Frøslev is Denmark's largest producer of NTR pressure-impregnated wood. We do the impregnation at our own plants under strictly supervised and environmentally sound conditions.

Pressure-impregnated wood has been through an impregnation process. The purpose of using wood preservatives for the pressure impregnation of wood is to protect the wood from microbial degradation by rot, fungal attacks, insect attacks and to extend the natural durability of the wood.

NTR pressure-impregnated wood has been impregnated according to Nordic Wood Preservation Council’s standards and is free of chromium and arsenic.

Impregnation all the way to the core

Preservation through pressure impregnation means that wood preservatives containing active substances against rot, fungi and insects are pressed into the tree. Impregnation ensures deep and full penetration into the sapwood, and the result is better protection against biodegradation than for example the natural protection and durability of the pine heartwood.

The lightest part of the wood, the sapwood, is saturated with impregnating liquid and is protected against biological organisms. Heartwood is not normally penetrated through impregnation as the structure and density of the heartwood and the content of resin and heartwood substances do not allow penetration of the impregnating agents.

Pressure impregnation is done in closed tank systems. Before impregnating the wood, it is verified that the wood is healthy (free of fungi and microorganisms). Similarly, the wood must be free of bark, dried and processed into the requested profile.

The wood is placed in the tank, and vacuum is used to extract moisture and air. The tank is then filled with impregnating liquid, and the active substances are pressed into the wood under pressure. Once the process is complete, the pressure tank opens, and excess liquid drips off. All liquid is collected and recycled. Before the wood can be delivered to the customer, the wood must be fixed for 48 hours so that it does not drip any more.

The Danish Environmental Protection Agency requires that Danish pressure-impregnated wood must be fixed before delivery and that the tree is placed with joists in between. There are no requirements for imported wood. In principle, it can be delivered un-fixed, wet and without joists which may involve an occupational health risk.

External quality control

Frøslev only uses water-proofing agents for which a risk assessment has been carried out and which have been approved by the Danish Environmental Protection Agency. This provides a guarantee for durability as well as environment. Danish Impregnation Control, an external quality control, and our internal control guarantee the quality of our process and products.

Good quality

The quality of impregnated wood is evident if the liquid has penetrated all the way to the heartwood. If the wood has only been immersed in impregnation liquid, the liquid will only have penetrated a few mm into the wood. To ensure good quality, make sure that the wood is produced in Denmark and has been impregnated according to the NTR standards. Then the quality is in order. 

Green or brown

Most associate pressure-impregnated wood with a greenish colour. The colour originates from the copper in the impregnation liquid. Over time, the green colour will fade.

However, it is also possible to pressure-impregnate wood to make it brown rather than green. The process is the same, only the liquid is brown. It gives the wood a warm, brown colour. As is with case with green impregnation, brown impregnation meets the NTR standards. This applies to wood for all purposes for which pressure impregnation is used.

The beautiful, brown colour should be treated with wood oil to preserve the colour. It gives the wood a deeper glow, the wood retains the brown colour for a longer time, and it becomes water-repellent and thus easier to clean and less susceptible to surface fouling.

Please note that neither green nor brown impregnated wood is to be considered surface-treated/-painted, and the colour will eventually patinate and turn greyer.

Wood for NTR pressure impregnation

Scotch pine is the most commonly used wood species for impregnation due to its high proportion of impregnable sapwood. However, other wood species such as spruce, Douglas and larch can also be impregnated.

NTR Labelling

All pressure-impregnated wood from Frøslev has been pressure-impregnated according to the NTR standards - the NTR symbol is therefore visible on our wood. NTR (Nordic Wood Preservation Council) is a voluntary labelling scheme and joint collaboration in the Nordic countries and is your guarantee that the wood has been impregnated with a product approved in the Nordic countries.

The scheme also covers an NTR classification within impregnation grades laid down by the NTR. The classes comply with current EN/DS standards. When the wood is NTR-labelled with the subsequent DK country code, it means that the wood has been impregnated with an agent approved in Denmark.

