hardwood deck biological durability
To be able to answer the question you need to know, the timbers durability class as given in BS EN350-2, you also need to know the Use class of the timber given in BS8417 all explained below.Biological performance is based on the resistance to microbiological attack of a species heartwood. Standards or reference documents used are listed below

The table below then gives you the expected service life.

Natural Durability classification schemes used in this part are described in the European standard EN 350 same as in EN 350-1;  Five natural durability classes are used.

Every timber species falls into a particular durability class based on years of field trials. The Heartwood part of the tree is the only element considered in durability classification – sapwood is excluded ( sapwood is deemed to be non-durable, i.e. Class 5). With the durability information, this then helps designers establish the best use of that specific species based on the durability characteristic.

The Natural Durability classes are given below:-

Durability Class 1:  Very Durable

Durability Class 2: Durable

Durability Class 3: Moderately Durable

Durability Class 4: Slightly Durable

Durability Class 5: Not Durable

Every  end use application is classified into four classes based on the British Standard BS8417 it groups the applications for treated wood into four ‘Use Classes.’ UC1, UC2, UC3 and UC4 explained below.

  • Use class 1 is for internal applications where there is a low risk of wetting.
  • Use class 2 for interior applications such as roof timbers where the wood may be exposed to wetting, condensation or damp over its service life.
  • Use class 3 relates to timber exposed to the weather but not in direct soil contact. Timber cladding must be treated to Use class 3 (UC3) or better. If timber cladding has been treated with a low-pressure non-leach resistant product, g. Protim, Vac Vac, Vacsol etc. the timber cladding must be coated with a suitable coating if used externally classified as U3 coated ( U3c)
  • Use class 4 applications are for timber used in permanent contact with the ground or fresh water.

Based on the information above ( from BS EN350-2 & BS8417) it is used to give the service life as in the table below.

Use Location Use Class UC Service factor code (see table below for explanation) Natural Durability Class and desired service life in years

15 years Service life

Natural Durability Class and desired service life in years

30 years Service life

Natural Durability Class and desired service life in years

60 years Service life

Dry- Internal joinery  and load bearing no risk of longhorn house beetle attack 1 A

B for load bearing

(joists / beams etc)

5 5 5
Internal joinery (Dry- Internal joinery  risk of longhorn house beetle attack and load bearing) 1 D for load bearing

(joists / beams etc)

3 3 3
External joinery above ground, e.g. timber cladding decking 3 C 4 3 2
Joinery below damp proof course or ground level, e.g. sleepers, fence posts 4 B/C/D 2 1 1
Timber in Fresh Water load bearing 4 D 2 1 n/a(very limited)
Timber in Salt Water load bearing 5 D 1 n/a (very limited) n/a (very limited)


For external cladding based on the natural durability, untreated applications use timbers that fall into Durability class 1, 2, 3 (use durability class 4 only as temporary)  any other durability classes will need to be preservative treated. Whereas for internal cladding, all durability cladding classes can be used without the need for preservative treatment.

EN 460 compares the requirements of Use Classes with the natural durability and indicates whether natural durability is sufficient for the end use or whether preservative treatment is required.

It is always important to check the expected durability class or preservative treatment type if you are working with organisations like the NHBC as they tend to have different specific classifications in their specification requirement, e.g. an extract from the NHBC Timber preservation specification is given ( natural solid timber) Chapter 3.3 link: NHBC/Standardsplus2017

Preservative treatment Biological

Three factors are important to consider when you decided whether to preservative treat a timber component or not as given below;

  • A component’s intended application and bio-deterioration risk as defined in British Standard Use Classes;
  • The consequences of premature failure and;
  • The natural durability of the wood to be used.

The table below gives the service factor codes relating to need for preservation based on the risk of failure if not treated.

 Service factor codes relating to need for preservation   (Source: BS8417)
Service factor code Description of risk and consequences of failure Need for Preservation
A Negligible risk of failure Unnecessary
B Where the risk of failure is low and preservation can be regarded as an insurance against the cost of repairs, and/or where replacement of timber or remedial action is not difficult or expensive. Optional
C Where the risk of failure is high and/or where replacement of timber or remedial action is difficult and expensive. 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 Essential

Difference between hardwoods, softwoods and modified timber

Buying well pressure treated timber- biological preservation-BS8417

Biological Treatment preservatives and processes -Pressure

How to Specifying treated timber


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