DE20116962U1 - Connection system for floor boards and floor boards therefor - Google Patents

Description

Munich, Our sign:

Applicant/Owner: Official reference number:

12 April 2002 PM5114 RF/rec

PALMBERG, Ulf 201 16 962.2

• Berlin

' Patent attorneys

European Patent Attorneys

Dipl.-Ing. Henning Christiansen

Dipl.-Ing. Joachim von Oppen

Dipl.-Ing. Jutta Kaden

Patent Attorney

Dipl.-Phys. Dr. Andreas Theobald

Alicante

European Trademark Attorney
Dipl.-Ing. Jürgen Klinghardt

Munich

Patent attorneys

European Patent Attorneys

Dipl.-Phys. Heinz Nöth

Dipl.-Wirt.-Ing. Rainer Fritsche

Lbm.-Chem. Gabriele Leißler-Gerstl

Dipl.-Ing. Olaf Ungerer

Patent Attorney

Dipl.-Chem. Dr. Peter Schuler

Arnulfstrasse 25

D-80335 Munich

Phone +49-10)89 -549 0750

Phone +49-(0)89-544 1380

Fax +49-10)89-5502 7555 (G3)

Fax +49-10)89-5441 3838 (G3)

Fax +49-(0|89-5490 7529 (G4)

mail@eisenfuhr.com

http://www.eisenfuhr.com

Patent attorneys
European Patent Attorneys Dipl.-Ing. Günther Eisenführ Dipl.-Ing. Dieter K. Speiser Dr.-Ing. Werner W. Rabus Dipl.-Ing. Jürgen Brügge Dipl.-Ing. Jürgen Klinghardt Dipl.-Ing. Klaus G. Göken Jochen Ehlers
Dipl.-Ing. Mark Andres Dipl.-Chem. Dr. Uwe Stilkenböhmer Dipl.-Ing. Stephan Keck Dipl.-Ing. Johannes MB Wasiljeff

Lawyers

Ulrich H. Sander

Christian Spintig \,

Sabine Richter

Harald A. Foerster

Hamburg
Patent Attorney

European Patent Attorney, Dipl.-Phys. Frank Meier,

Lawyers \

Rainer Boehm \

Nicol A. Schrömgens, LL. M.

CONNECTION SYSTEM FOR FLOORING BOARDS AND
PLANKS FOR HERE

SUBJECT OF THE INVENTION

The present invention relates to a connection system for floorboards, panels and discs, but mainly for parquet. The invention will replace the traditional tongue and groove technique. By providing slots or bevels in the projection corresponding to the tenon and corresponding cuts in the lower limiting edge of the groove, the planks can be connected in a substantially vertical movement or a transverse movement and then locked by a horizontal movement in the longitudinal direction of the planks. At the same time, the projection is hook-shaped so that the projection engages the groove. This will make the connection very easy to carry out, while at the same time providing a strong connection.

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STATE OF THE ART

The traditional tongue and tenon technique for floorboards has been known in the art for a long time. Furthermore, a number of technical alternative solutions with different shapes of projections and recesses are available. A more secure locking than with a tongue and tenon locking has been achieved by giving the projections different hook shapes. During assembly, the boards are tilted or angled against each other, connected and pivoted to obtain a flat floor surface and locked at the same time. Examples of this technique are shown, for example, in WO 94/26999, US Patent No. 5,797,237 and US Patent No. 6,006,486. It is also known to effect the locking with separate auxiliary locking elements. This is shown, for example, in SE B 512 290.

However, it is not always easy to join the boards together, as the full length of a projection on a board must be fitted into the recess of an existing board that has already been laid before the new board can be pivoted and locked into its final position. This is particularly difficult if the board is warped, not straight overall, and too long for one person to control both ends at the same time.

The inventor of the present application has realized that joining will become easier if the necessary angle between the planks is reduced and the planks can be joined with a largely vertical movement. In a further embodiment, the planks can be joined with a largely transverse movement. This is achieved by providing milled slots or bevels in the projections and in the lower edge of the recesses, which enables the plank to be joined in such a way that it can be inserted into the already laid plank more or less from above or from the side. The current plank can then be moved lengthwise to obtain the final locking thereto.

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The invention enables a simpler floor installation that can be carried out by one person. At the same time, the floorboard manufacturing process has not been significantly complicated.

