Care and Maintenance of Corrugated Iron

With a pedigree of almost two hundred years corrugated iron is a much undervalued material that was used extensively in traditional construction. The material is relatively light, portable and its adaptability means that it was often used to replace or cover a thatch roof, or construct a large agricultural or industrial building.

Materials
Corrugated iron was patented in 1828 with corrugations made in flat iron sheets by passing the thin plates through rollers to provide rigidity and additional structural strength. Until the 1920s corrugated sheets were manufactured by a broad range of ironworkers in various lengths, usually up to a maximum of ten feet. Sheet thickness varied from around 0.5 mm to 1.5 mm depending on the application. galvanizing of the iron with zinc became increasingly common at the end of the 19th century.

Sheet pitch and profiles
A wide variety of pitches was available, with standard dimensions varying from 25 – 153 mm. For domestic purposes, a 76 mm (3 inch) pitch was common. A range of profiles were manufactured, with firms such as FW Braby developing their own branded lines.

Coatings
A range of coatings was employed to protect corrugated iron from the elements. Oil (usually linseed) based paints were used extensively in conjunction with red lead paint, with pitch and bitumen to a lesser extent. The galvanizing process involved the iron sheet being dipped into a bath of molten zinc to form a protective layer on the metal surface. The zinc coating chemically protected the iron from corrosion. There were a number of patented ‘rustless coatings’ developed in the late nineteenth and early twentieth centuries then formed protective oxide layers on the surface of the metal and were often known as the Barff process after the inventor.

Construction details and techniques
Supplied in a range of sizes and profiles corrugated iron was frequently used for roofing and walling, and to a lesser extent for fencing and other innovative uses. Manufacturers quickly developed other components that allowed them to construct a building from a frame and corrugated sheet. This included ridging details, ventilators and windows. The retention of these pieces is especially important as most cannot be sourced now. Specialist fixings were required to suit the corrugated sheets, including hooks and bolts (usually galvanized) with specially shaped washers with which to effect watertight seals – particularly on varying profiles. Where these remain, they should be carefully saved, cleaned and set aside for reuse.

Maintenance issues
Corrugated iron buildings are particularly susceptible to damage from grass or other build up of soil / garden vegetation growing against the sheets and trapping moisture which accelerates corrosion. A sound understanding of the building is required. Entirely prefabricated buildings may have a visible manufacturers nameplate which could lead to a search of archive catalogs for referencing. Cleaning gutters and general maintenance is particularly required with corrugated iron structures as decay can happen quickly. Groundwater splashing onto walls also accelerates decay. Junction details, where ventilators and windows etc. have been inserted are problem areas. A good seal is difficult to achieve in the original construction and associated sealing materials often break down and do not perform effectively Corrugated iron roofs may also be found to overlie relatively intact thatch, retained for the insulation it provides, or an intact traditional cruck frame – both worthy of preservation. Corrugated iron has been the saviour of many vernacular buildings.

Corrosion
Since all metals corrode, the primary problem in dealing with corrugated iron is attempting to slow down the rate of corrosion. Corrosion can lead to coating loss, loss of structural strength or integrity, and serious corrosion of the iron can lead to the complete loss of the sheets. Since the sheets are not permeable to moisture, condensation is a problem, and wetting and drying cycles remain the primary cause of decay – internally and externally. The quality of early galvanizing was variable. This often fails, resulting in an oxidised surface (rusted appearance) which is aesthetically pleasing for many due to its natural coloration. In many cases the rusted roofs are seen as an integral part of the rural landscape. Rainwater run off over the rusty sheets onto masonry can be a problem. Where exposed stone is marked in this way the staining is virtually impossible to remove.

Coating failure
Coating failure is common on galvanized surfaces. Effective coating requires the use of a mordant wash to provide a chemical etch for the primer layer, or the use of an etching primer for galvanized surfaces. Often, neither was used. Although there are some firms still making corrugated iron sheets it is important to retain the original sheets as, in many cases, the matching profiles and sizes may be no longer available. It is technically feasible to re-galvanize existing sheets, and to patch repair them in some cases, although the thin gauge of metal makes this difficult to achieve. It is inadvisable to install new sheets directly on top of existing corroding sheets. This will trap moisture, and accelerate corrosion of the new sheet by creating a corrosion cell. Moisture movement and entrapment by capillary action between the sheets is also likely. Any repairs should follow the philosophy of the minimum intervention necessary to reduce the loss of historic material. Wholesale dismantling may be required for full repairs, and the services of an experienced professional should be sought to ensure that this does not place undue stress on other components. Firms experienced in the repair of traditional ironwork should be employed, but it should be appreciated that few will have experience of working with corrugated iron. Sheet metal workers who are sympathetic to the historic qualities of the material and posses a knowledge of how it performs may also be considered.

Painting
Over painting sheets should follow traditional practice. It may be possible to identify the original paint on existing ironwork and other local examples might provide an indication of the original color. The use of red lead for roofs and walls was common. This material is still available and its use does not require statutory consent to use. Green and red colors were fairly commonly applied, as were creams and browns in the twentieth century.