"RoofShield" Finally (nearly) watertight, again.

For those of you of a non-technical bent, Roof Shield is a nearly waterproof membrane placed above sarking boards (google?) and under slates, which permits airflow into roof spaces.
In England, slates are nailed to narrow battens, with gaps between them, whereas in Scotland the roof is covered in planking (sarking), prior to slating. Amazing what you learn, interrogating your builders!