I'm putting breathable underlay under a tiled roof. The slope of the
roof is very shallow - about 15 degrees. The instructions are to lay
the underlay loosely ie: let it droop between the rafters for a cold
roof and not to pull it taut. But I'm concerned that on such a shallow
pitch any dip is going to cause water to pond.

I know that the tiles are supposed to stop water penetration but again,
on such a shallow pitch, I think wind-blown rain is more likely to get
under them.

Should I be concerned about water penetration and, regarding laying the
breather membrane, how loose is loose?

Thanks for any advice.