Stick the first row of bird spikes towards the front of the ledge, so that the spikes fully protect the leading edge. Press down firmly. If the gap behind the 1st row is greater than 1/4 inch (6mm) birds may get in behind. It is therefore necessary to install another row of spikes into this space to block it. For very wide ledges, 3 or more rows of pigeon spikes may be required, remembering to leave as small a gap as possible between the tops of the pins of adjacent rows.