Parapet wall waterproofing for Jersey City rowhouses and walk-ups

The low brick wall around the edge of a flat roof takes more weather than almost any other part of the building. It's exposed on both faces and across the top, it soaks up rain, and in winter it freezes and thaws over and over. On 19th-century rowhouses in Hamilton Park and around Van Vorst Park, and on pre-war walk-ups near Journal Square, parapets are often the oldest masonry on the building and the least maintained.

Water that gets into a parapet doesn't stay there. It runs down inside the wall, below the roof line, and can surface on the top-floor ceiling, along an upper wall, or two floors down beside a window where it finally finds a way out. Owners blame the roof and pay for membrane patches that never touch the source. Our guide on why parapet walls are a leak source explains the pattern in more detail, and it's worth reading before you pay for another patch.

From the top. Coping stones, metal caps or terra cotta tiles sit on the parapet to shed water. When joints between stones open, mortar beds crack or a metal cap loses its fasteners, rain drops straight into the core of the wall. The top of the parapet is the most exposed surface on the building, so this is the most common entry point we find.

Through the face. Brick itself absorbs some water, but the mortar joints are the real path. Old lime mortar erodes, freeze-thaw opens hairline cracks, and wind-driven rain gets pushed deep into the wall. On the roof side, missing joints are often hidden behind old tar. Once water is inside the core of the wall, it can travel sideways and down for several feet before it shows up indoors.

At the base. The membrane on the roof should turn up the parapet and terminate high enough that splashing and ponding water can't reach the top of it. On many older roofs, base flashing stops only a few inches up, or it's been patched so many times that the termination is buried in mastic. Water gets behind it and runs down into the building.

• Open joints between coping stones
• Loose or rusted metal parapet caps
• Eroded or missing mortar joints on either face
• Base flashing terminated too low
• Cracked brick from freeze-thaw
• Failed through-wall flashing or none at all

The right fix depends on the condition of the masonry and how the parapet looks from the street. When the brick and mortar are sound and the parapet is hidden from view, the most durable approach is often to run a membrane up the inside face, over the top and down the outside a short distance, then cap it with new metal coping. Water then has no masonry to soak into from above.

Where the parapet faces the street or sits in a historic district, the brick usually has to stay visible. In that case we do brick repointing with a mortar matched to the old brick, reset or replace coping with proper joints, and install through-wall flashing under the coping so any water that gets past it is directed back out. Exterior work in Jersey City's designated historic districts goes through the city's Historic Preservation Commission review, and we tell owners up front when a parapet job will likely need it.

At the base, we raise the flashing and terminate it under a bar or into a reglet cut into the mortar, then seal it with counterflashing. Corners get reinforcing fabric, because that is where a raised upturn cracks first. That ties directly into roof flashing repair, which often happens on the same visit, and the finished base gives any future work on the roof a clean, high detail to tie into.

On Jersey City rowhouses, the parapet between two roofs is often shared, sitting right on the property line. Water getting into it can show up in either house, and fixing only your side may leave the neighbor's half still feeding the leak. We look at both faces when we can reach them, and we explain clearly which part of the wall is failing.

When the neighbor agrees, sealing the whole party-wall parapet in one job is cleaner and cheaper for both owners. When they don't, we can wrap and flash your side and the top, with a clean termination at the line. We'll be honest about how much that will help. Either way, we keep the details consistent so future work on either roof can tie into them. If staining continues down the facade below the parapet, the wall itself may need exterior wall waterproofing as well.