Proper sealing for brownstone roof dividing wall

Question

TL;DR: For a four-storey New York brownstone with flat roof how should the dividing parapet wall be covered/sealed?

The brownstone has a rubber-like membrane on the roof that is connected to brick parapet walls. The parapet walls have flags of stone on top of them. These stones aren’t in good condition and parts are flaking off in places. I think it is likely water is getting past the stone and going behind the membrane. In some places I see blisters in the membrane beneath these stones. One of these blisters recently cracked and caused a leak which I fixed with flashing cement and roofing fabric.

My neighbors’ roofs have different treatments.

• Some have this wall covered in cement and painted
• Some have clay coping tiles that look like they are designed for this application
• Some have a sheet metal covering
• Some have cuboid blocks similar to mine but they appear to be concrete instead of stone

My questions are:

1. Should be worried about the way my parapet wall has been capped?
2. Is the condition of the capping stones the likely cause of the blistering? If not, is there something else I should be looking at?
3. How should the parapet wall be capped?
4. To keep water off the place where the membrane joins the wall I am thinking of adding inverted U-shaped aluminum flashing over the stones held in place with flashing cement. Is this a sensible solution?

My roof showing the stones capping the parapet walls

Close up showing the condition of the capping stones - stone is flaking off

Blisters in the roof membrane at the parapet wall

Example of one of my neighbor's walls with clay coping tiles

Answer 1

My roof is extremely similar to the last picture. So similar, in fact, that I suspect I know where you live.

The way these roofs work is that that the roofing membrane forms a U shaped bathtub that conducts the water to the scupper at the low end.

The capping stones provide a semi-impervious cover for the exposed top of the walls, and, if done correctly, provide a drip edge so that the water drips off into the bathtub, and doesn't get wicked along the wall. They don't need to be perfect, and shouldn't be, as you want some possibility of having water evaporate out of the wall. There's a whole theory that older lime mortar was better than portland cement because it could handle being wetted and would expand and contract without cracking, but that's another discussion.

In any event, the walls were not really designed to have the entire parapet wall be watertight, and, what you are likely seeing is that some water got behind the wall and is causing it to blister. You will notice that on the terra cotta cap-stone roof, the membrane only goes up a certain distance and is held to the wall by flashing that is caulked -- this joint is protected by the stand-off of the capstone.

How that water got behind the wall could be due to a variety of things...

• excessive snow followed by rain or a thaw -- this can cause the water to spill over the bathtub
• driving rain at an angle
• a "fish mouth" bubble in the seams of the roofing material
• water coming from the other side of the party wall for the above reasons
• gap between the coping stones that is wicking water into the wall.
• a small puncture or nail hole in the membrane that only gets exposed in high intensity rains
• a temporary blockage in the drain that caused the water level to be higher

As an FYI, figuring out the cause of these issues is incredibly hard as the water can travel under the top layer and come out elsewhere. The roofing companies are notoriously expensive (thanks NYC Scaffold Law!), and, at least the last time I called one, they asked if I spoke spanish and offered me a position as foreman.

In any event, those coping stones look fine, and even if they weep water it is unlikely that that is causing the leak you experienced -- look for sources of bigger amounts of water.