil.
Quite widely used for cheaper greenhouses on account of low cost, but to achieve reasonably long life it must be treated or painted, preferably both. Being soft it is very easy to work and it does not split readily in the process. It has a strength/bulk ratio roughly similar to redwood.
Teak and oak
Both of these are specialized woods of very high durability, having bulk/strength ratios of around 721kg/m3 (40 —41 lb/cu ft). Easy to work, they are excellent where a greenhouse extremely resistant to varying climatic conditions is wanted, but when used in smaller sections they will make the greenhouse very expensive.
A greenhouse may be constructed of one type of timber, a variety of timbers, or a mixture of steel and timber.
Preservation of timber
This is a task best left to the timber or greenhouse supplier, but it is useful to know something about preservative treatment. Assessing the quality of timber is work for a specialist, but even the uninitiated can judge whether timber is free from small knots, large knots which may be loose, and ‘splits’, or whether it is too sappy (new). Many defects can be covered up with a coat of priming paint. Ideally timber should be both pressure treated and painted, unless of superior quality, although either will suffice in part. Paint protects the wood from the entry of moisture, which in turn protects it from decay, provided a perfect cover of paint is maintained. Preservatives impregnate the wood and immunize it against both the entry of moisture and decay, but obviously a painted greenhouse is more attractive than one which is merely preservative treated.
Western red cedar is usually treated with linseed oil or special paints which is simply brushed on when the wood is dry.
It should be realized that hand painting as a method of applying preservatives only results in penetration of the skin of the wood, which means that this will require fairly regular treatment.
Painting of timber
The painting of greenhouses is rather a boring task. Some manufacturers send their wooden greenhouses out either flat coated or fully painted, and the latter is to be preferred.
Newer paints based on organic oils and synthetic resin with pigments other than lead, have considerably better light reflective qualities than those based on lead. They are also longer lasting and do not powder or dissolve. They must, however, be applied to dry timber, otherwise they -will soon blister or flake off.
When painting wood it is usual to apply a priming coat to timber which has not previously been painted. This should be wire brushed before applying the primer if necessary. If you arc erecting the greenhouse yourself, it is important to put on the priming coat before erection, otherwise joint surfaces are left unprotected. Next comes the undercoat, which may be put on after erection but before glazing. The final coat is put on after glazing.
Repainting will be necessary every three or four years on untreated or poor quality timber, perhaps more frequently in industrial areas. It is largely because they do not need painting that aluminium alloy, superior or preservative treated timber, and treated metal houses are now so popular.
Whatever type of paint is used, it is better not to bring plants into a newly painted greenhouse for a few days after painting, since they could be damaged by fumes.