The word ‘clad’ is related to the word ‘clothe’, and cladding is precisely that: covering one material (or surface) with another. The drawing rooms of the eighteenth century upper classes were richly clad in mahogany and oak panelling, while American pioneers clad the outside of their houses with hand-split oak or cedar boards. In Australia, weatherboards were introduced within a few years of European settlement.
With a history dating back hundreds of years, the warmth and texture of timber cladding give it a timeless appeal. Construction Review shares some tips for making the correct choice for your exterior or interior cladding application.
In a nutshell, green building is about reducing the impact of buildings on the natural environment – humans included. In order to do this, it focuses on increasing the efficiency with which buildings use energy, water and materials, as well as reducing or replacing environmentally-damaging building practices and materials. Green materials include products that reduce material use, products with exceptional durability and low maintenance, certified wood products, rapidly renewable products, salvaged or recycled products, products which avoid toxic emissions, minimally-processed products and products that reduce environmental impacts during construction, demolition or renovation.
In modern buildings in Europe, America and Australia, external timber cladding is an increasingly popular choice for the external building envelope. Less developed on the African continent, timber – or timber composite – cladding presents numerous possibilities for architectural expression, as well as for providing a rapid, cost-effective solution for low-cost housing delivery.
Not only attractive and versatile, timber is also remarkably resilient and offers a higher strength-to-weight ratio than rigid building materials. Thanks to its sound dampening properties, it helps to keep noise out of buildings and reduce reflected noise in confined outdoor spaces. Provided it is harvested responsibly, it can also be environmentally friendly, being reusable, recyclable and low in embodied energy.
Choosing timber cladding
A number of factors need to be considered when choosing timber cladding: • Will it fit with the architectural style?
Is it available and affordable?
Can it be easily installed and maintained?
Will it cope with the environmental conditions (including rain, wind, steam, sun) to which it will be subjected?
Does it need to provide additional strength for the structure?
Do local government regulations or statutory requirements permit its use?
Exterior versus interior timber cladding
Often referred to as siding or weatherboarding, external timber cladding is available in a variety of finishes, including traditional shiplap, rough sawn planks, vertical or horizontal tongue-and-groove, or solid sheets (using materials such as marine ply and shutterply). Rainscreen cladding (popular in the UK and New Zealand) consists of timber cladding attached to a timber or aluminium fixing structure, and then fixed to the wall, with a ventilated and drained cavity separating the two.
Interior cladding, often referred to as panelling, provides acoustic and thermal insulation and, by regulating humidity levels, helps maintain a healthier living environment. Long evocative of the 1970s and 80s, timber cladding can still be used to contemporise an interior by inviting touch and providing a contrast to hard finishes and soft furnishings. Tongue-and-groove boards can be painted or stained in a contemporary finish to complement interior decor. Large wooden panels (solid or ply) can also be used in a variety of ways, such as at the head of a bed or as feature elements to create a certain atmosphere.
Timber cladding is being used more creatively than ever in interior applications, including traditionally taboo areas like bathrooms and kitchens. The key lies in getting the design right and constructing and installing the panels in such a way as to reduce the possibility of moisture ingress. Many timber claddings are available with factory-applied finishes and pre-manufactured fixing clips for easy fitting.
Designing for durability
Moisture can be one of the greatest enemies of timber cladding if it is not properly designed, manufactured and installed. The Australian National Association of Forest Industries offers the following practical tips for getting the most out of your external timber cladding (some apply to internal installations too):
Protect timber from direct rainwater by using large eaves or installing timber cladding on a verandah.
Ensure that no water leaks into the structure at corners, and where cladding meets doors, windows and other walls. Install flashing at corners and vertical joints.
Terminate the cladding at least 150mm above ground level to avoid moisture uptake, and slope the bottom edge inwards and upwards to form a drip initiator. If the cladding comes closer, slope the earth or pavement away from the wall to encourage runoff.
Make sure the cladding does not touch any porous materials (such as masonry), and allow a 5mm gap between cladding sections to prevent moisture transfer.
Seal the end grain.
Include a well-ventilated and drained cavity of at least 19mm behind the cladding.
The moisture content of timbers should be between 13% and 19%, less for heat-treated timber.
Use stainless steel or hot-dipped galvanised fixings – never copper. The extractives from the timber will react with the copper, causing copper fixings to corrode.
Install sarking (a waterproof but vapour-permeable flexible sheet material that is fixed directly behind timber cladding or under roofing material) in areas subject to high winds and wind updraughts, or where boarding is fixed diagonally or vertically. Sarking prevents wind- and storm-driven water penetrating the cladding by directing it back to the outside of the structure.
Fasten cladding so that the boards are able to shrink and swell individually and so reduce the chance of cupping, cracking and splitting. Don’t use nails to fix two adjacent boards together.
Use a water-repellent preservative on freshly-cut surfaces and on cladding that is to have a natural or stained finish.
Solid Timber Cladding
In addition to solid wood’s natural allure, the wide variety of widths, profiles and fixing options available makes solid timber an extremely popular cladding material. When choosing a hardwood cladding, one should ideally choose naturally durable species. Timbers that are classified as “slightly durable” (that is softwoods such as pine and all sapwoods) have low natural durability and should be CCA (copper chrome arsenate) preservative treated before installation outdoors.
An alternative to chemical treatments is heat treatment (known in British Standards as ‘thermal modification’), which involves heating timber to temperatures of more than 200°C to improve durability and stability. The heartwood of timber, which is classed as "durable" and "moderately durable" need not be treated for external cladding purposes.
It is also important to use the correct grade of timber for the application. For example, external walls subject to moderate to severe weather exposure should be clad in the highest grade of timber available.
