The information below sets out some important aspects that need to be considered at the design stage.
Cladding Design Considerations
Cladding profile styles
The texture, size, profile, orientation (horizontal, vertical, diagonal) of boards and the natural appearance or addition of colour offers infinite design flexibility and encourages innovation.
There is a wide choice of standard profiles available in softwoods, modified woods and hardwoods typically up to 150mm in width.
Styles can vary from one manufacturer to another so always obtain samples and agree quality parameters prior to confirming your selection.
If you require a bespoke profile or a specific species then contact a member of the TDCA; they will be more than happy to discuss your requirements.
Only specify cladding manufactured under a recognised quality scheme such as the TDCA CladMark/ISO09001.
Thickness is largely determined by the profile required. Guidance is available in British Standard BS8605:External Timber Cladding Part 1: Method of specifying.
Ease of maintenance
The ease of being able to access cladding for maintenance and to repair or replace boards that have been damaged or have failed is a key element in material choice.
If access is relatively easy and regular maintenance costs are acceptable then a wider range of options including lower durability species is available. Where maintenance is less easy or expensive or the consequences of failure are not acceptable then species with medium to high durability should always be selected.
The table below sets out the service factors that help to determine what level of durability is required. Service factor C is typical for cladding.
|Service Factor Code||Description of risk and consequences of failure|
|A||Negligible risk of failure. Enhanced durability optional.|
|B||Where risk of failure is low and use of materials with enhanced durability can be regarded as an insurance against cost of repairs, and/or where replacement of timber or remedial action is not difficult or expensive.|
|C||Where risk of failure is high and/or where replacement of timber or remedial action is difficult and expensive. Enhanced durability is desirable.|
|D||Where risk of failure is very high and/or where failure of timber components would result in serious danger to structure or persons. Enhanced durability is essential.|
- Timber cladding shall have a moisture content of 16% ± 3% at the time of installation.
- Wood expands across the grain, rarely along it.
- Species have different rates of movement - see cladding wood types.
- Oak, pine and spruce will experience a 1% dimension change for every 4% change in moisture content.
- Fungi become inactive below 5°C. The impact of climate change and warmer winters must be taken into account in specifying durability.
Wood is a natural material. It contains moisture and has the ability to increase and decrease its moisture content relative to its surroundings. Different species have different degrees of movement and this must be accounted for in cladding design.
Minimising the consequences of movement
As temperature and humidity levels change, wood expands or contracts to reflect (be in equilibrium) these changes. The moisture content of external timber cladding will fluctuate from around 12% in summer to 20% in winter or after heavy rain.
To minimise the effects of movement, TDCA recommends that at the time of installation timber cladding has a moisture content of around 16%. The only exception to this is where unseasoned wood is the desired cladding material e.g. “green oak” for which additional allowances are made for movement in its design and fixing.
As wood expands and contracts most movement takes place across the grain, rarely along it. Different timber species have different rates of movement and it is important to consider these before selecting a species for cladding and determining the board profile and layout – shrinkage may result in gaps that are unacceptable and compromise surface coatings, exposing bare wood.
Moisture and biological organisms
The risk of decay and surface moulds is significantly reduced if timber cladding remains below 20% during service. Design and installation should include a) measures to prevent the uptake of additional moisture through end grain and b) configurations that discourage water retention.
The threat of fungal decay is eliminated by the specification of timber cladding with appropriate durability for the desired service life required.
Design measures to minimise the effects of moisture or water include:
- Use species regarded as being dimensionally stable or with small to medium movement characteristics for cladding.
- Only wood that has been specifically processed for timber cladding should be specified.
- Design features such eaves and overhangs help to deflect rain. Where this is not feasible flashing should be used to protect the tops of cladding boards.
- Prevent splash lines by finishing cladding at least 200mm from the ground or a horizontal surface and where possible use a surface that diffuses falling rain and run off e.g. gravel.
- Board widths should generally be 4 to 6 times the board thickness. Cladding styles commonly do not exceed 150 mm in width and design detailing must include measures that minimise water penetration.
- Tongue and groove boards should have a tongue of sufficient length to prevent disengagement if shrinkage occurs.
- Surface coatings help to deflect water and reduce movement. If cladding is to be finished with a pigmented coating, application of the basecoat and at least one top coat is recommended before installation.
Timber cladding relies on good design and material specification to provide reliable performance. Durability can be either a natural characteristic of a particular species or it can be conferred on a species by preservative pre-treatment or a wood modification process.
It is important to remember that the sapwood of all wood species and the heartwood of many has insufficient natural durability for external applications but can often be made highly durable with a wood protection process.
