The invention relates to a process and an apparatus for detecting gas-permeable damaged points in an electrically non-conductive layer which can exist as a single layer or as a multi-layer composite with at least one conductive layer. Bone screws cooperating with the sleeves include at least one aperture or a plurality of ridges and grooves for close cooperation and engagement with the ridges and grooves of the anti-torqueanti-shear sleeves. The third member has protrusions provided at the longitudinal opposite ends of each of the front and rear side walls. The secured plank can be joined to another plank in side-by-side or end-to-end engagement at a ceiling by interengagement with the tongue portion at the side or end of the other plank.