All timber absorbs or looses moisture in response to changes in the humidity of its environment, which causes it to expand and contract slightly. Its moisture content (termed equilibrium moisture content, or EMC) may range from as little as 6% to over 20% (as does external cladding). Most timber cladding is supplied kiln dried or seasoned, and has an EMC of 10-15%. Unseasoned cladding should be fixed immediately after delivery and allowance made for shrinkage as it dries.
To avoid the appearance of small cracks or surface “checks” on natural-finish timber, a water repellent coating should be applied regularly. This will also preserve the surface colour of the timber, which will turn silver-grey if left uncoated. (Avoid using linseed oil as a natural finish as it encourages mould and fungi, which discolour the timber.) For long-term durability, apply an acrylic, polyurethane or oil-based paint or stain.
Choosing solid wood for interior cladding
When making a timber choice for interior panelling, it is important to weigh cost against factors like maintenance and lifespan. Tongue-and-groove pine is the cheapest and most common material for internal cladding, followed by Saligna – both of which are grown in South Africa. For a more durable and exotic-looking material, Kesh Rambharose of Country Woods in Durban recommends Northwest African hardwoods such as African Rosewood, Rhodesian Teak and African Mahogany, or the exceptionally stable Okoume wood from Gabon, Cameroon and parts of the DRC. Rosewood, kiaat, chamfuta and teak from Mozambique and Zambia are also popular interior cladding options.
Angelo Coppolo of Coppola Bros (Pty) Ltd in Durban has installed timber cladding at many upmarket venues over the years, including the Elangeni Hotel, the Beverley Hills Hotel and Zimbali Lodge. Coppolo recommends mahogany and oak for internal cladding, especially in areas subject to moisture. In such areas, though, even the most moisture-proof timber is at risk of decay if proper groundwork is not done to prevent moisture ingress. Installed ready-sealed, Coppolo Bros’ cladding requires no maintenance other than a wipe with a damp cloth. Unsealed cladding can be maintained by oiling it once a year with a quick-drying, flake-proof treatment such as Danish oil.
Plywood Cladding
Plywood consists of an odd number of wood veneers bonded together using a resin such as urea formaldehyde (for interior applications) or phenol formaldehyde (exterior). The grain of each layer being perpendicular to the next, plywood is more stable in both directions than the equivalent thickness of board in its solid form. Cheaper than most solid timber, plywood can be used for interior paneling and also as an economical exterior for residential, commercial, industrial and rural buildings. Its good acoustic properties, durability, rigidity and suitability for a range of finishes make plywood a popular cladding choice, especially in Australia.
Sealing the bottom edges of sheets in contact with the floor before installation is vital to prevent moisture ingress and delamination. It may also be necessary to treat plywood against fungal and insect attack using a CCA pressure treatment or equivalent. Finally, remember to accommodate a moderate degree of expansion in response to changing moisture levels.
Plywood for exterior cladding comes in three main varieties: Exterior Hardwood Plywood, Marine Plywood and Pine Shutterply. The first uses a hardwood species throughout its construction and is bonded together using an exterior grade waterproof and boilproof phenolic resin (WBP). Exterior hardwood plywood is useful for applications requiring the appearance of a natural veneer, but at a lower cost.
Marine plywood is manufactured from selected durable tropical hardwoods with a natural resistance to wood-destroying fungi. The hardwood veneers are bonded with phenolic adhesive for water resistance, heat resistance and strength. Marine plywood is used extensively in both the marine industry and for exterior applications.
Pine shutterply, whose appearance is secondary to its functionality, consists of a phenolic adhesive-bonded board covered with a C/C grade face which is often coated with an additional resin for improved water resistance. While shutterply is designed primarily for temporary moulding, it can also be used for cladding steel-frame structures and for rustic interior panelling and ceiling cladding.
Fibreboard Cladding
Fibreboard or particleboard consists of wood chips or fibres coated in a resin and compressed into board form – be it hardboard, chipboard or medium-density fibreboard (MDF). Fibreboard is mostly used for interior cladding applications, but an exterior-grade version has been developed in Australia and is suitable for most exterior cladding applications. Board ends are supplied slightly bevelled, and the face, edges and ends are factory prime-coated. For exterior cladding using fibreboard, all openings should be flashed.
Vapour-permeable sarking should also be used directly under the cladding in walls that are subject to high wind conditions and wind updraughts. Finish coating should be applied to this type of cladding within sixty days of fixing.
For internal cladding purposes, MDF is an economical and easily workable alternative to solid- or plywood cladding. PG Bison’s version, BisonBord, is exceptionally strong and easily machined, and is used extensively (both in its raw state and as a substrate) in shopfitting, kitchen manufacturing and the furniture industry.
Its surface is ideal for priming, painting, printing, veneering and laminating. PG Bison’s BisonBord V313 has further additives for increased moisture resistance in areas such as bathrooms and laundries. Both types of MDF can be wrapped in a PVC or acrylic wrap for a decorative finish.
Wood plastic composite
First introduced as a timber substitute for external decking and outdoor furniture, wood plastic composite (WTC) or timber plastic is beginning to make its mark in the cladding market. WTC profiles are manufactured by an extrusion process using a mixture of recycled plastics and hardwood wastes with various additives. The resulting standard- or timber-look product sports numerous benefits, including:
resistance to heat, cold, weathering and UV radiation
a range of colour-fast colours, including traditional timber finishes
a life span of several decades
low to no maintenance
excellent insulation properties
availability in solid or hollow section
horizontal, vertical or diagonal fitment
As demand increases for faster and more eco-friendly building methods, there is little doubt that timber cladding will feature strongly as a material of choice across the building spectrum – from high-end residential and commercial buildings to low-cost housing projects. Wood plastic composite cladding is an area with particular potential for growth.