SUMMARY OF THE INVENTION

The invention therefore relates to a connection system for floorboards, comprising boards to be connected to one another. The first board has a projection which fits into a recess in the second board. According to the invention, the projection on the first board has hooks or hook-shaped projections with slots or bevels arranged therebetween, and the recess in the second board is delimited in the upward direction by a solid edge and in the downward direction by an edge with heads which are of such a size that they can be inserted into the first board through the slots.

Preferably, the lower edge of the recess has shoulders between the heads.

The invention further relates to a corresponding plank for such a connection system.

The invention is defined in the appended claims 1, 11, 21 and 30, whereas preferred embodiments are described in the dependent claims.

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SHORT DESCRIPTION OF THE DRAWINGS

The invention is explained in more detail below with reference to the accompanying drawing figures.

Fig. 1 is a partial sectional view of two planks, viewed from the shorter side, which are not connected to each other, according to an embodiment of the present invention;

Fig. 2 is a view similar to Figure 1 with the planks connected to each other;

Fig. 3 is a perspective view showing the recess on a long side of a plank of the first embodiment;

Fig. 4 is a perspective view from the other side showing the hooks on the other long side of a plank of the first embodiment;

Fig. 5 is a view corresponding to Figure 3 with planks connected to each other before they are moved into their final position;

Fig. 6 is a view corresponding to Figure 4 with interconnected planks in their final position;

Fig. 7 is a partial sectional view similar to Fig. 1 of two planks of a second embodiment of the present invention;

Fig. 8 is a view similar to Figure 7 with planks connected to each other;

Fig. 9 is a perspective view showing the recess on a long side of a plank of the second embodiment;

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Fig. 10 is a perspective view from the other side showing the hooks on the other long side of a plank of the second embodiment;

Fig. 11 is a partial sectional view similar to Fig. 1 of two planks of a third embodiment of the present invention;

Fig. 12 is a view similar to Figure 11 with planks connected to each other;

Fig. 13 is a perspective view showing the recess on a long side of a plank of the third embodiment;

Fig. 14 is a perspective view from the other side showing the hooks on the other long side of a plank of the third embodiment;

Fig. 15 is a partial cross-sectional view similar to Fig. 1 of two planks of a fourth embodiment of the present invention;

Fig. 16 is a view similar to Figure 15 with planks connected to each other;

Fig. 17 is a perspective view showing the recess on a long side of a plank of the fourth embodiment;

Fig. 18 is a perspective view from the other side showing the bevels on the other long side of a plank of the fourth embodiment;

Fig. 19 is a partial sectional view similar to Fig. 1 of two planks of a fifth embodiment of the present invention;

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Fig. 20 is a view similar to Figure 18 with planks connected to each other;

Fig. 21 is a perspective view showing the recess on a long side of a plank of the fifth embodiment;

Fig. 22 is a perspective view from the other side showing the bevels on the other long side of a plank of the fifth embodiment.

DETAILED DESCRIPTION OF THE EMBODIMENTS OF THE INVENTION

The invention will be explained by means of embodiments in which the floorboards are made of wood. It should be noted that other materials such as laminates, plastic, etc. could also be used.

The term "hook" is generally understood to mean a projection with a profiled shape whose function is to engage a cavity. The term "head" is generally understood to mean a projection that protrudes from a surface. The term "slot" or "notch" is generally understood to mean a space between two hooks or two heads, respectively. The term "solid" is generally understood to mean a straight surface that is not interrupted by a notch. The term "non-solid" is generally understood to mean a surface that is interrupted by a slot or notch. The terms "horizontal" and "vertical" are used to describe directions related to a plank that is on a horizontal surface, such as a typical

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floor. However, it goes without saying that the invention can also be used for floorboards for wall and ceiling panels in any direction.

In Figures 1 and 2 two planks A and B are shown in a cross-sectional view, viewed from their shorter sides. The planks have, as is usual, an upper wear layer 8 of hardwood and a carrier layer of simpler wood. It should be noted that planks A and B are identical, but Figures 1 and 2 only show the connection between the right long side of plank A and the left long side of plank B.

Figures 3, 4, 5 and 6 show perspective views of the planks. Instead of a known tenon or tongue, plank A has hooks 1 with slots or notches 2 milled in between. Plank B has a recess 11 which corresponds to a known groove. The recess is limited on the upper side by a solid or continuous edge 3 and on the lower side by a non-solid or interrupted edge which has heads 4 with slots milled in between. Preferably, these slots do not extend over the full depth of the recess 11, but a shoulder 7 remains. The function of the shoulder 7 is explained below.

