WaterProofing (and WeatherProofing) gaps in roof

Question

Our house roof consists is made up of roofing metal and cement sheets. Out of the total area of the roof 50% is covered with metal sheets. This part of the roof also has good slopes for the rain water to drain off (This is because this part has a small tent shaped attic). The other 50% roof comprises of cement sheets. This part of the roof is more like flat surface with minimal slope for the rain water to drain off. There is a gap between where the metal and cement sheets meet. As they both have uneven surface and they do not match, hence even in the slightest rains there is leakage from this part of the roof. This is further aggravated by the difference in slope between these 2 surfaces.

How do I fill this gap with water proof material ? As this is exposed to the sun it would also need to be heat/sun resistant and crack resistant due to change in the metal length due to heat.

Note: Completely redoing the roof is not possible as the building itself is more than 80+ years old and other financial constraints.

Answer 1

The best way to remedy this is to remove the portion of metal over the transition to cement roof, add a transition flashing that goes over the cement roof. It should cover at least the first row of exposed cement tile at the top and the metal is folded so it turns up the higher pitch, but goes under the metal roof at least as high as a few inches above the first row of fasteners at the bottom of the metal. Do not fasten the metal flashing through to the cement tile. The metal needs to be bent so it presses in place onto the cement tile. You may be able to remove just enough metal fasteners, if they are exposed to ease up the metal roof enough to slip the transition flashing under it without removing all of the metal. Your biggest issue will be working off the cement roof without cracking the tiles. A picture of the trouble spot may help fashion a better answer.