Headlap is the amount of overhang that the bottom of one tile covers the top of the tile not immediately beneath it but the one beneath that:
As mentioned already, all tiles removed, battens removed, new battens at closer centres, re-hang tiles not forgetting you'll need more tiles to go back on than came off.
![slate-headlap.jpg](http://www.nachi.org/images10/slate-headlap.jpg)
As mentioned already, all tiles removed, battens removed, new battens at closer centres, re-hang tiles not forgetting you'll need more tiles to go back on than came off.