Semi-detached homes share a wall. That wall extends up through the roof, creating a junction where your roof meets your neighbour's roof. In roofing, we call this the party wall. It's the single most important detail on a semi-detached re-roof, and it's the one most likely to be done wrong.
We tore off and re-roofed a semi-detached in Scarborough — 20 squares, relatively simple roofline. The party wall made it complicated.
What we found at the party wall
When we stripped the shingles along the party wall, we expected to find step flashing — individual L-shaped pieces of metal woven into each course of shingles, directing water away from the wall junction. That's the correct installation.
What we found was caulk. Just caulk. Someone had run a thick bead of roofing sealant along the wall junction and shingled over it. No metal flashing at all. The caulk had cracked and separated in multiple places. Water had been getting behind it for years, running down the party wall inside the wall cavity.
The decking along the party wall was stained but not yet rotten. Another year or two and we would have been replacing framing.
Why party wall flashing matters
The party wall on a semi-detached is a vertical surface that interrupts the roof plane. Every course of shingles that meets that wall needs a piece of step flashing behind it — one for each course, overlapping like shingles themselves. At the top, counter flashing covers the step flashing to prevent water from entering above it.
This is the same technique used where a roof meets any wall — a dormer, a chimney, an addition. The difference on a semi-detached is that the party wall belongs to two houses. You can only flash your side. And the flashing has to create a clean termination line that doesn't rely on the neighbour's roof being in good condition.
What makes it hard
You can't touch the other side. On this job, the neighbour's shingles came right up to the party wall from their side. Our flashing had to tuck under a cap that sits on top of the wall, without disturbing the neighbour's shingles. If their roof is old and brittle, even careful work near the junction can crack their shingles.
The party wall cap. Most semi-detached homes have a metal or cement cap on top of the party wall where it protrudes above the roofline. That cap is shared property. If it's deteriorated, replacing it requires coordination with the neighbour and potentially their consent. On this job, the existing cap was galvanized steel in decent shape — we sealed over it with ice-and-water shield and new counter flashing.
Water direction. The party wall has to shed water onto your roof, not your neighbour's and not into the wall. The step and counter flashing direct water outward onto the shingle surface, where it runs to the eave normally. Get the overlap sequence wrong and you're directing water into the wall cavity.
The repair
We installed full step flashing along the party wall — one piece per shingle course, 26 pieces total. Each one overlaps the one below by at least 3 inches. We ran ice-and-water shield 12 inches from the wall across the entire length. New counter flashing over the step flashing, mechanically fastened to the wall and sealed at the top.
The rest of the roof was straightforward: new synthetic underlayment, IKO Cambridge in Dual Black, new drip edge on all exposed edges.
What semi-detached homeowners should know
If your semi-detached roof is over 15 years old, the party wall flashing should be inspected. You can sometimes see it from the attic — look at the wall junction from inside and check for water staining, daylight, or missing metal.
When you get a quote for a semi-detached re-roof, the party wall should be called out as a specific line item. If the quote doesn't mention it, the crew may not be planning to address it properly. Ask: "How are you handling the party wall?" If the answer involves the word "caulk" without the word "flashing," get another quote.