rough call - lightweight cladding and monolithic finishes

Architect's memo 60: October 1999

Last month's memo regarding lightweight cladding and monolithic finishes prompted several questions as to what is required in a coating for such a system.

As usual, the answer is not simple. The basic fibre-reinforced sheet is very durable and two coats of a good quality, general-purpose acrylic will give years of service.

Zinc, including galvanised nails, however, is a very reactive metal and will corrode in our mainly marine environment unless protected with a special primer. If the galvanising is damaged when the nail is driven home, another dimension of steel corrosion is added.

The jointing materials are often the weakest link in the system and waterproof coatings in this area can increase their performance.

For all of the above technical considerations, the main driver is aesthetics.

The method of building does not lend itself to producing absolutely flat surfaces with uniform porosity. Smooth, low build coatings will highlight any differences present and consequently the eye needs to be tricked into not seeing them by the use of textures.

Resene produces a range of textures for this purpose from the deep tufted Resene Resitex Coarse, Resene Resitex Medium and Resene Resitex Standard through to the fine textured Resene Sandtex.

The inherent texture of the coarser products provides the best disguise for surface irregularities but also provides a surface more likely to harbour windblown dirt.

The finer products, especially the Resene Sandtex, may not have sufficient inherent texture to disguise adequately and require specific application techniques such as 'Crows Footing' in order to 'trompe l'oeil' - that is, fool the eye.

Aggregate-based textures may not suit all tastes and an alternative can be offered with Resene Thixalon 5. Resene Thixalon 5 has no inherent texture but has a consistency that can produce textures off a roller that have a smoother profile than aggregate-based products.

Whatever the texture chosen, it is imperative that a sample panel of the desired texture is prepared and agreed upon before coating commences. This becomes a yardstick against which progress can be assessed.

It is also of note that, in order to reduce heat build-up, the sheet manufacturers recommend a minimum light reflectance value of 40% for finishing colours. For guidance, that means colours not darker than BS 10A05 (Resene Delta), 22B19 (Resene Spun Pearl), 08C35 (Resene Twine), 16D41 (Resene Gulf Stream) and 20E51 (Resene Danube).

A full specification would therefore call for coating of all galvanised fixings (Resene Galvo-Prime); a coat of waterproofing membrane (Resene X-200); and an agreed texture sufficient to disguise any surface imperfections and to bring out the best in the building system.

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