Just in case anyone else is wondering -I have never had a nailed felt roof leak -I bought a summer house and had it fitted and they used staples and after first winter you could see the water marks on inside of roof.
I had to put more felt on and used clout nails as normal - been fine - So if anyone buys anything with a felt roof make sure fitters (if you get one) do not use staples and the rain went down the staple leg - They stapled it because roof was so thin a clout would have gone through.
Two layers of felt fixed that though - It turned out to be a good buy as it's now filled with the wifes rubbish.