17 and 18 is approximately 1 63 inches 4 13 cm the legs 17 and 18 are caused to spread apart upon insertion into the soft insulating concrete by sloping the interior surfaces of opposed sides 22 and 23 of legs 17 and 18 respectively toward one another the portion of soft concrete entrapped within the hollow enclosure of the legs as the nail is driven pushes against these sloped surfaces forcing them apart the entrapped material resists against subsequent withdrawal of the fastener the use of such overlapping channel shaped legs as a shank the width of which is the same at the bottom as at the top has the advantage over for example a cone shaped shank such as shown in u s pat no 3 466 967 that no void in the soft cement is left at the point of insertion following penetration of the shank in fig6 the shank of the nail of fig3 has been inserted through center hole 11 of tab 10 and the nail driven through base ply 24 positioned upon a layer 25 of insulating concrete e g a mixture of portland cement expanded vermiculite air entraining agent and water legs 17 and 18 have spread to better lock the nail within the concrete layer the inserted nail tab combination shown in fig6 is ready for the usual layer of hot fluid asphalt applied e g by mopping over the entire upper surface of the nail tab and base ply openings 12 in combination with the stand off projections.