Membrane to block grit from old torching (Ed.)

[scottandcath]

Hi,

I have a slate roof and in the loft space I can see the slates and the original cement type stuff. This is slowly coming down and covering everything in grit and dust. I'm going to put something (breathable sheeting I presume) over the rafters to stop this happening.
So my question is - what sheeting should I use and would a staple gun be sufficient to attach it to rafters?

Thanks

 

[Wayners]

Rolls of breathable membrane. Cheap as you can get I guess