FAQ

Timber burns, but predictably: a charred layer forms on its surface, which slows down charring and protects the inner cross-section. That is why the fire resistance of a CLT structure can be calculated with a standard method – according to the new Eurocode 5, for up to 120 minutes.

Its reaction-to-fire class (D-s2, d0) is the same as that of solid timber and glulam. Where the requirements are stricter, gypsum board cladding or a fire-retardant coating helps. We prepare the fire safety design with a partner office experienced in CLT. More: the new Eurocode 5 and CLT.

If it stays dry, a very long time. CLT may only be used in service classes 1 and 2 – in heated or covered, protected places; there the moisture content of the timber stays below 20%, so wood-destroying fungi do not attack it and insect damage is not a concern either.

A good example of how long a well-protected timber structure lasts is the seven-storey timber granary in Geislingen, Germany, built in 1445. The key is design: the structure must be permanently protected from moisture.

A short shower does no harm, but CLT must be protected from moisture from delivery to installation, and until the permanent weather protection is complete: installed elements are covered, stored elements are kept off the ground under tarpaulins.

The transport film must be removed quickly, because condensation underneath can cause blue stain or mould. If an element does get soaked, it must be dried quickly but gently, and the building must be well ventilated during screed or plastering work. We plan moisture protection together with production and the assembly sequence.

No. CLT elements are usually connected with self-drilling, fully or partially threaded timber screws, but a screw behaves differently in CLT than in solid timber – because of the gaps and the cross layers.

That is why only fasteners whose load-bearing values for CLT are confirmed by a European Technical Assessment (ETA) may be used, and every connection must be designed by structural calculation. The type, number and spacing are specified by the designer – they cannot be swapped on site.

In our view, yes. In Hungary, where we are based, the construction documents must clearly specify the name, position, size and technical properties of every material, structure and product to be installed (Government Decree 191/2009, Section 22(3)). For bolted, riveted, welded or glued structures, the structural design must also include connection detail drawings at a scale of 1:20 – this also applies to CLT, which consists of glued elements with screwed connections. In other countries the national rules apply.

The new execution part of Eurocode 5 (EN 1995-3:2025) confirms the same Europe-wide: before execution, the drawings, the technical specification and an execution specification checked for completeness must be available on site.

For the structural design for construction we also prepare the connection detail drawings, and in the fabrication drawings we specify the fasteners element by element, with a bill of materials.

Yes. Not only walls, floors and roofs can be made of CLT, but also stair flights, balcony slabs, lintels and columns. Like the other elements, a stair flight is made to exact size in the factory.

One thing needs attention, though: CLT is made of spruce, a softwood, and stair treads are subject to heavy wear. A CLT stair therefore needs a hard-wearing tread surface, for example a hardwood (oak or beech) covering – it is no coincidence that parquet is usually made of hardwood, too. The CLT provides the structure, the covering a durable, attractive walking surface.

It is worth deciding the structure of the stair – CLT, reinforced concrete or gypsum concrete – already at concept stage.

Good, if it is well designed. A CLT element is light and stiff, so on its own it is not enough for sound insulation – good results come from the build-up.

For walls, good airborne sound insulation is achieved with double- or multi-leaf build-ups and a flexible lining wall, which also serves as a service zone. For floors, a floating screed, impact sound insulation and, if needed, a heavy fill or a suspended ceiling interrupt the path of sound – so impact sound insulation can even exceed stricter requirements. Acoustic design is carried out by our partner office.

CLT can be insulated on the outside without thermal bridges, with many materials: wood fibre or mineral wool insulation, an external thermal insulation composite system or a ventilated facade. On the inside, an insulated service zone can further improve insulation – passive house build-ups are not unusual.

In terms of vapour, CLT acts as a vapour retarder (μ = 30–80), so depending on the outer layers, an internal vapour barrier may even be omitted – this must be verified by calculation for each build-up. The high heat storage capacity of timber gives a more pleasant indoor climate in summer.

Yes. The pipes of underfloor heating go into the screed or dry screed system laid on the CLT floor, not into the timber element itself; the structural calculation takes the additional load into account. For ceiling heating or cooling, the panels go into the service zone or a suspended ceiling.

During the first heating season, it is best to heat up gradually and ventilate, so that the timber slowly reaches its final moisture content – this means fewer drying cracks.

CLT is a structural element, not a furniture panel. Just as exposed concrete shows the formwork joints and fixing points, visible CLT shows the joints between panels and the gaps between boards. This is a natural part of the structure, not a defect.

Manufacturing accuracy matches this: according to industry guidance, the tolerance for thickness and width is ±2 mm, and for length ±3–5 mm depending on size. For a building element this is excellent accuracy, but not cabinet-maker accuracy.

Timber continues to dry after installation, so drying cracks may appear on the surface. This is the case with every solid timber product and does not affect the load-bearing capacity of the structure.

If you want a furniture-quality surface: we order the CLT in non-visible quality and, at the end of construction, add a thin three-layer solid wood panel of the best surface quality to the visible side. The thinnest panels are about 12–13 mm thick. The surface is uniform, shows no cracks and is not damaged during construction.

Industry guidance distinguishes three surface qualities. Non-visible quality is for covered surfaces: the surface is levelled, gaps are up to 6 mm, and other wood species may be mixed in. Industrial visible quality is planed or sanded, made of one wood species, with gaps up to 4 mm. Residential visible quality has the strictest requirements: gaps up to 2 mm, dark or loose knots repaired, discolouration up to 5%.

Manufacturers may also have their own surface categories. Without a separate agreement, the simplest quality is delivered – so visible surfaces must be specified in the design and in the order.

A resin pocket is a resin-filled cavity that occurs naturally in spruce. It does not affect strength; according to industry guidance it is permitted in non-visible and industrial visible quality, but in residential visible quality only in small sizes – up to 5 × 50 mm – and larger ones are repaired by the manufacturer.

Many people would like an industrial visible surface in living spaces, too. In our experience, resin can melt out of the pockets in warm conditions and drip – in living spaces this is not a good solution.

A good alternative is non-visible CLT covered with a three-layer solid wood panel – see the question about visible CLT surfaces.

It is not required. Untreated timber is a natural surface; over time its colour darkens under the influence of light. If you want to slow this down, or would like a surface that is easier to clean, CLT can be stained, painted, varnished or oiled.

Where fire safety requires it, the surface can be made flame-retardant with a fire-retardant coating.

Usually not. During construction, mainly the outer layers absorb moisture, which then gradually dries out during use. As with every solid timber product, drying cracks appear; according to industry guidance they are characteristics of the product and cannot be avoided. Glue lines and finger joints are not considered defects either.

The best way to reduce cracking is gradual drying: heat up slowly during the first heating season and ventilate regularly. If a crack is unusually large, ask a structural engineer for an opinion. If you do not want to see any cracks on the visible surface at all, non-visible CLT covered with a three-layer panel is a good solution (see the question about visible CLT surfaces).