The connection is made as follows:

Plank A is laid and must be connected to the next plank B.

The plank B is fitted from above in such a way that the heads 4 pass through the slots 2 of the plank A. By slightly pivoting or angling the plank B and holding the plank B at a suitable distance from the plank A, the shoulders 7 are allowed to pass the hooks.

Finally, the continuous or solid edge 3 of the plank B will abut against the top 5 of the hooks 1 and the shoulders 7 can

under the hooks 1 by pivoting and pressing the two planks towards each other.

The plank B can then be moved lengthwise into its correct position in relation to the walls and the other floorboards. The heads 4 are also moved to a position below the hooks 1 to form a firm connection. The connection can be further strengthened by adhesive which is applied, for example, to the top 5 of the hooks before the plank B is mounted. The shoulders 7 can, for example, have a length of approximately 2 mm. It is also conceivable to mill the shoulders 7 completely down to the bottom of the recess. The plank B can then be laid vertically as a whole, but at the same time the connection is weakened since the entire head 4 will generally not be arranged below a hook 1.

The top surface 5 of the hook 1 preferably extends at an angle like the corresponding bottom surface 6 of the recess 11. A suitable angle is about 45°. The hooks may also have other profiled shapes than the configuration shown with straight edges and a flat outer end surface, for example convex or concave rounded shapes or other curved or hooked configurations, some embodiments of which are described below. The recess will have a more or less complementary shape. The main point is that the configuration should result in the planks A and B not being able to be pulled aside from each other without being rotated after they have been connected.

Figures 7 to 10 show a second embodiment. Corresponding parts are designated by the same reference numerals as in Figures 1 to 6, with Figure 2 additionally in front. In this embodiment, the shape of the top of the hooks 21 and the corresponding downward-facing inner side of the edge 23 have a different design. Instead of being straight and running at an angle, the sides are designed with horizontal surfaces,

such that the hook 21 has a flat top surface 25 and the inner side a corresponding flat abutment surface 26. When the planks are to be moved horizontally relative to one another, they will be guided both by the upper flat contact surface at 25 and 26 and by the lower flat contact surface on the lower side of the hook 21 and the corresponding upwardly facing inner side of the edge 23. The guiding results in there being no difference in height between the upper surfaces of the planks when the hook 21 should be moved relative to the recess 211.

The shoulder 27 is shown straight in the figures, but may also have a rounded upper edge. This will facilitate the joining process, but will also result in a slightly looser joint.

The connection is similar to the known procedure and is carried out as follows:

The plank 2A is laid and is to be connected to the next plank 2B. The plank 2B is laid on the existing floor close to the plank 2A in such a way that the heads 24 are arranged just in front of the slots 22 of the plank 2A and the hooks 21 are arranged just in front of the shoulders 27. It is now easy to press or knock the planks together, for example by means of a wooden hammer or a block similar to the case with the known groove and tenon. This will cause the hooks 21 to pass under the continuous or solid edge 23, pass the shoulders 27 and enter the recess 211.

Just as the hook 21 passes under the edge 23, a slight rotation or lifting or bending will occur between the planks to allow passage. However, this is automatic and is very difficult for the floor fitter to notice.

The plank 2B can then be moved longitudinally into its correct position in relation to the walls and the

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other floorboards. The heads 24 will also be moved to a position under the hooks 21 to form a solid connection. The connection can be further reinforced by adhesive, which is applied, for example, to the top 25 of the hooks before the floorboard 2B is mounted. The shoulders 27 can, for example, have a length of approximately 2 mm.

A third embodiment is shown in Figures 11 to 14. Corresponding parts are designated by the same reference numerals as in Figures 1 to 6, with Figure 3 additionally preceding them. Here, the hook 31 is instead provided with a downwardly facing projection 315 which fits into a corresponding groove 316 in the upwardly facing inner side of the recess 311. The connection is carried out in essentially the same way as is the case in the above second embodiment.

A fourth embodiment is shown in Figures 15 to 18. Corresponding parts are designated by the same reference numerals as in Figures 1 to 6, with Figure 4 added in front. Here, the plank 4A is provided with a projection, but the slots are not milled through the projection, but are formed as chamfers as shown at 48. Between the chamfers 48, the projection is shaped in a similar manner to a whole hook 41. The projection has a continuous, solid upper edge 42 (Fig. 18) with the same flat top 45 as in the second embodiment explained above. The plank 4B has a shape identical to that of the second embodiment with a continuous or solid edge 43, a recess 411, heads 44 and shoulders 47. The shoulder 47 is shown straight in the figures, but may also have a rounded upper edge. This makes the connection process easier, but results in a slightly looser connection.

