BM TRADA experts’ knowledge covers best practice principles, fault finding and diagnostic work on timber cladding and decking boards. They carry out in-situ cladding and decking inspections in a variety of locations.

Cladding and decking inspections and on-site surveys

BM TRADA undertakes cladding and decking inspections on a variety of installations for clients throughout the UK and abroad. These surveys are typically carried out on behalf of suppliers of the boards, installers or building owners where there is a concern about:

  • The quality of the timber used in an installation
  • The quality of workmanship on-site
  • Excessive movement or distortion of boards
  • The discoloration of boards
  • The condition of cladding and decking and the timbers to which they are fixed.

Our expertise in the use of timber for these applications is demonstrated through our external timber cladding and timber decking publications.

Our timber team also carries out practical research to understand the causes of in-service discoloration of unfinished cladding and the most appropriate means of its removal and treatment to prevent its recurrence.

Your timber cladding FAQs answered

Timber cladding is popular with building owners and architects seeking to maximise timber's unique visual appeal and environmental credentials. It is used for internal or external applications, although you must consider the durability of the timber when selecting material for external use.

Timber cladding has long been used to construct the external walls of buildings, either as the primary form of structure and enclosure or merely to enclose a separate structural frame.

As construction in the UK now embraces the concepts of sustainable development, 'dry' construction (construction without the application of plaster or mortar) and increased prefabrication, and because of the variety of profiles possible with timber cladding, its popularity is likely to rise even further.

Stainless steel or non-ferrous fasteners are preferred to prevent long-term corrosion and possible chemical reaction with some timber species.

Softwoods can generally be fixed with suitably sized ring-shank nails, but it is recommended that hardwoods are fixed with screws and washers through oversized holes where double fixings are used. There are options for discreet fixing, but this is an area where specific advice should be sought from an expert source.

Some species commonly used as uncoated cladding are prone to extractive staining. As the boards weather, they can leach out extractives that can stain other boards, or the surfaces of other materials below or adjacent to them.

Measures that can be taken include careful building design such as water-shedding details to direct rainfall away from the cladding and protecting absorbent surfaces around the cladding from the runoff during the initial weathering process.

Horizontal boards cannot be easily bent around tight radial curves, particularly if they have a tongued and grooved profile. It is therefore preferable to use vertical boards, with horizontal battens that curve. This curvature can be achieved, for example, by building the battens up in layers thin enough to be bent cold, before fixing them into position by gluing and screwing.

There are Building Regulations in place which are intended to restrict the spread of fire. Depending on its location, wood cladding may require pre-treatment with a fire retardant, which must be compatible with any preservative treatments and coatings. Timber cladding may not be acceptable close to boundaries or above a certain height. It is recommended that you check with the Local Authority Building Control.

Some woods have high extractives content, and this may take several months for the rain to 'wash out'. Other staining may be due to excessive wetting, chemical staining, or pollution. There are a number of ways of removing stains, from washing to the application of chemical cleaners. The method of cleaning depends on the cause of the staining, and this can be identified by on-site inspection/investigation.

There are many different profiles, but it is recommended that boards should be 150mm maximum width, to limit the effects of any moisture movement across the board. Tongued and grooved boards should be further limited, preferably to 100mm (max 125mm), with only kiln-dried wood used.

Board thickness is dependent upon the profile. It is also recommended that any individual part of the board, e.g., the tongue or top edge of a featheredge board, should not be less than 9mm in thickness.

Softwood is the most common choice for cladding in the UK, with home-grown species increasingly selected. A variety of hardwoods and modified woods are also suitable, however.

When selecting species, consider the natural durability of the wood and the options for preservation and coating. Durable species may be more expensive to specify at the outset but may require less maintenance than a cheaper, less durable species.

Appearance (usually in respect of the size and frequency of knots) is also an important consideration and will determine which species to choose.


Your timber decking FAQs answered

The use of timber for external decking has a long history ranging from its use as Bronze Age walkways in the fens, to the verandas, railway platforms, piers and jetties of the nineteenth century. In the major timber growing areas of the world, such as North America and Scandinavia, wood was used for sidewalks in the cities and lasted for many years before eventually being replaced by paving.

The deck, however, largely developed as an uncovered version of the verandas typical of frame houses of the tropics and the Southern states of America. They are an integral feature of open-air living which ties into the lifestyle of these areas. However, timber decks are now common all over the world, including the UK.

All timber deck structures should be designed to give a long-term life expectancy. However, surface checks and splits are inevitable as the timber weathers. Any decking will benefit from regular cleaning.

Decks using durable or copper organic treated timbers should not need any maintenance other than regular cleaning with a stiff broom brush. Decks in shaded areas, such as under trees, may require more cleaning, as the deposits from vegetation combined with moisture may promote algae growth.

