On Sun, 24 Mar 2019 17:48:54 +0000, Bob Minchin
wrote:
snip

Felt is a waste of time and effort.

Well, I've had the shed over 45 years and I can't remember having to
re-felt it very often?

Use EPDM and do it once only

A quick check, 3 x 4m of 1.2mm is around £100, plus adhesive, carriage
and Vat etc.

An advantage looks like it's reasonably easy to apply and durable so I
might check it out ...

On similar lines I was thinking of laying some woven fibreglass mat
directly over the OSB?

Cheers, T i m