Impregnation grades

• NTR grade A: Impregnated wood for use in and near soil, in fresh water (lakes etc.), for repairing after fungal attacks and for structures with a possible safety risk.
• NTR grade AB: Impregnated wood for cladding and fences without ground contact
• NTR grade SPRUCE: impregnated spruce for cladding to be painted and for fencing that are surface-treated (impregnation acts as a primer against rot and fungi).
• NTR grade M: Unfortunately, the product has been discontinued.

Pressure-impregnated wood has not always had the best reputation in terms of environmental impact. Among other things, this had to do with the treatment of railway sleepers and telephone poles that were impregnated with chemicals other than those used today. They were therefore considered environmentally hazardous waste.

Fortunately, that is not the case today!

In the past, creosote, arsenic and chromium were used in impregnated wood - this is NO longer the case for Danish impregnated wood. 

The use of arsenic for impregnated wood is prohibited in Denmark. The ban came into force back in 1997. Over time, chrome has been phased out and replaced by copper.

Today, copper and a number of organic substances (fungicides) are used for impregnation in Denmark where the copper content is between 0.1-0.2% of the weight of the wood. Copper compounds only trigger a classification as hazardous waste in terms of health hazards when the concentration is higher than either 20% or 25%. The green or grey/green colour of pressure-impregnated wood comes from the copper. The content of boron is considered to be very low and does not trigger any classification. Danish impregnated wood is thus NOT classified as hazardous waste according to the Danish Environmental Protection Agency.

This also applies to all wood that has been pressure-impregnated at Frøslev. Our wood is also either FSC- or PEFC-certified.

However, never forget...

• Impregnated wood may not be used in wood-burning stoves or bonfires.
• Dispose of pressure-impregnated wood properly.
• When disposing of pressure-impregnated wood: comply with proper waste management according to municipal instructions.

Maintenance

Wood is a living material that is affected by the elements. Continuous maintenance extends the life of the wood. Make sure that the pressure-impregnated wood is completely dry before it is surface-treated.

Wood must be installed correctly - please follow the installation instructions for Frøslev system fences.

If the brown shade in brown-impregnated wood is requested preserved, continuous care is necessary with wood preservatives for coloured wood.

The products must be installed, built-in and treated in accordance with our instructions applicable at the time of purchase, i.a. including:

 Adjusted boards/timber (sawed, processed or otherwise) must receive a finishing treatment with a suitable primer on cut surfaces (such as GORI Transparent Primer 11 which is a water-based primer).

Wood that is installed at particularly exposed places must be constructively protected against the effects of the elements, for example:

• Vertically mounted posts must be covered with post covers.

• Vertical posts must always be installed with the shortened end facing up.

• With eaves from the roof or some other construction.

• Ensure necessary drainage around/below the posts to make sure that water can be drained off the post (in clay soil/limestone layers etc.).

• Wood impregnated according to NTR grade AB must be installed at least 20 cm from the ground surface, and we also recommend sealing the lower edge against the ground surface, grass or other planting.

• For other matters, please see the instructions from Træbranchens Oplysningsråd Træinformation

Wood is a living material that is affected by the elements. Continuous maintenance extends the life of the wood. Make sure that the pressure-impregnated wood is completely dry before it is surface-treated.

Wood must be installed correctly - please follow the installation instructions for Frøslev system fences.

If the brown shade in brown-impregnated wood is requested preserved, continuous care is necessary with wood preservatives for coloured wood.

The products must be installed, built-in and treated in accordance with our instructions applicable at the time of purchase, i.a. including:

 Adjusted boards/timber (sawed, processed or otherwise) must receive a finishing treatment with a suitable primer on cut surfaces (such as GORI Transparent Primer 11 which is a water-based primer).

Wood that is installed at particularly exposed places must be constructively protected against the effects of the elements, for example:

• Vertically mounted posts must be covered with post covers.
• Vertical posts must always be installed with the shortened end facing up.
• With eaves from the roof or some other construction.
• Ensure necessary drainage around/below the posts to make sure that water can be drained off the post (in clay soil/limestone layers etc.).
• Wood impregnated according to NTR grade AB must be installed at least 20 cm from the ground surface, and we also recommend sealing the lower edge against the ground surface, grass or other planting.
• For other matters, please see the instructions from Træbranchens Oplysningsråd Træinformation