A more thorough clean can be done with a pressure washer or mechanical brush. Care should be taken, however, as some machinery may damage the surface of some softer timbers.

There are a number of decking stains, decking oils, water repellent treatments and anti-slip surface products on the market. These will need maintenance as per the manufacturer's instructions. Such finishes are often not required, but are used to change the appearance of the deck, or improve slip resistance.

The slipperiness (slip resistance) of a deck depends on a complex set of factors, some of which can be controlled by selection of appropriate materials, good design and maintenance.

Surface water

Ensuring board gaps of at least 8mm and providing the deck with a slight fall promotes drainage. Board surfaces can be profiled to provide grooves into which water drains, or chamfered to promote drainage to their edges. A ribbed surface on the underside of the board (not the face) facilitates water drainage away from the supporting joist.

Direction of pedestrian travel

Walking across a grooved surface provides greater friction and makes for a less slippery surface; walking along the grooves may reduce friction.

Algae and build-up of plant debris

Algal build-up results in the timber surface becoming more slippery when wet, while the build-up of debris prevents water drainage of water, making the deck more slippery. Regular cleaning of decks in shade or under trees is particularly recommended.

Proprietary anti-slip inserts and anti-slip deck coatings are also available.

Further guidance on avoiding a slippery deck can be found in the BM TRADA publication Timber decking: The professionals' manual 3rd edition.

It is advised that timber posts are kept out of ground contact and out of the splash zone where possible.

If they are in ground contact, they should go into concrete foundations that provide drainage at the base. This can be done with the use of loose fill around the post, and a drainage hole in the concrete footing.

If you do wish to use timber in ground contact, then you will need to consider the appropriate species and preservative treatment type and application. These can vary considerably.

The Wood Protection Association provides a specifiers' guide to wood treatment, or treatment manufacturers may be able to provide the relevant information. Treatment options will range from approximately 15 to 45 years' service life.

For decking over or next to water, the use of durable or very durable species such as Ipe, Jatoba Balau and Cumaru, to name a few, is recommended.

If you are using treated timbers, then the treatment should be copper organic to a suitable standard. The Wood Protection Association produces a specification and practice document if you need further guidance, or alternatively you can contact the timber preservative manufacturers.

See BM TRADA's Wood Species database to find more timbers which are durable or better. Alternatively, many supplier websites will list timbers and their properties.

If you are unsure of a timber species' suitability for decking, and cannot find information regarding its properties, contact the Timber Technical Helpline using the information above.

It is recommended that these gaps are 8mm or more, depending on the board profile and moisture content at time of installation.

If these gaps are less than 6mm, the boards may not shed water efficiently and, as a result, could become slippery.

Gaps wider than 10mm may also be an issue, particularly with potential health and safety issues, e.g., stiletto heels, pram or bicycle wheels, roller blades etc. However, there is no regulation dealing with this.

Some board profiles will have the edges of boards cut to an angle so the bottom of the gap is larger than the surface. These may be used where a smaller gap on the top surface is needed.

Further guidance on board profiles and spacing can be found in the BM TRADA publication Timber decking: The professionals' manual 3rd edition.

Boards can be fixed to joists by various means – such as nailing, screwing or proprietary clips. The use of stainless steel fixings is recommended to avoid unsightly staining, although zinc coated or epoxy coated fixings can also give a good service life. Be aware that plated fixings can react with tannin in timber, causing corrosion of mild steel and staining of wood.

For the substructure, hot-dipped galvanized mild steel fixings and brackets may be adequate. Remember, however, that stainless steel and galvanized materials should not come into contact with each other due to electrolysis reactions. Nails are not appropriate for fixing any part of the substructure.

See the BM TRADA publication Timber decking: The professionals' manual 3rd edition for detailed advice on fixings.

This issue more often presents itself when durable timbers are used on balcony decking on a rendered building. Typically, it happens when durable timbers are left unfinished.

As the boards weather, they can leach out water soluble extractives that can stain other boards, or the surfaces of other materials below or adjacent to them. Special care should be taken when using timber above absorbent materials, as stains may be difficult to remove. It is advisable to protect these surfaces from the runoff during the initial weathering process. This process is temporary and should stop once the boards have begun to weather down to a silvery-grey colour.

Yes. The following is recommended:

  • Timber should be oversized in order to provide an extra safety factor.
  • If timber is to be left untreated, a durable or very durable species should be used, or care should be taken with the correct treatment specification.
  • Consideration should be given to liquids or items falling between the gaps of the decking – there are propriety products that can be inserted between the boards if this is a concern.
  • Health and safety measures such as guard rails should be incorporated.
  • Planning requirements should be considered.