How can I determine where the water is entering?

I am assuming this happens every time it rains or only when heavy rain!
Although the 10yr ‘ snags cover’ is out of date the builders may still be responsible for bad designed roof or fobbing you off. However you will need proof of contact when it first occurred informing them of the fault. Seek legal advice on this or contact your local building control see if they can advise on this point.

Yes it may appear to the builder its ok but he should be checking when it rains which obvioulsy I assume is when the leak shows. It could be a broken or cracked tile further up the roof letting rain in on to the felt underlay and running down between rafters to the eaves where it can puddle and eventually over flowing into the roof void instead of guttering where the felt should be overlaped in to the gutter approx 3 inches.

If there is a chimney stack in line with the leak it could be badly fitted so its a case of they will have to look when its raining lifting the tiles at the leak point and two above to see if the rain is flowing down the felt. Looking under tiles at eaves finding correctly laid will not show what is causing the leak. So get some legal advice if you have any communication evidence with the builders within the 10yr guarantee period. It appears to me they haven’t investigated it in a proper manner if all they have done is look in the roof void and with it felted it would not reveal day light where a tile/slate is broken or cracked, that’s the problem with under felt. I feel they should have stripped out the row up the roof between the rafters to see what’s happening. The other point is interlocking tiles are a bugger to find which is causing ingress of rain. If a well known property developer then not a good advert for them is it.

BB
December 2010