I fear shingles may be almost as bad, lots of nooks and crannies to hide in. I use Onduline, lasts years and is very good:
![](http://www.ascot-timber.co.uk/ascotstables/looseboxes/images/eastdroof.jpg)
Not my picture but it shows the material very well.
Last very well and is easy to work with. You can get it in most DIY sheds.
Jerry