NO345746B1

NO345746B1 - A self-extinguishing cross laminated timber (CLT) element

Info

Publication number: NO345746B1
Application number: NO20191468A
Authority: NO
Inventors: Geir Jensen; Alexander Jensen
Original assignee: Fss Tre As
Priority date: 2019-12-11
Filing date: 2019-12-11
Publication date: 2021-07-12
Also published as: EP4072854A1; NO20200247A1; NO345852B1; WO2021116305A1; NO20191468A1

Description

The present invention is related to a self-extinguishing timber structure comprising one or more cross laminated timber (CLT) elements, each CLT element comprising a plurality of lamellae and each lamella comprises one or more board sections. The lamella and board sections are arranged such that during an enclosure fire, the at least one of the plurality of lamellae and/or at least one of the board sections char seamlessly, and/or delaminates at different points in time than the adjacent lamellae and/or boards.

BACKGROUND

A wall element of solid timber boards with a thickness exceeding e.g. 50 mm when subjected to a room fire, may self-extinguish in the decay phase after room content is burnt away. This is due to a char layer which is always protecting fresh wood behind it and is well documented. Solid homogenous wall elements of thicknesses 50-200 mm are not available today due to high cost, less practical to manufacture without warping and susceptibility of developing cracks in use. The most common timber wall elements used today are cross-laminated ones (CLT).

Cross-laminated timber (CLT) is a wood panel product made from gluing layers of solid-sawn lumber together. Each layer of boards is usually oriented perpendicular to adjacent layers and bonded to the wide faces of each board, usually in a symmetric way so that the outer layers have the same orientation. Regular timber is an anisotropic material, meaning that the physical properties change depending on the direction at which the force is applied. By gluing layers of wood at right angles, the panel is able to achieve better structural rigidity in both directions. It is similar to plywood but with distinctively thicker laminations (or lamellae).

Traditional cross laminated timber (CLT) elements, when not protected by gypsum or calcium silicate, cementitious boards or fire protective paint (i.e. on wood surfaces facing the room) are practical up to a certain point only. Further development of the fire after such protective layers delaminate becomes less predictable in terms of self-extinction (see below).

All CLT elements have layers of wood (lamellae), typically glued together. During a fire, as the protective layer or some areas of the first timber lamella is burnt through, the remaining of the first lamella may be abruptly detached from the wood element and drop down simultaneously. Because the room fire is uniform at this stage, this detachment will occur about the same time for all first lamella of the CLT wall elements in the room (ceiling CLT elements may delaminate a bit earlier).

This simultaneous separation of the protective layers or the first timber lamellae will expose uncharred fresh surface of the next lamella layers of the CLT element, which will cause the fire heat release rate (HRR) to regain substantially by radiation and reradiation. The HRR is the rate of heat generated by fire. It is a measure of the heat that is available in every square meter of surface absorbing heat within a particular surface. The HRR may be measured in Joules per second or Watts.

By regaining the HRR, the room temperature may increase so reignition and secondary flashover occur. As a result, flame impinging on facade out from window increases as well. Inside the burning room, the fire may cause increased charring rate each time a CLT lamella delaminates. The fire eventually does not decay and do not self-extinguish. Full burn-through of a CLT element is a critical fire compartmentation failure which often leads to total loss and pose risk of loadbearing collapse.

Full scale fire tests have shown that due to the unwanted delamination of the CLT lamellae, at least two walls and ceiling of a room need to be protected by several layers of gypsum or other protection rated for e.g. 60 min fire resistance to ensure self-extinction in case of fire.

If delamination does not occur abrupt and simultaneous, the number of exposed wood walls in a room may be increased to three or all four, depending on type of ceiling and ventilation openings of room on fire. This is what the market and architects are craving for and yet out of reach.

However, it is neither the mean HRR nor the total energy of the CLT elements that prevent fire extinction or cause secondary flashovers. It is the post-flashover peaks of HRR that define the difference of solid timber and CLT. The peaks of HRR are caused by abrupt delamination of large areas. Without the HRR peaks, or if peaks are evened out and reduced sufficiently, the CLT will self-extinguish similar to solid timber.

Furthermore, tests have shown that the effect of current CLT is delicate: even a small reduction in delaminated area can reduce the HRR peak sufficiently for the room fire to self-extinguish.

Currently, the best fire-resistant glues to effectively mitigate the delamination effect are rarely applied due to secondary disadvantages. Those products have several disadvantages such as: harmful to environment, extended hardening times, application difficulties and/or increased cost. Some have proposed to use solid timber instead of or as first lamellae of CLT or using high density wood specimens.

