Two households, twice the holes
A two-family house is not a bigger house. It is two houses stacked, and it has two of everything that has to come through the roof — on exactly the same amount of roof.
- 1 The ridge. One ridge over two households, and one of them pays to replace it.
- 2 The field. The part everyone looks at, and rarely the part that actually failed.
- 3 Valleys. Most of them were cut in later, when the house was divided into flats.
- 4 The chimney. Often two of them, and often neither one carries anything any more.
- 5 Penetrations. Two kitchens and two bathrooms mean twice the holes on the same roof.
- 6 The overhang. Where heat from a flat you do not control comes back down as ice.
Count them from the street
Two kitchens and two bathrooms mean two plumbing stacks, sometimes four. Two heating systems mean two flues. Two bath fans, two dryer vents, and on many of these houses a second electrical mast as well.
Every one of those is a hole cut on purpose and then sealed. They are not defects — they are how a building works. But a roof with eight penetrations has eight chances to fail where one with four has four, and it costs nothing to know which you have.
The rubber collars are the short clock
The flexible collar around a plumbing stack ages faster than the covering around it. It cracks at the fold where the pipe meets the flange, and it does it quietly. It is the single most common source of a ceiling stain that appears years before the roof itself is finished.
Better collars cost very little at installation and push that first leak past the middle of the roof’s life instead of before it. On a building with twice the penetrations, that choice is worth twice as much.
What to ask
Ask what goes back around each penetration, named as a product rather than implied. Ask whether anything can be removed — abandoned flues and dead vents are common on converted houses, and a hole that is closed properly needs no maintenance at all.
And ask whether the bath fans actually exhaust outside. On this housing they frequently discharge into the attic, which puts moisture exactly where it does the most harm.