Laying underfelt - how loose should it be?

[James]

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.

[keith_765]

Just roll it out taut. you'll find it sags on its own. Try to lay it with a sag and you will find that the next layer will not follow the same sag. also when the wind blows the underlay vibrates.