The connection is similar to the method used in the second and third embodiments and is carried out as follows:

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Plank 4A is laid and must be connected to the next plank 4B.

The plank 4B is laid on the existing floor close to the plank 4A in such a way that the heads 44 are arranged just in front of the bevels 48 of the plank 4A and the hook-shaped projections 41 are arranged just in front of the shoulders 47. It is now easy to press or knock the planks together by means of a mallet or a block, similar to the case of the known groove and tenon. This will cause the hook-shaped projections 41 to pass under the continuous or solid edge 43, pass the shoulders 47 and enter the recess 411. The heads 44 do not require a slot, but will slip under the bevels 48.

Just when the hook 41 passes under the edge 43, a slight lifting or bending occurs between the planks, which allows passage as explained above.

Thereafter, the plank 4B can be moved longitudinally to its correct position in relation to the walls and the other floor boards as explained above. The heads 44 will also be moved to a position below the hooks 41 to form a solid connection. The connection can be further reinforced by adhesive, for example applied to the top 45 of the hooks before the plank 4B is mounted. The shoulders 47 can, for example, have a length of about 2 mm.

A fifth embodiment is shown in Figures 19 to 22. Corresponding parts are identified by the same reference numerals as in Figures 1 to 6, with Figure 5 additionally in front. Here, as in the fourth embodiment, the plank 5A is provided with a projection, whereby the slots have not been milled through the projection, but are chamfered or designed as chamfers, as shown at 58. Between the chamfers 58, the projection is provided with a downward-pointing projection 515, which, as in the third embodiment, fits into a corresponding groove 516 in the upward-pointing inside of the recess 511. The projection has an un-

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broken, continuous or solid upper edge of the same shape as in the third embodiment above. The plank 5B has a shape which is identical to the third embodiment. The shoulder 57 is shown straight in the figures, but may also have a rounded upper edge. This will facilitate the joining process, but will result in a slightly looser joint.

The connection is carried out in substantially the same manner as in the second embodiment explained above, wherein the heads with the grooves 516 slide under the chamfers 58.

After connection, a small clearance remains between the planks A and B in the carrier layer 9, as can be seen from Figure 2. The wearing layer 8 should preferably show no remaining gap between the planks. If the planks should warp after connection, or if the underlying floor is uneven, the planks can pivot together about a point located at the top 5, 25, 45 of the hooks and the bottom 6, 26, 46 of the recess 11, 211, 411. It is advantageous for this point to be located as high as possible, thereby avoiding the formation of any gap in the wearing layer when the angle between the planks changes.

In the embodiments shown, the widths of the hooks and the intermediate slots or bevels are the same. This can be varied as long as the heads have a width corresponding to the slots or bevels. The heads (and the hooks) should have a width of about 8 mm and the slots or bevels (and the shoulders) a width of about 10 mm. Since the tenon on the short side is 6 rhm, the displacement will be at least 6 mm. If the heads are made wider, for example 20 mm, the total surface area locking the boards will be reduced.

The shorter sides of the planks can be fitted with known tongue and groove joints (or other well-known systems) in all embodiments. This is in connection with the first embodiment.

3, 4, 5 and 6 by a groove 10. The corresponding pin is arranged at the other short end and is therefore not visible.

The embodiments shown can be varied, for example, by modifying the thickness of the wear and support layers, the angle and design of the hook, the width and length of the planks, etc., all of which will be obvious to those skilled in the art. The scope of the invention is limited only by the appended claims.