Neither is viable however, as the benefits of stiffness, accessibility, ease of CLT production and low cost then may be lost. In addition, thick first lamellae to face fire may at the same time lead to unacceptable thickness increase of CLT elements. This will again have limited effect since the extinction is prevented during charring time of that first lamella only. Typical 40 or 50 mm is the maximum thickness of wooden boards for CLT. Thicker boards will reduce stability, cause warping and are prone to developing cracks right after installation.

A strong incentive for using CLT in buildings are to allow the wooden surfaces to be exposed.

EP 3.424.657 A1 relates to a fire resistant plywood panel and a method for improving fire resistance of a plywood panel. The method comprises receiving a plywood panel, the plywood panel comprising a first veneer layer, a second veneer layer, and adhesive in between the first veneer layer and the second veneer layer, the first veneer layer forming a first surface of the plywood panel and having grains oriented in a first grain direction. The method comprises sanding the first surface of the plywood panel in a first sanding direction that forms an angle of at least 30 degrees with the first grain direction, and after said sanding the first surface, applying fire retardant onto the first surface having a temperature of at least 15 °C. The plywood panel is treated with fire retardants between layers to extend the time of burn-through.

In view of the above, the aim of the present invention is to provide a wood structure or a wood element that resolve or at least mitigate one or more of the abovementioned problems related to fire.

The major aim of the present invention is to provide a wood structure or element that in case of a fire will have increased probability of self-extinction.

SUMMARY OF THE INVENTION

The present invention is related to a self-extinguishing cross laminated timber (CLT) element comprising a plurality of lamellae arranged to form the CLT element, each lamella comprises one or more board sections arranged side by side to form one lamella.

At least one lamella of the plurality of lamellae comprises at least one or more board sections arranged with different thickness in relation to adjacent board section, such that during an enclosure fire, the at least one of the one or more board sections char seamlessly, and/or delaminates at different points in time than the adjacent board section.

The term adjacent is directed to a neighboring board section having substantially same orientation and located within the same lamella or a neighboring lamella.

The term cross laminated wood element may be a CLT element or wood panel made from layers of board sections or solid-sawn timber joined together. Each layer may comprise one or more board sections of solid or semi-solid lumber and may be oriented perpendicular or at least with an angle to adjacent layers and glued on the wide faces of each board.

The plurality of lamellae may be attached together by graded edges, adhesives, glass-fiber reinforced adhesive glue, dowels, interlocking, joinery, nails or other means of bonding lamellae to each other.

In case of a fire, each board section and lamella delaminate layer by layer, as the fire burns into or through the cross laminated timber element, as disclosed in prior art.

The present invention provides a self-extinguishing cross laminated timber (CLT) element by avoiding that most of the lamellae delaminates at the same time and expose the fresh lamella wood layer behind and causing the heat release rate (HRR) to increase. Thus, the char layer of the present invention will be attached to the CLT element for a longer time, thus protecting the fresh wood layer behind.

Since the delamination of the layers (lamellae) may occur at different points in time, the CLT element of the present invention may respond to fire similar to a solid timber, and by a protective char layer that ensure self-extinguishing and prevent secondary flashovers.

According to the present invention, the mating surface between the at least one or more board sections and the adjacent board sections, comprises neighboring board sections that has different thickness in relation to each other.

Each of the one or more board sections may comprises an assembly of boards or at least one of the one or more board sections may comprise an assembly of boards.

A board section may therefore comprise one board or comprise two or more boards arranged in a series/layers to form one board section. The boards arranged in an assembly of boards are also referred to as sub-boards.

The at least one lamella may comprise a first assembly of boards and a second assembly of boards arranged alternating side by side to form the lamella. Each of the first assembly and the second assembly of boards may comprise an upper subboard arranged above a lower sub-board.

The upper sub-board of the first board assembly may have a thickness different than the upper sub-board of the second board assembly, such that the upper sub-boards of the first and second board assembly are staggered in relation to each other.

The upper sub-board of the first board assembly may have same thickness as the lower sub-board of the second board assembly, and the lower sub-board of the first board assembly, has same thickness as the upper sub-board of the second board assembly.

The width of upper and lower sub-boards dictates the shape of the assembly which is then preferably rectangular or square cross-sectional shape.

