In article , The Natural Philosopher wrote:
However the felt is not supposed to stop water, but airflow. Its
NOT a good idea to have sagging felt with puddles forming in it
around your timber battens.

That's what the counter battens are for. We used to allow low
pitched roofs built like this, but only with ply over the rafters
to support the felt

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Tony Bryer SDA UK 'Software to build on' http://www.sda.co.uk
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http://www.sda.co.uk/qsedbuk.htm