Timber treatment

Timber is vulnerable to attack from insects, damp and fungi and needs protection from all of these to extend its working life. All Imprest products are treated using a vacuum-pressure-vacuum method and we also offer timber treatment as a separate service.


Preservatives and hazard classes

We have 2 treatment tanks and water-borne, chromiumfree wood preservative – Impralit®-KDS. Please click here to read more about Impralit®-KDS. Chemical is safe for the environment and give the necessary protection depending on the hazard class. For different products, markets & customers we are using different treatment processes but the majority of our timber is being treated according to European Standard EN 351-1 to meet the following hazard classes:
  • HC2 – for outside undercover usage
  • HC3 – for outside non ground contact usage
  • HC4 – for outside in ground usage.
During the summertime untreated products are being treated with anti-stain preservative by dipping in order to avoid bluestain & mould.

Treatment available in three colours:

  • Green ("Park Green"), HC4
  • Brown ("Evening Brown"), HC4
  • Grey ("Vintage Grey") HC3


We are controlling our treatment quality by measuring the volume of the preservative that we pump into the timber and/or by using long pressure times. The actual service life of the timber depends on a proper treatment process, on the moisture content of the timber that should be under 28% and finally on the wood species. Pine is more easy to treat but spruce has better natural resistance.

Enclosed is a small overview about different treatment classes based on EN 351-1:

Species  Hazard class  Analytical
Tolerances1   Pre vacuum  Pressure  Final
Service life2
Spruce HC2 3 mm lateral 75 30 min 30-45 min      

15-20 min
5-10 years
Spruce HC3 3 mm lateral 75 45 min 45-90 min 10-15 years
Spruce HC4 6 mm lateral 75 45 min 180-360 min 10-15 years
Pine HC2 3 mm lateral 90 20 min 30 min 10-15 years
Pine HC3 6 mm lateral 90 30 min 45 min 15-20 years
Pine HC4 Full sapwood 90 30 min 60 min 15-20 years
HC3 Full sapwood 90 30 min 45 min 15-20 years
HC4 Full sapwood 90 30 min 60 min 15-20 years

1 % of test specimen that must conform
2 Imprest guarantees service life only if the timber has been kiln dried prior to the treatment

       Wood is put into tank & pre-vacuum begins.
Air & moisture is pulled out from wood cells.
     Tank is filled up with impregnation liquid.
     With hydraulic pressure liquid is pressed into wood.
     Tank is emptied.
     Final vacuum pulls out unnecessary impregnation liquid.


Treatment certificates

Since last year Imprest is NTR A and NTR AB-certified.  The NTR certificate for wood impregnation confirms that
our company is entitled to impregnate sawn and round wood with the chemical Impralit-KDS according to the requirements of the NTR (Nordic Wood Preservation Council), which is responsible for ensuring the proper and responsible use of wood preservatives and execution of the vacuum-pressure method.

Our products are certified as following:
Preservation classes (EN 335): NTR AB (UC3) and NTR A (UC4)
Penetration class (EN 351): NP 5
Wood preservative: Impralit-KDS

Imprest is also proud to be a member of Wood Preservation Association (WPA). Imprest Premium Range products are certified under the WPA auditing scheme and are valid to carry “Benchmark” logo. That means our treatment operations have been assessed and certified as meeting requirements of WPA Benchmark Quality Scheme for the preservative impregnation of the softwood components in accordance with the standards and practices defined in the WPA manual. Imprest is the first non-UK producer to be awarded this accolade.