The upper and lower sub-board of the first assembly of boards may be respectively a thin board bonded to a thick board. In the second assembly of boards, the arrangement is opposite such that the upper sub-board is thick and bonded on top of a thin board. The first and second assembly of boards are then bonded together at their sides such that the upper layers of sub-boards has a staggered relation in relation to each other. Hence, the upper layer of the sub-boards of the first and second board assembly has different thickness.

During an enclosure fire the upper layer of the sub-boards, having respective thin and thick layers arranged alternating, the thin layers will delaminate first, and the thick layers delaminate at a later stage. Thus, avoiding a sudden and simultaneous delamination of the upper layer.

Furthermore, each of the first and second assembly of boards may comprise the upper sub-board having a varying thickness from one side of the assembly of boards to another, and the lower sub-board having a varying thickness in opposite direction than the upper sub-board.

The upper sub-board may have a decreasing thickness from the one side of the assembly of boards to another, and wherein the lower sub-board may have an increasing thickness from the one side of the assembly of boards to another, in opposite direction of the upper sub-board.

The first and second assembly of boards may have the same assembly of sub-boards with varying thickness, such that when they are bonded together and arranged alternating first and second assembly of boards, the upper layer of the sub-boards are arranged in a staggered manner in relation to each other. The thin side of the varying thickness upper sub-board of the first assembly of boards, is facing the thick side of the varying thickness upper sub-board of the second assembly of boards. The opposite applies for the lower sub-board where the thick side of the first assembly of boards is facing the thin side of the lower sub-board of the second board assembly.

During an enclosure fire the upper layer of sub-board char seamlessly, meaning that the charring advances at a moderated or almost fixed rate across the sub-board due to the varying thickness. The lower sub-layer (below the upper sub-layer) of subboards may be arranged in a staggered manner such that the seamless charring will continue into this layer.

The one or more board sections may have same thickness such that the at least one lamella of the plurality of lamellae has an even thickness across the CLT element.

Each lamella of the plurality of lamella are oriented perpendicular to adjacent lamellae, and wherein each lamella may comprise a number of board assemblies according to the embodiments described above.

Each lamella may have even thickness across the entire lamella, meaning that the board or assembly or boards of the lamella, all have same thickness. The lamella may be oriented perpendicular to adjacent lamella and bonded to the wide faces of each board, in a symmetric way so that the outer layers (lamellae) have the same orientation.

The self-extinguishing cross laminated timber (CLT) element may constitute one or more CLT sub-elements. The sub-elements may be connected such that they form the self-extinguishing cross laminated timber (CLT) element.

The self-extinguishing cross laminated timber (CLT) element may comprise a first sub-element and a second sub-element, wherein the first assembly of board is arranged on the first-sub element and the second assembly of boards is arranged on the second sub-element, and wherein the first assembly of board has different thickness in relation to adjacent second assembly of boards.

The self-extinguishing cross laminated timber (CLT) element and/or sub-element may be provided with a locking system provided on at least two opposite ends of the CLT element and/or sub-element, for connection to another symmetric or asymmetric CLT element and/or sub-element.

The locking system may comprise a first receiving structure and a second mating structure arranged at opposite ends, the first receiving structure comprises an interface complementary to the second structure.

The CLT elements may be connected by graded edges, click boards, finger joints, joinery, adhesives, glass fiber-reinforced adhesives, glue or combinations thereof.

The intention of the present invention is to allow local delamination to occur at different points in time to even out the fire HRR over time and thus allowing new char layer to gradually build on next layer of fresh wood.

Furthermore, the intention is also to ensure delamination of a single lamella to occur over a period of time rather than abrupt, to further even out the HRR and avoid HRR peaks.

A major impact of the present invention may be to enable self-extinction, prevent regrowth of fire, prevent secondary flashovers and prevent increased exposure to façade.

Further objects, structural embodiments and advantages of the present invention will be seen clearly from the following detailed description, the attached drawings and the claims below.

FIGURES

The invention will now be described with reference to the attached figures, wherein:

Figure 1 shows a prior art CLT element comprising five layers (lamellae).

Figure 2 a) shows a section of a CLT element comprising three lamellae, each lamella comprising plurality of board sections and each board section comprises assembly of boards.

Figure 2 b) shows a cross section of the CLT element section in figure 2 a).

Figure 3 a) shows a first and second assembly of boards, where each assembly comprises sub-boards with varying thickness.

Figure 3 b) shows a cross section of the CLT element in figure 3 a).

Figure 4 shows graphically a HRR rate over time during a room fire, the first in a room of solid wood panels and the second in a room of CLT panels where lamellae delaminated.