Almost every performance issue of timber used in construction occurs because:
- it was installed at the wrong moisture content for its application and movement occurs as it adjusts to its local environment
- the component(s) subsequently became wet due to poor detailing, workmanship or maintenance.
Movement causes defects and moisture penetration and retention are the conditions that encourage fungal attack, decay and insects.
In a temperate maritime climate like the UK has, durability (the ability to resist wood destroying organisms) is the critical consideration when choosing cladding. Only timber species designated as suitable for above ground outdoor use shall be used in cladding construction. Use outdoors and above ground is termed as Use Class 3 in standards (BS EN 335 Part 1) and this class is split into coated and uncoated applications. Therefore the species chosen must be suitable for use in Use Class 3.
The risk of insect attack is often considered small in the UK save for those localised areas where the house longhorn beetle, death watch beetle and termites exist. Powder post beetle is a threat to Oak sapwood but this can be eliminated by specifying that all sapwood shall be excluded. If this is not viable, then like all other species, oak can be made resistant to insect attack by pre-treatment with a preservative.
Climate Change factors are assuming an increasing importance in the development of British Standards. According to Defra, those involved in designing buildings with a life in excess of 60 years must take into account the consequences of a warmer, wetter and occasionally more extreme climate than in the past. Increasing warmth and wetness are the conditions that encourage decay and insect attack. As a consequence, specifying timber cladding with more than sufficient ability to deal with climate change is recommended.
The TDCA recommends the use of industrial wood protection processes only to enhance the long-term performance of timber that lacks adequate natural durability.
The durability ratings given in BS EN 350 part 2 relate to the heartwood part of the timber only. The sapwood of all species, whether hardwood or softwood, is not durable and needs protection. If for any reason, sapwood cannot be excluded or minimized to an acceptable level, then enhancing durability should considered. The options are:-
- the application of an appropriate wood preservative in an industrial process e.g TANALISED timber
- A modification process to change the properties of the timber to make it more durable e.g. ACCOYA, THERMOWOOD or THERMO-ASH.
Where building regulations require fire performance in accordance with British Standards (Typically Class 0 or Class 1) or Euroclass B or C, the TDCA recommends pre-treatment with a quality assured flame retardant applied by a processor approved by the product manufacturer. The principal source of guidance on industrial flame retardants is the Wood Protection Association (WPA). WPA publishes a specification manual that provides guidance on how specifiers can ensure the products they use are fit for purpose.
Designers should satisfy themselves that the description of the performance achieved in fire test “classification reports” corresponds with the species and size of cladding that they plan to specify.
To obtain a copy of the WPA Flame Retardant Specification manual visit the WPA website.
The benchmark for service life of wood in construction is set in BS 8417: Timber Preservation - Code of Practice. This standard defines three service life categories: 15, 30 and 60 years. The desired service life required for timber cladding should always be included in the specification.
The industry default specification for pressure treated cladding is 30 years. When relying on naturally durable heartwood, the desired service life will depend on the durability class of the species selected.
Desired service life categories are not guarantees of performance but indications of the expectation against which the durability performance benchmarks are set. They assume good design detailing, installation practice and normal conditions of use. Other factors, such as degree of exposure of the building to the elements, mechanical damage, vandalism or failure of other elements, may limit the service life of any construction.
Impact on adjacent surfaces
Leaching of extractives from some species can cause water marking and stain adjacent materials.
- Leachate is acidic and will cause corrosion and surface staining when in contact with metals that at not corrosion resistant e.g. non ferrous steel.
Timber is an organic material. As such it contains a wide range of natural chemicals. Because of their ability move as a liquid these chemicals are collectively referred to as “extractives”. Extractives tend to be more prevalent in the heartwood of a species where their concentration can be a principal cause of the darker colour of the heartwood of a naturally durable species like oak and western red cedar. Water soluble extractives such as Tannin can be leached from cladding by rain. This can leave affected surfaces with an uneven watermark until the surface has been fully leached.
Extractives like tannin in Cedar or Oak or lapachol in Ipe can stain adjacent surfaces like flashings, rendered walls and masonry which can be unsightly. Affected surfaces can be cleaned with dilute bleach, household detergent or a pressure spray. Testing a small area first is recommended.
Tannins in wood are acidic and as such leachate and the wood itself may react with certain metals that they come into contact with and cause corrosion. Contact with iron for example manifests itself in black discolouration. This is typically occurs when the wrong type of fixing is used and the surface of the cladding is covered with black spots and stains from each fixing point.