LVL Handbook EUROPE

7. DURABILITY

Figure 7.3.

Moisture protection treatment on LVL beams.

WeatherGuard® Metsä Wood

7.2.4 Discussion of durability class

definitions

The biological durability classification based on EN 350 is

poorly applicable to LVL or other engineered wood products.

The biological durability of LVL and other wood-based pan-

els (uncoated or overlaid) is dependent on the end use condi-

tions, and this is where the EN 335 use classes provide useful

guidance.

EN 350 durability classes are defined for the heartwood of

wood species in use class 4. However, there is no information

on the suitability of different natural durability classes for dif-

ferent end uses. CEN/TS 1099 gives guidance for plywood, but

does not provide information on the suitability of different nat-

ural durability classes for different end uses.

LVL and other wood-based panels contain sapwood. The

fact that sapwood is always regarded as non-durable argues

against using the EN 350 classification as, based on this, LVL

and all wood-based panels are universally classed as ‘not du-

rable’ (class 5).

EN 350 states that the durability classes are designed to in-

dicate the expected level of performance of wood when used

in contact with the ground (service conditions as described for

use class 4 in EN 335) and that in other use classes the service

conditions can result in wood performance that differs from

that implied by the durability classification. Based on this, the

standard may not be directly applicable to use class 3, 2 and 1

conditions and should therefore not be used as the biological

durability criteria. The natural durability of LVL can differ from

that of the wood species used in its manufacture. Additional

factors contributing to durability include the thickness of the

plies, the structure of the board, and the characteristics and

quantity of the binder.

In conclusion, it is recommended to declare the product as

durable in use class 1 and 2 (or service class 1 and 2 according

to EN1995-1-1) conditions.

7.3 STRUCTURAL WOOD PROTECTION

The best way to manage the durability of LVL, as well as all

other timber structures, is structural protection. This means,

for example, long enough roof overhangs and high foundation

levels and moisture isolation between the foundation and sole

plates. Structural protection keeps the structures in service

class 2 (use class 2) or dryer conditions, which wood products

can resist well.

7.3.1 Resistance against temporary moisture

exposure of structural LVL products

Exposure to rain, splashing as well as water convection from

other structures shall be avoided. The designer must pay atten-

tion to the details of the construction to ensure that no water

pockets will be formed. The product may be exposed to weath-

er for a short period during installation. During the erection

of a building, structural LVL products and elements, which are

structurally glued from LVL components, have good resistance

to temporary exposure to water without damage or decay. This

requires, however, that it is ensured that the products can after-

wards dry to the desired moisture content before the structural

envelope is closed. The integrity of the glue bonding is main-

tained according to the assigned service class throughout the

expected life of the structure

LVL products swell when the moisture content increases, and

shrink when the moisture content decreases. A part of the swell-

ing is permanent and the extent of these dimensional chang-

es depends on the grain direction. Wetting can cause perma-

nent deformations and impair the visual appearance of surface

veneers, such as colour changes due to water staining, surface

cracks and falling knots due to drying shrinkage after wetting

Joints with mechanical connectors, such as bolted connec-

tions, may become loose due to swelling and shrinking cycles.

Drying shrinkage after severe wetting may cause cracking,

which often reduces the load-bearing capacity of dowel-type

connections, notched beams and beams with holes.

7.3.2 Resistance against UV radiation

Like all wood products, non-treated surfaces of LVL will slow-

ly fade to grey due to the action of UV radiation from the sun.

This greying does not affect the strength properties.

If this natural greying is not desired, an adequately pigment-

ed coating system or a coating containing special additives

must be applied. The higher the proportion of pigments, the

greater the protective effect. The coating layer must be of suffi-

cient thickness to fulfil the minimum protection requirement

across the entire surface. More information is provided in the

technical data sheets of the coating manufacturers

7.3.3 Chemical durability

The main components of wood: cellulose and lignin, react

in opposite ways to acids and alkalinity. Cellulose is not very

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LVL Handbook Europe