Figure 5 shows graphically the Mass Loss Rate (g/m2’*s) over time, during a room fire in a room with respective solid wood panels CLT panels. ;Figure 6 shows graphically the Mass Loss Rate (g/m2’*s) over time of figure 5, where the Mass Loss Rate of the present invention is shown.

DETAILED DESCRIPTION OF THE FIGURES

In the following, embodiments of the invention will be discussed in more detail with reference to the appended drawings. It should be understood, however, that the drawings are not intended to limit the invention to the subject-matter depicted in the drawings.

Figure 1 shows a prior art cross-laminated timber (CLT) element 10. The CLT element 10 is a wood panel product made from layers (lamellae) 11 of solid-sawn lumber joined together. Each lamella 11 comprises boards 12 oriented perpendicular to adjacent lamella 11 and bonded on the wide faces of each board 12. The boards 12 of each lamella 11 are bonded side by side providing a bond line 18 between the boards 12. The lamellae 11 are further arranged symmetric such that outer layers (top and bottom) have the same orientation. An odd number of lamellae 11 (such as 3, 5 or 7 layers) are most common, but there are configurations with even numbers as well (which are then arranged to give a symmetric configuration). The figure 1 shows CLT element 10 having 5 layers or lamellae 11. The outer lamellae 11 are arranged symmetric about the center lamella, and all lamellae has same thickness.

The figure 2 a) shows an embodiment of the present invention where the CLT element comprises three lamellae 11, each lamella 11 comprises a plurality of board 12 sections and each board 12 section comprises an assembly of two sub-boards (upper and lower) arranged one over another.

Thus, the one or more boards 12 section may comprise one board or an assembly of boards 13, 14. The sub-boards may be an upper sub-board 15 arranged above a lower sub-board 16. The at least one lamella 11 may comprise a first assembly of boards 13 and a second assembly of boards 14 arranged alternating side by side to form the lamella 11. At the bond line 18 between the first assembly of boards 13 and the second assembly of boards 14, the thickness of the boards are different or staggered.

As shown in figure 2 a) and 2 b) the upper sub-board 15 of the first assembly of board 13 may be a thin board, arranged above a lower sub-board 16 which is a thick board, both sample embodiments 160 mm wide and respectively 30 and 50 mm thick, so assembly is rectangular 160x80 mm. The first assembly of board 13 is arranged to be bonded to the second assembly of boards 14, where the second assembly of boards 14 comprises an upper sub-board which is thick and a lower sub-board which is thin (opposite than the first assembly of boards). When bonded, the upper sub-board 15 of the first and second assembly of boards 13, 14 will have a thin board facing a thick board.

During an enclosure fire the upper layer of the sub-boards 15, having respective thin and thick layers arranged alternating, the thin layers will delaminate first, and the thick layers delaminate at a later stage. This arrangement avoids the sudden and simultaneous delamination of the upper layer of sub-boards 15 throughout the CLT element.

Figure 2 a) and 2 b) shows a three-layer (lamellae) CLT element, where each layer comprises a plurality of first and second assembly of boards 13, 14 arranged alternating. The middle layer is arranged perpendicular and bonded together with the upper and lower layer (lamella).

In a sample embodiment, each of the first and second assembly of boards 13, 14 may comprise the upper sub-board 15 having a varying thickness from one end of the assembly of boards 13, 14 to another, and the lower sub-board 16 having a varying thickness in opposite direction than the upper sub-board 15.

Figure 2b) also shows that the self-extinguishing cross laminated timber (CLT) element 10 may constitute one or more CLT sub-elements 20. The sub-elements 20 may be connected/bonded at an interface 21, between the sub-elements 20, such that they form the self-extinguishing cross laminated timber (CLT) element 10.

The self-extinguishing cross laminated timber (CLT) element 10 may comprise a first sub-element 20 and a second sub-element 20, wherein the first assembly of board 13 is arranged on the first-sub element 20 and the second assembly of boards 14 is arranged on the second sub-element 20, and wherein the first assembly of board 13 has different thickness in relation to adjacent second assembly of boards 14.

Figure 3 a) shows a first assembly of boards 13 and a second assembly of boards 14. Each assembly of boards 13, 14 comprises an upper sub-board 15 and a lower subboard 16 having varying thickness.

The upper sub-board 15 may have a decreasing thickness from the one end of the assembly of boards 13, 14 to another, and wherein the lower sub-board 16 may have an increasing thickness from the one end of the assembly of boards 13, 14 to another, in opposite direction of the upper sub-board 15. The upper sub-board 15 and lower sub-board 16 are bonded together at a bond line 18.

