Cladding Wood Types

Natural durability classes of heartwood to fungal decay  (Source: BS EN 350-2)
 Class 1  Class 2 Class 3   Class 4  Class 5
 Very durable  Durable  Moderately durable  Slightly durable  Not durable

To achieve a 15 year desired service life specification or higher using natural durability, you need to use heartwood with appropriate durability otherwise durability would need to be improved using a suitable wood protection product / technology - ref: BS8417 or the WPA (Wood Protection Association) Manual.

Natural durability class  Need for treatment / modification   Desired service life / years  
 Occasionally wet1 Frequently wet 
 1 Suitable without preservative treatment or modification  >60  60
 2 Suitable without preservative treatment or modification  60  30
 3 Treatment recommended for certain end uses e.g. tall building otherwise often considered suitable without  30  152
 4 Preservative treatment or modification required  152  <152
5 Preservative treatment or modification required <152 <152

1 Cladding with maintained coating will fit into this category.
2 Preservative treatment or modification of lower durability species is designed to extend service life to 15/30 years or more.

Principal softwood species used in the UK for cladding

Softwood_cladding_species.png (2)

.Notes:

  1. The natural durability of some species varies, in which case the timber should be assigned to the lowest durability class within it's range. Natural durability classes refer to heartwood only. The durability of sapwood is minimal whatever the species.
  2. BS 8417 gives two treatability classes. These align with BS EN 350-2 as follows: permeable = Class 1 and Resistant = Class 2, 3 and 4. Both systems are based on the treatability of the sapwood.
Treatability classes (Source: BS EN 350-2)   
 Class 1  Class 2 Class 3   Class 4
 Easy to treat  Moderately easy to treat  Difficult to treat  Extremely difficult to treat

Principal hardwood species used in the UK for cladding

Hardwood_cladding2.png (2)

v = variable
Note 1: The natural durability of some species varies, in which case the timber should be assigned to the lowest durability

Wood Modification

Wood modification involves the action of a chemical, biological or heat upon a suitable species to deliver enhanced performance. The principal aim is to improve durability – resistance to fungal decay - and modified wood processes achieve this without the need to use biocides in contrast to wood preservatives.

Most modified woods used in cladding are generally rated as Very durable (BSEN350:2 Class 1) or Durable (Class 2) and capable of delivering a desired service life of 30 years or more. Durability against insect attack is also conferred.

Modified woods are usually more dimensionally stable than unmodified timber. Because modification processes usually change the entire cross section of the timber, products can be machined after the modification process without exposing unprotected areas. Other characteristics like strength and density can be affected but this is not normally an issue cladding applications.

It should be noted that much of the performance assessments of modified wood is often based on laboratory tests and so the degree of performance certainty associated with wood preservatives based on field test data and long term use has yet to evolve. The performance properties of modified woods are known to change in relation to the species, the process and variations in process.

The modified wood processes most suitable for cladding are:

Heat – the thermal modification of wood

Impregnation – modification of the water binding cells by a chemical.

Principal modified wood brands used in the UK for cladding

Modified_cladding.png (1)

The data in this table is based on laboratory tests and has been verified by the manufacturers of the individual brands. The modification processes change the source species into a higher value commodity and effectively create a new species with enhanced durability and stability characteristics. This is a key factor in improving coating performance compared to the base material.

KEY
 + = Good  ++ = Very good  TBC = To be confirmed