Three-decker roofs: why it is a different job
The three-decker is the building type this region is made of, and it is the reason so much local roofing advice does not apply here. Its main roof is flat or nearly flat, its covering is a membrane rather than shingle, and it carries a second, smaller roof over the porch stack at the back.
What you actually own
Three stacked apartments in a wood-frame house, built somewhere between 1890 and 1930, with a shallow or flat main roof and open porches running up the rear. The main roof does not shed water the way a pitched roof does — it holds it and drains it deliberately, through a scupper cut in the parapet or an internal drain piped through the building. That single fact reorganises everything: the material has to be continuous and sealed, the seams matter more than the field, and every penetration is a potential entry point.
The porch stack carries its own roof. It is smaller, lower-slope, usually older than the main covering, and almost always out of sight from the street. Where it ties into the wall behind it is where water most often gets into the structure — and it does not show on the porch, it shows as a stain on a ceiling a floor below.
The arithmetic is different too
A membrane roof runs fifteen to twenty-five years depending on the product and the quality of the installation. A good shingle roof on a pitched house runs twenty to thirty. Owners working from the shingle number are budgeting for a cycle their building does not have, and are frequently surprised twice: once by the timing, and once by the fact that the crew who did the neighbour’s Cape does not do this work.
The cost drivers are also different. Not waste factor and valley work, which barely apply, but drainage, insulation and the parapet detail. Insulation matters more here than on a pitched house because on a flat roof it sits inside the assembly rather than in an accessible attic — meaning replacement day is the only practical moment to improve it, and the only moment it is affordable.
Choosing who does it
The useful question is not how long a company has been in business but how many low-slope roofs its crews have installed in the past year. Flat roofing is a seam-and-detail trade; the skill lives in the hands doing the parapet and the drain, not in the name on the truck. Ask for the membrane by product, ask what happens to the drainage, and ask whether the porch roof is in the number.
Next step: How long a roof actually lasts here
A contractor who works only in shingle is not the person to ask whether your flat roof needs a membrane — sometimes a perfectly honest adviser, but the incentive is what it is. On a building this common in the region, it is worth getting one of the three quotes from a firm whose main work is low-slope.