In a sample embodiment of principle shown by figure 3 a) the thin edge is 15 mm and the thick edge is 65 mm, both boards 120 mm wide, so the assembly of boards provides a rectangular board section of 120x80 mm.

The first and second assembly of boards 13, 14 may have same arrangement of upper and lower sub-boards 15, 16. When they are bonded together at a bond line 18 and arranged alternating first and second assembly of boards, the upper layers of the sub-boards 15 are arranged in a staggered manner in relation to each other. The thin end of the varying thickness upper sub-board 15 of the first assembly of boards 13, is facing the thick end of the varying thickness upper sub-board 15 of the second assembly of boards 14. The opposite applies for the lower sub-board 16 where the thick end of the first assembly of boards 13 is facing the thin end of the lower subboard 16 of the second board assembly 14.

As shown in figure 3 b) the CLT element may comprise three layers (lamella) 11 where all lamella 11 comprises a plurality of first and second assembly of boards 13, 14 arranged alternating. In this embodiment, the first and second assembly 13, 14 of boards has same structure. The middle lamella 11 is arranged perpendicular to the upper and lower lamella 11 and all lamellas 11 has the same structure with varying thickness sub-boards 15, 16.

During an enclosure fire the upper layer of sub-board 15 char seamlessly, meaning that the charring takes place at a fixed rate across the sub-board due to the varying thickness. The lower sub-layer (below the upper sub-layer) of sub-boards 16 are also arranged in a staggered manner such that the seamless charring will continue into this layer.

From publication Wiesner: "Structural capacity in fire of laminated timber elements in compartments with exposed timber surfaces", Engineering Structures Publication 179. 2019, following graphs demonstrates the HRR over time for a wall element.

The first graph a) Beta1, in Figure 4 shows a graph of the Heat Release Rate (HRR) over time (duration) for a wall element of solid wood during a room fire. The HRR is a rate of heat produced by the fire and is available to all exposed surfaces. The accumulated heat exposing wood surfaces will ignite them at certain level of heat or radiation. HRR are measured in Joules per second or Watts (Megawatt).

The graph shows that the HRR increases rapidly in a room fire when the outer surface of the solid wood element gets involved. The HRR increases and has a peak of about 5 Megawatt at 10-20 min., during which a char layer has been built on the outer surface of the solid wood element. Due to the char layer of the wood which is always protecting the fresh wood behind, the HRR rate will drastically drop after approximately 20 min, when all the contents in the room is burnt away. The added heat to the solid wood surface will decrease and the HRR rate will drop drastically, causing the fire to self-extinguish in the decay phase.

The graph b) Beta2, of figure 4 shows a graph of the Heat Release Rate (HRR) over time (duration) for a wall element of CLT panel during a room fire. The CLT panel which comprises a plurality of layers, will have the same fire development as for the solid wood in the decay phase. The HRR increases quickly to a peak of about 5 MW in about 10-15 min. Due to the development of the char layer (first layer), the HRR drops when all the contents in the room is burnt away at about 15 min. When the first layer is burnt through the remaining of the first layer may be abruptly detach from the CLT panel and drop down simultaneously. Because the room fire is uniform at his stage, this detachment will occur about the same time for all first layers of the CLT panels in the room.

This is shown in greyscale (“char fall-off) in the graph b) Beta2. The delamination of the first layers will cause the fire HRR to regain substantially by radiation and reradiation. By regaining the HRR, the room temperature may increase, reignition and secondary flashover may occur. As a result, flame impinging on facade out from window increases as well. Inside the burning room, the fire may cause increased charring rate as layer by layer of the CLT elements delaminates as shown in the graph. The fire eventually does not decay and do not self-extinguish. Manual fire extinction must be performed in order to extinguish the fire. Final burn-through of a CLT panel is a critical fire compartmentation failure which often leads to total loss and pose risk of loadbearing collapse.

As disclosed above, it is neither the mean HRR nor the total energy of the CLT panels that prevent fire extinction or cause secondary flashovers. It is the peaks of HRR that define the difference of solid timber and CLT. The peaks of HRR are caused by abrupt delamination of large areas. Without the HRR peaks, or if peaks are evened out and reduced sufficiently, the CLT will self-extinguish similar to solid timber. Furthermore, tests have shown that the effect of current CLT is delicate: even a small reduction in delaminated area can reduce the HRR peak sufficiently for the room fire to self-extinguish.

