Harry Bloomfield wrote:

On 02/02/2004 The Natural Philosopher a wrote :

Don't boher. it is only there to stop pressure build up blowing
slates/tiles off...


That may be one reason, but I think another reason was so that roofs
could be made lighter/cheaper, with less overlap needed if there was a
layer of felt underneath.


I don't think so.

My roof with reasonable Tyvek overlap., leaked i heavy rain before
tiloing through the odd hole.

After tiling, even in huge winds, there is no leak. Thats whey you have
a three tile overlap - to stop the wind and driving rain.


The felt helps, not because it is waterproof, but because it is
windproof - it slows the draughts down so they can't carry the water up
the tiles. This also helps stop the negative pressure effects - where
vaccum in a strong gust sucks the tiles off.