The high tensile strength of steel and its high strength/bulk ratio make it an ideal structural material for all buildings, and greenhouses are no exception. Many commercial greenhouses now use steel for the gutters (now often alloy) and the supports, for purlin posts and rails, side and end supports, roof trusses and pitch frames whether they are cladded (the astragals holding the glass) with wood, steel or alloy.
The main problem with many steel frame glazing bars lies in the fact that steel of sufficient thickness, unlike aluminium alloy or expanded metal, cannot readily be pressed into a shape to allow a glazing method which secures the glass effectively without the use of a putty or bitumen type of glazing system.
Steel must of course be properly treated against corrosion, otherwise it will quickly rust, and hot dip galvanizing is essential after the steel has been cut to size, shaped and all holes bored. Aluminium paint is also very effective as a rust preventive for steel, provided the steel is reasonably clean before the aluminium paint is applied. Enamel is also used for painting over galvanized steel. Stainless steel can also be used with a coat of enamel.
Personal experience over many years with the steel components of commercial and amateur greenhouses has shown the inadequacy in many cases of the metal galvanizing treatment. A further problem with steel, especially if used in conjunction with aluminium alloy, is the bi-metallic corrosion caused by electrolytic action. Even the water running from alloy glazing bars on to galvanized steel gutters can give problems. All manufacturers now ensure that a plastic seal prevents the actual contact of the two metals, although nothing can be done about the shed water. Fortunately this is a problem which does not usually occur with amateur-sized greenhouses, as gutters on alloy greenhouses are also invariably of alloy.
Earlier types of alloy were subject to corrosion, especially in areas of salty air or industrial pollution. Even now considerable powder deposit may form on the alloy, but this need not give rise to concern. Aluminium alloy has an excellent strength/ bulk ratio and, being a light metal, it can readily be extruded into sections of various shapes, the sections all being designed to give additional strength. It can also be shaped into complicated sections to form drip channels and to allow either clip or groove glazing, this latter being a system which has gained considerable favour in recent years since it gives a really tight seal and prevents air losses.
Direct contact with steel should be avoided, and where this is impossible it is most important that the steel should be protected with bitumen paint. Where aluminium alloy gutters are used, drip channels should be provided underneath to prevent the condensation droplets damaging crops, the condensation occurring owing to the high thermal conductivity of alloy. With commercial greenhouses it is advisable to use gutter support braces to ensure that the alloy gutter in a long section house does not sag, which can happen with snow load or under a worker’s weight when doing repairs.