Claims (38)

1. Connection system for floorboards, containing boards (A, B; 2A, 2B; 3A, 3B) which are to be connected to one another, wherein a first board (A; 2A; 3A) has a projection which fits into a recess ( 11 ; 211 ; 311 ) of a second board (B; 2B; 3B), wherein the projection on the first board (A; 2A; 3A) contains hooks ( 1 ; 21 ; 31 ) with slots ( 2 , 22 ; 32 ) therebetween and wherein the recess ( 11 ; 211 ; 311 ) of the second board (B; 2B; 3B) is delimited on the upper side by a continuous edge ( 3 ; 23 ) and on the lower side by an edge which has heads ( 4 ; 24 ), wherein the hooks ( 1 ; 21 ; 31 ) on the first plank (A; 2A; 3A) are dimensioned such that they are able to pass between the heads ( 4 ; 24 ) on the second plank (B; 2B; 3B). 2. Connection system according to claim 1, wherein the hooks ( 1 ; 21 ; 31 ) have profiled upper sides and the continuous edge ( 3 ; 23 ) has a correspondingly profiled lower side. 3. Connection system according to claim 2, wherein the upper side of the hooks ( 1 ) is provided with a straight upper side ( 15 ) running at an angle and the continuous edge ( 3 ) is provided with a correspondingly shaped, downwardly facing inner side ( 6 ). 4. Connection system according to claim 2, wherein the upper side of the hooks ( 21 ) is provided with a flat upper side ( 25 ) and the continuous edge ( 23 ) is provided with a correspondingly shaped, downwardly facing inner side ( 26 ). 5. Connection system according to claim 2, wherein the underside of the hooks ( 31 ) is provided with a projection ( 315 ) and the continuous edge is provided with a correspondingly shaped, upwardly facing inner side ( 316 ). 6. Connection system according to claim 1, wherein the lower edge of the recess ( 11 ; 211 ; 311 ) shows shoulders ( 7 ; 27 ) between the heads ( 4 ; 24 ). 7. A connection system according to claim 6, wherein the shoulders have a rounded upper edge. 8. Connection system according to claim 1, wherein the hooks ( 1 ; 21 ; 31 ), the slots ( 2 ; 22 ; 32 ) and the heads ( 4 ; 24 ) have substantially the same size in the longitudinal direction of the planks. 9. Connection system according to claim 8, wherein the heads ( 4 ; 24 ) are slightly narrower than the slots ( 2 ; 22 ). 10. Connection system according to claim 1, wherein the short sides of the planks (A, B; 2A, 2B; 3A, 3B) are provided with grooves ( 10 ) and tenons which fit into each other. 11. Plank for a connection system, which has a projection on a first longitudinal side of the plank and a recess ( 11 ; 211 ; 311 ) on a second longitudinal side of the plank, wherein the projection fits into the recess ( 11 ; 211 ; 311 ), wherein the projection on the first longitudinal side has hooks ( 1 ; 21 ; 31 ) with slots ( 2 ; 22 ; 32 ) therebetween, and wherein the recess ( 11 ; 211 ; 311 ) in the second longitudinal side is delimited on the upper side by a continuous edge ( 3 ; 23 ) and on the lower side by an edge with heads ( 4 ; 24 ), wherein the hooks ( 1 ; 21 ; 31 ) are dimensioned such that they are able to fit between the heads ( 4 ; 24 ) of another identical plank, thereby enabling identical planks to be joined together. 12. Plank according to claim 11, wherein the hooks ( 1 ; 21 ; 31 ) have profiled upper sides and the continuous edge ( 3 ; 23 ) has a correspondingly profiled underside. 13. Plank according to claim 12, wherein the upper side of the hooks ( 1 ) is provided with a straight, angled upper side ( 5 ) and the continuous edge ( 3 ) is provided with a correspondingly shaped, downwardly facing inner side ( 6 ). 14. Plank according to claim 12, wherein the upper side of the hooks ( 21 ) is provided with a flat upper side ( 25 ) and the continuous edge ( 23 ) is provided with a correspondingly shaped, downwardly facing inner side ( 26 ). 15. A plank according to claim 12, wherein the underside of the hook ( 31 ) is provided with a projection ( 315 ) and the continuous edge is provided with a correspondingly shaped, upwardly facing inner side ( 316 ). 16. Plank according to claim 11, wherein the lower edge of the recess ( 11 ; 211 ; 311 ) has shoulders ( 7 ; 27 ) between the heads ( 4 ; 24 ). 17. A plank according to claim 16, wherein the shoulders have a rounded upper edge. 18. A plank according to claim 11, wherein the hooks ( 1 ; 21 ; 31 ), the slots ( 2 ; 22 ; 32 ) and the heads ( 4 ; 24 ) have substantially the same size in the longitudinal direction of the planks. 19. Plank according to claim 18, wherein the heads ( 4 ; 24 ) are slightly narrower than the slots ( 2 ; 22 ). 