Looks like we have done the same thing but with different results... The felt looks ok , been up on the roof to check it out, no obvious issues and nice and flat (best roofing felt job I have done) . I think I am going to do a water test with the hose to see if that can show the issue.
I have a chalet which is 4.5 *4.5 metres. The original supplied felt lasted one year before it split. I took it off and replaced it with IKO super roof felt from Wickes which was their 'better' and more expensive product . I did a 6 inch overlap , nailed every 6 inches and a small amount of...