Internal and external cladding: what you need to know

Wood behaves differently outdoors because it is exposed to the changing seasonal elements, whereas inside it is protected and remains relatively dry. That’s why additional factors must be considered when working with timber cladding outdoors. Here, the Timber Decking & Cladding Association explains the facts you need to know.

Timber is hygroscopic – it absorbs moisture from the air. Varying levels of moisture in wood can cause it to expand and contract. This movement occurs across the grain rather than along it, so in terms of cladding boards, it affects width and thickness. This is not a problem so long as you allow for such movement in the design and installation of the exterior timber cladding.

Summary of generic cladding profiles and recommended dimensions.

The first crucial point both suppliers and installers should understand is that an external timber cladding profile could be distinctly different from one suitable for internal use, especially if it is tongue-and-groove style (T&G).

The tongue needs to be wide/long enough so that it will not disengage as the timber contracts. T&G is described as a complex, interlocking profile in BS8605 External Timber Cladding Part 1: Method of specifying.

To allow for movement the standard gives guidance on board width and thickness dimensions to accommodate such movement and limits the width of T&G boards to 125mm (unless ‘very small movement’ timber is used).

The TDCA is an official distributor for BSI Standards. You can purchase any British Standard from the TDCA website at a 15% discount (members 20%). This can be found online at

The movement that occurs in external use timber is not substantial and is essentially predictable, depending on the movement class the species is assigned to. Incidentally, these values do not change with age.

Moisture and installation

BS8605 also gives guidance on suitable movement gaps, which is very helpful when installing cladding. As you’d expect, the wider the board the larger the movement gap must be.

Movement classification below fibre saturation point (FSP) based upon research by the Building Research Establishment (The movement of timber, technical note 33 [14]). There is no movement above FSP.

The standard recommends that moisture content in the wood at the time of installation should be 16% (+/-4%). However, it is traditional to supply featheredge boards as green (wet) meaning the moisture content will be much higher than this.

BS8605 states that green timber is only suitable for overlapping profiles and that shrinkage must be allowed for, and provides a table of shrinkage values for particular species.

Knowing these values will allow you to determine the optimum overlap required and pass the information on to your buyer. Special fixing guidance may also be given.

As timber dries you get differential movement around and across the grain. This can cause cladding boards to ‘cup’ away from the centre growth rings. If a cladding board is nailed with the heart side against the battens, the edges will curl forward as it dries. If nailed with the heart side facing out, the edges push back against the battens and the board will stay relatively flat.

Any non-symmetrical profiles should, if possible, be machined so that the heart side is front facing.

For sawn boards or symmetrical profiles, look at the curved growth rings on the end of the board.


It goes without saying that outdoor cladding needs to be more durable than internal cladding, because a wetter environment presents a greater risk of decay.

Where board on board arrangement is concerned this mounting should be avoided.

There are many naturally durable cladding hardwood species and a selection of softwood species which offer variety in grain and colour. Low durability species can be made more suitable for exterior cladding use by either treating with an appropriate wood preservative, applied under controlled factory conditions, or using a modified wood product.

To sum up, cladding profiles suitable for internal use don’t always translate into being suitable for external use. Yet the market continues to supply T&G profiles that are too wide and whose tongues are too short for outdoor use. Rarely do we see cladding marked up or segregated as fit for indoor or fit for outdoor use. The TDCA believes that this issue must be addressed across the industry. It is the suppliers’ responsibility to ensure the profiles targeted for outdoor use are suitable for use as such.

By taking note and considering the profiles you sell for exterior cladding use, you can reduce the potential for costly and time-consuming movement related complaints.

The TDCA provides guidance on all timber cladding-related matters and is happy to help advise every part of the timber cladding supply sector, as well as the end user.

Know your terminology

‘Movement’ is a term used to describe ongoing dimensional change after the timber is dried. ‘Shrinkage’ refers to the reduction in size that occurs as the timber is dried from its initial green/wet condition following felling.

Initial shrinkage is always greater than subsequent movement.