Hey guys, so you should have a gutter bond between the two properties.
You should have every 3rd to 4th gauge nailed. This means every 3rd or 4th course of tiles need to be nailed. If this has not been done itís a massive cause for concern.
In regards to the join between both properties there are various forms of bonding gutter, but essentially they are all used for the same purpose, to weather a junction between differing roof coverings Ė for example, between double lapped plain tiles and single lap tiles. The most common situation is where an old roof is replaced on a semi-detached or terraced house and the junction between the new roof and an existing neighbouring roof needs to be weathered (i.e. sealed).The wrong way of weathering the junction between roof coverings is to finish the new tiling close to the neighbouring tiling and then mortar-bed some form of covering over the joint, for example paving stones, copings or even a row of tiles. Although commonly done, this is rarely successful or adequate. The integrity of the joint relies on the mortar bedding, so even if it is waterproof to start with, it will inevitably start to leak as soon as any small cracks appear in the mortar bed.
The correct way to weather the junction is to install some form of gutter, or channel, between the different tiling. This can be formed from lead or be a manufactured, pre-formed product.
i would need to come and investigate to diagnose the problem sufficiently.
What I will say is the finish on the tiles in that picture is completely wrong.
feel free to email me if you have anymore questions.