My house and verandah have separate corrugated iron roofs. The verandah was added later, and the sheets extend a short distance below the sheets of the roof. The overlap between these two levels is not sealed. The house roof is quite a steep pitch, while the verandah roof is reasonably flat. In the north queensland wet season, water travels up the verandah roof and leaks under the house roof iron onto the verandah below. We have a lot of trees overhanding our roof, and because of the flat pitch of the verandah roof, they sometimes come to rest just below the join, allowing water to bank up there and making the problem worse.
I would like to seal the join between these two layers and stop this happening, but the fix is made more complicated by the fact that the curves of the corrugated iron on the house and verandah levels are not perfectly aligned, so the shape of the space between the two roofs varies, making the weatherproof foam and rubber inserts that are usually suitable for this purpose, unsuitable.
I have considered using some kind of weatherproof tape but due to the misaligned corrugations, I am not sure that I would be able to make the tape curve as necessary and make a good seal. I have also considered some kind of waterproofing membrane treatment but I don't know much about them. Does anyone have any suggestions?
When you say, "short distance below the sheets of the roof", do you mean a continuation of the existing roof or it is under an eave butted to a wall?
It is like the top diagram. It doesn't butt against the wall.
Step 1: From inside the main roof space make sure the corrugate is úp-turned @60 degrees.
Step 2: On the outside with a wire brush remove as much dirt and grime as possible dry the area with a heat gun (important to remove any moisture).
Step 3: Silicone the external join...you're going to need heaps so don't skimp. (after the silicone has been applied you can spray soapy water over it to prevent silicone from sticking to your spreader)
Step 4: After you've siliconed the external (when it's dry) silicone from the inside too.
All of that but I'd prefer to hear Poly urethane used as silicone sealants breakdown/dry out/pull away.