Why does wood rot

Wood in the garden: choice and lifespan

Wood is an organic building material and as such is subject to nature's cycle of materials. Some types of wood are naturally more resilient and robust than others - you should choose durable wood for outdoors.

Wood conveys a feeling of warmth and sympathy and gives the house and garden a friendly face. Residents and visitors feel at home as soon as they enter the property. Pests and moisture can add to the natural material wood. The risk of infestation depends on the location of the components. In the interior, wood is largely protected from destruction. Outside, wind and weather gnaw the wood, wood-hostile insects have free access, and contact with the ground attracts harmful fungi.

Standards classify wooden components in so-called hazard classes (GK): the higher the class, the greater the biological hazard to which the wood is exposed. Risk classes GK 2 to GK 4 apply to wood outdoors. Wood also belongs to the highest risk class if dirt can accumulate in cracks and joints and store a lot of moisture there.

How endangered is wood really? *

Hazard classWash-out stressAreas of applicationRequirements for wood preservatives
1no exposure to precipitation and splash waterInterior components (roof structures, ceilings, interior walls) and components subject to equivalent loads, relative humidity below 70%Preventing insects
2no exposure to precipitation and splash waterInterior components, relative humidity over 70% and interior components (e.g. in the area of ​​showers) are covered to be water-repellent. External components without direct exposure to weatherPreventive against insects, fungal
3Exposure to precipitation and splash waterExterior components without ground and / or water contact, interior components in wet roomsPreventive against insects, anti-fungal, weather-resistant
4Constant contact with earth and / or waterExternal components with and without sheathing (e.g. concrete)Preventive against insects, anti-fungal, weather-resistant, rot-resistant

* Hazard classes according to DIN 68800, part 3
Wooden components also belong to grade 4 if a lot of dirt is presumably deposited in cracks and joints and a particular amount of moisture is stored there.

How you can protect wood

If you observe two basic rules, wood can withstand environmental influences by nature and requires little maintenance, even outdoors. Experts from the Wood Sales Fund advise: Firstly, use durable types of wood, such as oak, robinia or larch, and secondly, increase the service life of the components through structural wood protection - e.g. protect edges with sheet metal covers or bevel fence posts so that water can easily drain off. Under these conditions, wood can shine in its natural tones and usually does not need a coating. Over time, the wood surface acquires a silver color, it does not affect the functionality of the component.

Heartwood: This is how well it resists fungal attack

The heartwood of some tree species contains substances that are toxic to pests. Types of wood with natural durability such as robinia, oak or larch are suitable for wooden parts directly in the ground - such posts and palisades, paving, bed borders and terrace substructures do not require any treatment with wood preservatives. However, the sapwood of all types of wood is not considered permanent, so you should only use sapwood-free material - even if it is made of durable woods.

Wood speciesDurability classHints
Douglas fir3 or 3-4cultivated from North America or Europe
Spruce wood4reacts sluggishly to humidification
Pine wood3-4Resin-containing, impregnated, can also be used in hazard class 3 and GK 4
Larch wood3-4Resin-containing, for GK 4 woods with a high proportion of sapwood, pressure-impregnated as a precaution
Fir wood4reacts slowly to humidification, pressure-impregnated, suitable for GK 3 and GK 4
Americ. Red oak4not suitable for components outdoors
Oak wood2Ingredients cause metals to rust and components to dirty
Robinia wood1-2Ingredients cause metals to rust and components to dirty
Afzelia1Imported wood; very resistant, well suited for weathered components
Bongossi1 or 2 VImported wood; very permanent in contact with water, twisty
teak wood1 or 1-3Imported wood; Plantation wood does not always have the same natural durability as jungle wood

1 = very permanent
2 = permanent
3 = moderately permanent
4 = not very permanent
5 = not permanent
V = unusually high variability

Source: Holzabsatzfonds / Infoholz

Dry wood resists pests

Wood-destroying fungi and insects need certain conditions for their work, above all sufficient wood moisture. If this is not well above 20 percent in the long term, mushrooms hardly stand a chance. Load-bearing and stiffening wooden components must be installed dry. Subsequent drying-related shrinkage, twisting and cracking can be minimized and mold growth can be prevented.

The following applies to small structures such as garden sheds - they ensure human life and building quality. In addition to the permitted wood moisture content (it must not exceed 20 percent), the standards also regulate material thicknesses and necessary treatments:

Weathered glued laminated timber (glulam) must not be thicker than 33 millimeters - intense sun and direct weathering increase the risk of cracking.

Wood-based panels may be used as load-bearing or stiffening, for example for roofs of carports or paneling of balconies - if the panels are standardized or generally approved by the building authorities. The use of wood material class 100 G with factory-made, standardized chemical wood protection is required. Exception: individual general building inspectorate approvals for certain products, such as three- and five-layer panels, permit the waiver of preventive chemical wood protection.

Mechanical fasteners such as dowels, screws or sheet steel require corrosion protection - the factory coating protects hot-dip galvanized sheet steel connectors and fasteners for load-bearing connections, if necessary under the roof and without ground contact. Everywhere else you have to choose more extensive protection for sheet steel parts. In practice, stainless steel is mostly used despite the higher costs; it does not contaminate the construction with rust flags and also excludes corrosive influences from the tannic acid of individual woods, for example oak.

Gently fend off pests on the wood

Constructive, i.e. structural wood protection preserves wood from weather and moisture damage without chemicals. Wooden parts are installed in such a way that water does not reach the surface or flows away quickly and moisture can ventilate. Wood destroyers such as fungi and insects need wood moisture to survive - if it is above 20 percent for a long time, fungal attack must be expected. Conversely, wood is considered safe if it is installed above the ground or water and the wood moisture content generally remains below 20 percent - constructive wood protection keeps the material dry. The most important measures:

• Roof overhangs, canopies and covers
• Bevelling (chamfering) wooden edges
• Avoid horizontal wooden surfaces
• Raising wooden components
• Use half-timber that does not crack
The structural and constructive measures against insects include:
• Cover wood
• Install wood sustainably dry
• Keep wood dry

Protect wood chemically

Preventive chemical wood protection should only be used if
• All possibilities of structural wood protection have been exhausted or are not possible due to the construction
• Durable types of wood are not available in sufficient quantity or quality
• Durable types of wood are out of the question for financial or ecological reasons

Wood preservatives for load-bearing and / or stiffening components must be generally approved by the building authorities for the area of ​​application and the intended application method and permanently meet the respective requirements. Example: In hazard class GK 3 (weathered without contact with the ground), heartwood of at least durability class 2 or wood preservatives with the test rating insects preventive (Iv), fungus-resistant (P), weather-resistant (W) for load-bearing and / or stiffening components are considered to be standard-compliant.

Impregnation extends the lifespan of wood. The best protective treatment in hazard class 3 is boiler pressure impregnation, but wood professionals can also protect the wood by hand using a brush or spray method. However, these carpenter-like methods do not provide any deep protection, you have to check the wood regularly and treat shrinkage cracks afterwards. If wood is used in hazard class 4 (weathered with contact with the ground), the wood preservative should penetrate particularly deeply into the wood - this is best achieved with pressure impregnation. If you want to be on the safe side, buy impregnated round and sawn timber with the quality mark RAL-GZ 411 "Impregnated timber construction elements"; they also meet the requirements of GK 4.

Preventive chemical wood protection is most effective when it is applied after the beam - i.e. after cutting, drilling or milling. Otherwise, cuts or holes should be protected afterwards with a brush or spray tool.