It goes over it. If it went under it and water got through a broken slate or tile and ran downwards towards the valley it would run off the membrane and under the lead rendering it useless so it lays over the lead and water will then run onto the lead and away.
ive read somewhere that the lead should sit directly onto the valley board with no felt under it?
someone also suggested nailing a cement board on top of the lead valley to help when bedding the tiles,but this seems to defeat the object as you would be nailing through the lead?
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