20. A plank according to claim 11, wherein the shorter sides of the planks are provided with grooves ( 10 ) and tenons which enable identical planks to be connected to one another. 21. Connection system for floorboards, comprising boards ( 4 A, 4 B; 5 A, 5 B) which are to be connected to one another, wherein a first board ( 4 A; 5 A) has a projection which fits into a recess ( 411 ; 511 ) in a second board ( 4 B; 5 B), wherein the projection on the first board ( 4 A; 5 A) has bevels ( 48 ; 58 ) with hook-shaped sections ( 41 ; 51 ) lying therebetween and a continuous upper edge ( 42 ), and wherein the recess ( 411 ; 511 ) in the second board ( 4 B; 5 B) is delimited on the upper side by a continuous edge ( 43 ) and on the lower side by an edge with heads ( 44 ), wherein the hook-shaped sections ( 41 ; 51 ) on the first plank ( 4A ; 5A ) are dimensioned such that they are able to pass between the heads ( 44 ) on the second plank ( 4B ; 5B ). 22. Connecting system according to claim 21, wherein the hook-shaped sections ( 41 ; 51 ) contain profiled upper sides and the continuous edge ( 43 ) of the recess ( 411 ; 511 ) has a correspondingly profiled lower side. 23. Connection system according to claim 22, wherein the upper side of the hooks ( 41 ) is designed with a flat upper side ( 45 ) and the continuous edge ( 43 ) of the recess ( 411 ) is designed with a correspondingly shaped, downwardly facing inner side ( 46 ). 24. Connection system according to claim 22, wherein the underside of the hooks ( 51 ) is provided with a projection ( 515 ) and the continuous edge of the recess ( 511 ) is provided with a correspondingly shaped, upwardly facing inner side ( 516 ). 25. A connection system according to claim 21, wherein the lower edge of the recess ( 411 ; 511 ) shows shoulders ( 47 ; 57 ) between the heads ( 44 ). 26. A connection system according to claim 25, wherein the shoulders have a rounded upper edge. 27. A connection system according to claim 21, wherein the hook-shaped portions ( 41 ; 51 ), the bevels ( 48 ; 58 ) and the heads ( 44 ) have substantially the same size in the longitudinal direction of the planks. 28. A connection system according to claim 27, wherein the heads ( 44 ) are slightly narrower than the chamfers. 29. A connection system according to claim 21, wherein the shorter sides of the planks are provided with grooves and tenons which fit into one another. 30. Plank for a connection system, which has a projection on a first longitudinal side of the plank and a recess ( 411 ; 511 ) on a second longitudinal side of the plank, the projection fitting into the recess ( 411 ; 511 ), the projection on the first longitudinal side containing bevels ( 48 ; 58 ) with hook-shaped portions ( 41 ; 51 ) lying therebetween, and the recess ( 411 ; 511 ) of the second longitudinal side being delimited on the upper side by a continuous edge ( 43 ) and on the lower side by an edge provided with heads ( 44 ), the hook-shaped portions ( 41 ; 51 ) being dimensioned such that they are able to pass between the heads ( 44 ) of another identical plank, thereby enabling identical planks to be connected to one another. to become. 31. Plank according to claim 30, wherein the projection has a profiled upper side and the continuous edge ( 43 ) of the recess ( 411 ; 511 ) has a correspondingly profiled lower side. 32. Plank according to claim 31, wherein the upper side ( 45 ) of the projection is flat and the continuous edge ( 43 ) is provided with a correspondingly shaped, downwardly facing inner side ( 46 ). 33. Plank according to claim 30, wherein the underside of the hook-shaped sections ( 51 ) is formed with a projection ( 515 ) and the continuous edge of the recess ( 511 ) is formed with a correspondingly shaped, upwardly facing inner side ( 516 ). 34. A plank according to claim 30, wherein the lower edge of the recess ( 411 ; 511 ) has shoulders ( 47 ; 57 ) between the heads ( 44 ). 35. A plank according to claim 34, wherein the shoulders have a rounded upper edge. 36. A plank according to claim 30, wherein the hook-shaped portions ( 41 ; 51 ), the bevels ( 48 ; 58 ) and the heads ( 44 ) have substantially the same size in the longitudinal direction of the plank. 37. A plank according to claim 36, wherein the heads ( 44 ) are slightly narrower than the bevels. 38. A plank according to claim 30, wherein the shorter sides of the planks are provided with mortise and tenon joints enabling identical planks to be joined together.