Why does flashing fail before the shingles do?
Flashing sits at the joins between different materials, where movement, temperature swings and trapped water concentrate. Shingles lie flat and shed water by gravity. Flashing has to hold a seal across a gap that changes with the seasons, and its sealants dry out and its metal rusts or bends.
A brick wall and a wood roof expand and contract at different rates, and every winter's freezing cycle works at the joint. Sealant that was fresh when installed will crack after years of that flexing.
That is why roofers prefer to rebuild metal joints than to smear new sealant onto old. Sealant is a short-term measure on a joint that is moving.
Is caulk or roof cement a real repair?
Only as a temporary measure. Roof cement and caulk can slow a leak, but they dry, crack and shrink, and they cannot replace metal that is rusted or the wrong shape. A lasting repair re-forms or replaces the flashing so water runs over it by design.
Multiple layers of tar on a joint are a clue that the joint has been leaking for years. They also make a proper repair harder, because the old material has to be removed first.
Ask the roofer what they will use and whether the old material comes out. If the answer is only "seal it," ask what happens when the sealant fails.
What makes party-wall flashing special on row houses?
A party wall is shared with the neighbor, so the joint belongs to both roofs, and water that enters at one can show up in the other house. The wall is brick, so the metal has to be set into mortar, and the work has to be coordinated so that neither side's roof is left exposed.
Rows are often roofed at different times by different owners. Your joint may be new while your neighbor's is old, and water can travel along the wall to find the weak side.
It helps to speak to the neighbor before work, and to ask the roofer to check both sides of the wall where they can safely see them. The guide on replacing a roof on a Trenton rowhouse or twin explains the coordination.
How long should new flashing last?
It depends on the metal, the installation and the exposure, and no honest figure can be given without seeing the joint. Good metal set correctly in a sound wall can outlast the shingles around it. Ask the roofer which metal they use and what workmanship terms they offer in writing.
The weak point is often the sealant at the top of the flashing, not the metal. A well-detailed joint hides that sealant behind a cap or sets the metal into a cut groove, so it is protected from sun and water.
If the roof around the joint is also old, it may make sense to do the flashing as part of a larger job. See roof repair and roof replacement for the way those choices fit together.
What should I ask a roofer who proposes to re-flash my roof?
Ask which joints will be replaced, what metal will be used, whether the old material comes out, how the metal will be fastened and sealed, and whether any masonry work is needed. A clear answer to each, in writing, tells you whether the repair is a rebuild or a coat of sealant.
Flashing quotes vary widely because they can mean very different things. One may replace every piece of metal and cut new grooves in the brick, while another may brush on a sealant and call it flashing. If a quote is much lower than another, compare the lists line by line.
It is reasonable to ask for photographs of the finished joints, particularly at the party wall and chimney, which are hard to see from the ground. Ask also about any workmanship terms. This line cannot state a warranty on a roofer's behalf, and the written quote is where such terms belong.
Finally, ask what the roofer will do if the old joint turns out to be worse than it looked. Brick, mortar and wood behind old flashing can be in poor shape, and a good scope names an allowance and a way of handling extra work.
