to be eliminated with the object of reducing or eliminating play between the strips along the vertical regions thereof the present invention provides that the arms of the clips have reliefs or domes such as those represented by reference numerals 29 and 30 in fig3 to reduce or eliminate the vertical play of the strip within the grooves for the clips as mentioned hereinbefore the precise shape of the lateral edges of the strips can vary between wide limits examples being shown in fig5 to 10 thus for example in fig5 two adjacent strips 31 and 32 have mating profiles substantially in the form of a projecting 33 and recessed 34 acute angle which are disposed so as to produce the aforementioned overlap between the upper wing 35 and the vertex of the projection 33 in order to fulfil the characteristics of the present invention fig6 shows an embodiment in which the strips 36 and 37 have a zigzag profile in other words a double angle by means of which each of the edges has a projection and a recess respectively mating with those of the adjacent strip the projection 38 and recess 39 being shown in the case of the strip 36 and the lower projection 40 and recess 41 in the case of the strip 37 mating with 39 and 38 respectively of the strip 36 in this case also the upper edge 42 will overlap the projection 38 in fig7 the adjacent strips 43 and 44 have mating profiles formed by.