Hoisko CLT

10 21 2020 | Fire protection | Fire regulations | wood construction

Cooperation in CLT fire testing to meet B-s1, d0 fire classification

Nordtreat has tested and classified CLT fire protection treatment in accordance with European class B-s1, d0 in close cooperation with Hoisko CLT. The aim of the partnership is to ensure that only reliably fire-protected and at the same time M1-class CLT elements are used in the growing wood construction market.

CLT elements made of cross-glued solid wood are already highly fire-resistant structures when used as they are. However, many applications require fire protection treatment of visible CLT surfaces in accordance with the requirements of European class B-s1, d0.

There is as yet no European product standard for CLT that permits the CE marking, and therefore the conformity of the product’s fire retesting treatment must be verified in some other way. This is important because, at present, building control authorities may accept surface treatments that do not fully meet the requirements of the fire class required for the building in question.

Hoisko CLT house at Tuusulan asuntomessut

Hoisko CLT, Housing Fair Tuusula 2020




Nordtreat and Hoisko CLT wanted to find a solution to the aforementioned problem of reliable fire protection verification. The partners share the view that due to cost or for other reasons, fire protection treatment whose compliance cannot be verified should not be approved in CLT construction. For this reason, it was decided to test and certify fire protection for CLT structures at the Forest and Wood Products Research and Development Institute (MeKA) in Latvia.

For the testing, several angles were built from 90-mm thick CLT panels manufactured from spruce by Hoisko CLT. These were fire-retested by applying a total of 350 g/m2 NT DECO fire retardant. The fire resistance of the structure was tested in standardised tests (e.g. SBI) and found to be compliant with the European class B-s1, d0.


Hoisko CLT production pressure

Manufacture of CLT elements, Hoisko CLT




Demand for strong and stable CLT elements is growing rapidly. Sales manager Jukka Matti-Tuuri of Hoisko CLT, Finland’s largest CLT manufacturer, says that the company’s production volume is now growing rapidly: 

“We are happy to help structural designers implement the most functional and cost-effective CLT solutions. The optimal use of the product is not yet known to all designers.”


Hoisko CLT truck handling


The strong demand results in particular from the desire of municipalities to invest in carbon-neutral construction in building new daycare centres, schools and care sector facilities. However, CLT is also ideal for residential construction, as a load-bearing and single-material structure for sustainable detached houses, terraced houses and blocks of flats.

At present, the need for fire protection of CLT applies in particular to interior cladding, i.e. surfaces visible on walls, ceilings and floors made of CLT. In those cases, the structure has either a 100 mm thick CLT element on the inside and behind it an insulation layer or, more commonly, a thicker (200–240 mm) CLT structure that is also suitable as it is as a load-bearing wall structure.

When the treatment of CLT with NT DECO is carried out in accordance with the manufacturer’s instructions for testing, the result is:

    • Fire protection according to European class B-s1, d0 (EN 13501-1)
    • Environmentally friendly fire protection (the fire retardant does not contain generally harmful substances such as boric acid, boron, bromides, formaldehyde or ammonia)
    • The best M1 emission class for building materials with fire-retardant material (research-proven clean air quality)
    • The natural wood patterns of CLT show through in the desired translucent shade (thousands of colour options)
    • Cost-effective treatment (one product, no separate primers or surface paints)

In addition, when using NT DECO the inherent breathability of the solid wood structure is maintained.


CLT element lifting - Hoisko CLT




Hoisko fire protects its CLT elements according to Nordtreat’s certified treatment method. Because CLT elements are quite large and the volumes are increasing, the treatment is also carried out by a Nordtreat-trained party if necessary.


“Fire-retardant CLT elements supplied by Hoisko meet the requirements of European class B-s1, d0. The processing is always documented, and the customer receives a certificate of certified processing.”

Jukka Matti-Tuuri, CLT Finland Oy


In Hoisko’s CLT projects, CLT fire protection is always carried out under controlled conditions, i.e. in halls where indoor air humidity, temperature and other conditions are optimal for the absorption of the fire retardant.


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