Specification Guide

When specifying timber, consideration should be given to the species (natural durability) and the Use Class, i.e. where it is going to be used. The natural durability of timber may be improved by preservative treatment to achieve the required Use Class (conferred durability).


Use Guide Table

1 Above ground, covered. Permanently dry. Insect risk
2 Above ground, covered. Occasional risk of wetting.
3A Above ground, not covered. Exposed to short spells of wetting (several days).
Normally coated if exposed directly to bad weather.
3B Above ground, not covered. Exposed to frequent wetting (several weeks)
4 In contact with ground or fresh water. Permanently exposed to wetting.
5 Submerged or partially submerged in salt water and subject to attacks from marine invertebrates.


Classification House



Classification Table


Recommended shaded blue

A Domestic pitched roofs all structural timber 1      
B Condensation risk pitched roofs all structural timber 2      
C Tiling / slating battens 2      
D Flat roof timbers 2      
E Barge boards, fascia boards 3A      
F External frame - inner leaf - except sole plates 2      
G Frame sheathing - plywoods 2      
H External cladding 3      
I Battens for fixing external cladding 3      
J First floor joists 1      
K External joinery 3A      
L Surrounds for non-wood windows 3A      
M External doors 3A      
N External door frames 3A      
O Ground floor joists 2      
P Sole plates above DPC 3 / 4      
Q Sole plates below DPC 4      
R Decking 3B      
S Fence posts 4      
T Swimming pool surrounds (external) 4      
U Timber garden products 3 / 4      


Suggested Standard Specification Clause
Architects can refer to their NBS standard clauses. A useful phrase could be as follows:


"Preserve timber to hazard class... using a suitable preservative for that use class. Preserve any cut ends with appropriate cut end preservative. “You may specify the timber type and the preservative yourself.


More comprehensive information is available from our Technical Services section concerning use classes, timber species and service life. We will be happy to direct you to your nearest preserver. The above is intended as a quick guide only. Please check for specialist applications. Always check for your particular end use if in any doubt.