The Natural Philosopher wrote on Tuesday (03/02/2004) :
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.

Our roof has a three tile overlap at any place you care to look, but we
have no underfelt. We can see the under sides of the tiles from the
loft. I was rather puzzled by the fact that the exposed inside tile
hedges had been flaunched with a very crumbly mix of sand and cement,
much of which has fallen away over the years.

We are located in a very exposed hill top location and despite the lack
of any under felt, I have never known water to be driven in by the
wind, though it is very draughty up there. Each tile (at a guess) is
flat and about 12- 14" long.

--

Regards,
Harry (M1BYT) (Lap)
http://www.ukradioamateur.org