The Trouble with Flat Roofed Houses...

The Trouble with Flat Roofed Houses...

Flat Roofs stress city sewage system during storms

The recent record-breaking rains not only left in their wake a swath of flooded homes, mildewed and moldy basements, damaged foundations, eroded streets, but they put tremendous pressure on local sewers, some of which backed up. It’s bad enough to be flooded with rainwater, but sewage is a nightmare.

Storm water and Sewer water

Where the city has separated storm drains and sewer drains, the rain water runoff goes into the gutters and into the storm drains, remaining separate from the sewer drains so that in heavy rain the sewers don’t back up.

storm_sewer.The sewer drains are separated from the storm drains so they only collect water that comes out of buildings via the plumbing. This includes not only water from toilets, sinks, washing machines, and dishwashers, but also from rain from the roofs of flat roofed buildings. This is all water in the sewer portion of our water bill and is processed at the water treatment facility at Deer Island.

Flat Roofs drain rain and snow to the Sewer, not to Storm Drains

Flat roofs are slightly concave – just enough to funnel rain water and melting snow into a central drain that runs all the way down through the center of the building and directly into the municipal sewage system, not into the storm drains. Theoretically, water, snow, and ice do not build up on the roof, which could lead to dangerous conditions and create structural stress on the house.

However, during heavy rains when sewers are already stressed, adding the extra rainwater from flat roofed buildings to the already overworked sewage system is dangerous and unnecessary. But how much rainwater could that really be?

Flat Roofs Add over 10 million gallons to the Cambridge sewer system

If we have an estimated 2000-3000 flat roofed homes in Cambridge, with an average roof area of 1500 square feet on each, and last Monday and Tuesday’s downpour dropped about six inches of water on us, that comes to about 750 cubic feet of water per roof. On 2000 flat roofed homes, that represents 1.5 million cubic feet of water going into the city storm drains (in addition to the regular sewage) – 11.25 million additional GALLONS of water going through our already stressed sewer system. And that is just from residential flat roofs in Cambridge, it does not even take into account flat roofs of industrial and commercial buildings, nor some pitched roofs that still dump into the sewer through basement connections.

Flat Roofs can also allow snow and ice buildup to cause structural damage

If you’ve been a environmentally conscientious doobie and put insulation between your topmost floor ceiling and your roof, chances are the snow and ice on your roof won’t melt. Rather they will freeze over the central roof drain and block water from going down the drain, which means all that snow and ice will back up on the roof and continue freezing, which will allow even more ice and snow to accumulate. This can cause roof joists to crack under the weight of ice and snow buildup, and water which can’t drain to build up higher than the flashing and make its way into the house around the chimneys.

Flat roofs are liabilities to homeowners and to the Municipal Sewer System

Homeowners are at risk for structural damage due to snow and ice buildup on flat roofs, they run the risk that the central drainpipe breaks, causing potentially enormous and expensive water damage inside the walls and on all floors of the building. And all of us Cambridge residents pay sewage charges for water that comes into the sewage system from flat roof drains instead of running into the ground as it would off a pitched roof.

This additional water significantly increases the load on Municipal Sewage systems and it could lead to failures during intense snow or rain storms.

The International Panel on Climate Change forecasts significant increases in our snow loads (heavier, denser snow due to milder winters) and more intense precipitation – both rain and snow.

What’s wrong with this picture?

Picture the quintessential photo, postcard, drawing, painting, print - of wintertime in New England. In this picture, I bet all the houses have pitched roofs – for the snow and ice to run off. Steep pitched roofs are an integral part of snowy climates from New England to Switzerland.

Why, then, do so many homes here in Cambridge have flat roofs? It would seem counter-intuitive.

With climate change predicting more precipitation, the drainage problem is only going to get worse. Flat roofs just don’t make sense in Cambridge.

The City of Cambridge could be proactive and allow flat roof homeowners to pitch their roofs 45 degrees to allow snow and rain to run off to storm drains and not cause structural damage to the building.


The problem is not necessarily that there are flat roofs in Cambridge; the problem is that stormwater from flat roofs is tied into the sewer system. Flat roof runoff does not have to be connected to the sewer system. The problem can be solved by connecting roof stormwater runoff to the drainage system (provided of course that no wastewater from internal plumbing is tied into the pipe that conveys roof runoff).

thanks for your comment.... but I understand re-directing the existing run-off pipe away from the sewer line would be quite costly as most triple-deckers have a rainwater column that ties into the waste-water column before running out of the house to the sewer. In some cases it could mean running a whole separate column just for rain water. There is no incentive for homeowners to go through the expense and construction headaches associated with such a project. Furthermore, it would create areas of concentrated water outlets that would directly affect the foundations of the owners and their immediate neighbors, and could cause erosion problems on sloped properties, plus all sorts of other issues.

The advantages of allowing owners of triple-deckers to raise a pitched roof are:
A] distributing rain dumping (typically 6 gutters and at least 4 downpouts),
B] allowing snow to shed off rather than build up (my flat-roofed neighbor chiseled off 50,000lbs of ice + wet snow off his roof in January 2009, after the roof started leaking and cracking!),
C] the homeowner can gain some attic space
and for the city, this added attic space would increase property tax revenues.

The cost of pitching a roof may be greater than re-roofing a flat roof and diverting the rain pipe, but the owner gains peace of mind and attic space (better insulation too.