how can i fix felt in my loft that is slowly turning to dust without costin?

I have to disagree with Dave, the tiles are there to weatherproof a roof, the felt is there to waterproof it. Some tile are porous and will let water through; this is why the felt is lapped in to the gutters and causes damp problems when it slips behind and collapses in to the cavity.
The only sound way to effect a repair is to remove the tiles and replace the felt.

Gearbox
March 2010

INSERT INSULATION BETWEEN THE RAFTERS THE FELT IS NOT TO MAKE YOUR ROOF WATERTIGHT IT IS ONLY FOR INSULATION AND TO STOP DRAFTS FROM GETTING UNDER THE TILES/SLATES.
(Roofer)

David Cummings.
February 2010

The only sure way to fix it, is to get some quotes from some reputable roofers to remove the tiles\slates and replace the felt with new. It may cost a bit, but its better than having the water get in and rotting the rafters.

Homer.
February 2010