Why Important
Prior to the 19th Century, lead was rare and prohibitively expensive, therefore Lime mortar fillets were used to protect the junction between the roof and the wall.
What to look for
Mortar fillets can absorb water, and a small amount of dampness in the wall at this junction is likely, but in a well-ventilated, unused roof space this is acceptable.
Damp may be more problematic where there is a room in the roof. If persistent dampness occurs, it may be worth considering the introduction of a lead flashing which will involve stripping some of the slates.
When to look
Although mortar skews are the more traditional detail, they need regular maintenance, and on an inaccessible roof the choice to replace in lead may be pragmatic.
Lime mortar fillets need regular maintenance and would be limewashed every 2-3 years with a strong wash to fill any cracks caused by movement of the timber structure expanding and contracting or through frost damage.Post WINTER
WTF?
How to fix
Modern mortar fillets should be formed in a strong hydraulic lime mortar because of their degree of exposure.
Avoiding creating problems
Do not use cement mortar, because the mortar will be too hard and likely to fail by cracking when the materials below move. Strong cement mortars are also likely to cause erosion of adjacent stone in the long run.
Historical background
Other issues regarding all types of Flashing