In order to prevent rot and insect damage, wood must be properly treated with a suitable preservative.

The level of preservative needed is determined by the end use of the wood. For example, timber used for decking around a swimming pool requires a different level of treatment to timber used for floor joists.

To make it easier to select timber with the correct level of treatment, timber is categorised into four Use Classes dependent upon its end application.

DEFINITION OF USE CLASSES

USE CLASS 1:
Situation in which the wood or wood-based product is under cover, fully protected from the weather and not exposed to wetting.

USE CLASS 2:
Situation in which the wood or wood-based product is under cover and fully protected from the weather, but where high environmental humidity can lead to occasional but not to persistent wetting.

USE CLASS 3:
Situation in which the wood or wood-based product is not covered and not in contact with the ground. It is either continually exposed to the weather or is protected from the weather but subject to frequent wetting.

USE CLASS 4:
Situation in which the wood or wood-based product is in contact with the ground or fresh water and thus is permanently exposed to wetting.

The diagram below shows different areas of the house and garden where the timber might be used and highlights into which Use Class each application falls into. Click on the letters to reveal the Use Class.

Domestic pitched roofs all structural timber

Use Class 1

Condensation risk - pitched roofs, all structural timber

Use Class 2

Tiling/Slating battens

Use Class 2

Flat roof timbers

Use Class 2

Barge boards, fascia boards

Use Class 3

External frame - outer leaf

Use Class 3

External frame - inner leaf - except sole plates

Use Class 2

Frame sheathing - plywoods

Use Class 2

External cladding

Use Class 3

Battens for fixing external cladding

Use Class 2

First floor joists

Use Class 1

External joinery

Use Class 3

Surrounds for non-wood windows

Use Class 3

External doors

Use Class 3

External door frames

Use Class 3

Ground floor joists

Use Class 2

Sole plates above DPC

Use Class 2

Sole plates below DPC

Use Class 4

Decking

Use Class 3/4

Fence posts

Use Class 4

Swimming pool surrounds (external)

Use Class 4

Timber garden products

Use Class 3/4

 

Component

Use Class

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

3

F

External frame - outer leaf

3

G

External frame - inner leaf - except sole plates

2

H

Frame sheathing - plywoods

2

I

External cladding

3

J

Battens for fixing external cladding

2

K

First floor joists

1

Component

Use Class

L

External joinery

3

M

Surrounds for non-wood windows

3

N

External doors

3

O

External door frames

3

P

Ground floor joists

2

Q

Sole plates above DPC

2

R

Sole plates below DPC

4

S

Decking

3/4

T

Fence posts

4

U

Swimming pool surrounds (external)

4

V

Timber garden products

3/4

 

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