Figures 3 and 4 are taken from the publication: "Analysis of cross-laminated timber charring rates upon exposure to non-standard heating conditions”, by Bartlett, University of Edinburgh.

Figure 5 shows graphically the Mass Loss Rate (MLR) in weight (in grams) per square meter per second over a time duration (in minutes). The solid line disclosing a MLR for a room fire in a room having solid wood panels. The MLR increases rapidly as the solid wood panel takes fire and establishes a char layer. When the char layer has been established, the MLR drops significantly in accordance with the drop of the HRR, which eventually leads to self-extinction of the fire.

The dotted line in figure 4 shows a traditional CLT panel with a plurality of layers, where the MLR decreases after a char layer has been built up on the outer surface of the CLT panel. Due to the delamination of the outer layer, the MLR increase again when the reignition and flashover starts on the fresh wood behind the outer layer, causing the MLR to increase until a new char layer has been established. The dotted line in figure 5 shows the time when the delamination occurs, resulting in an increase in MLR just after the delamination.

Figure 6 shows the same graphs of figure 5, but with the graph of a CLT panel according to the present invention included. This graph in thick solid line qualitatively illustrates the effect of the invention by the embodiment shown by figure 2 a) and b). Due to the avoided sudden delamination of the outer layer the peaks of the MLR may be prevented by allowing delamination to occur at different times in order to even out the fire HRR over time and thus allowing new char layer to gradually build on next layer of fresh wood. A curve to qualitatively illustrate the effect of embodiment as of figure 3 a) and b) would be smooth and overlap fully the thin solid line (not shown). Hence, the CLT wood panels of the present invention allows delamination of a single lamella (layer) to occur over a period of time rather than abrupt, to further even out the HRR and avoid HRR peaks, thus also the Mass Loss Rate (MLR).

Claims

1. A self-extinguishing cross laminated timber (CLT) element (10) comprising a plurality of lamellae (11) arranged to form the CLT element (10), each lamella (11) comprises two or more board sections (12) arranged side by side to form one lamella, characterized in that at least one lamella (11) of the plurality of lamellae (11) comprises at least one or more board sections (12) arranged with a different thickness in relation to adjacent board section (12), such that during an enclosure fire the at least one of the two or more board sections (12) char seamlessly, and/or delaminates at different time point than the adjacent board section (12).

2. The self-extinguishing CLT element (10) according to claim 1, wherein at least one of the two or more board sections (12) comprises an assembly (13, 14) of boards.

3. The self-extinguishing CLT element (10) according to claim 1 or 2, wherein the at least one lamella (11) comprises a first assembly (13) of boards and a second assembly (14) of boards arranged alternating side by side to form the lamella (11), wherein each of the first assembly (13) and the second assembly (14) of boards comprises an upper sub-board (15) arranged above a lower sub-board (16).

4. The self-extinguishing CLT element (10) according to claim 3, wherein the upper sub-board (15) of the first assembly (13) of boards, has a thickness different than the upper sub-board (15) of the second assembly (14) of boards such that the upper sub-boards (15) of the first and second assembly (13, 14) of boards are staggered in relation to each other.

5. The self-extinguishing CLT element (10) according to claim 4, wherein the upper sub-board (15) of the first assembly (13) of boards has same thickness as the lower sub-board (15) of the second assembly (14) of boards, and the lower sub-board (16) of the first assembly (13) of boards, has same thickness as the upper sub-board (15) of the second assembly (14) of boards.

6. The self-extinguishing CLT element (10) according to claim 3, wherein each of the first and second assembly (13, 14) of boards comprises the upper subboard (15) having a varying thickness from one side of the assembly (13, 14) of boards to another, and the lower sub-board (16) having a varying thickness in opposite direction than the upper sub-board (15).

7. The self-extinguishing CLT element (10) according to claim 6, wherein the upper sub-board (15) has a decreasing thickness from the one side of the assembly (13, 14) of boards to another, and wherein the lower sub-board (16) has an increasing thickness from the one side of the assembly (13, 14) of boards to another, in opposite direction of the upper sub-board (15).

8. The self-extinguishing CLT element (10) according to any one of the preceding claims, wherein the one or more board sections (12) has same thickness such that the at least one lamella (11) has even thickness across the CLT element (10).

9. The self-extinguishing CLT element (10), wherein each lamella (11) of the plurality of lamella (11) are oriented perpendicular to adjacent lamella (11), wherein each lamella (11) comprises board sections (11) according to any one of claims 4 -7.