When I was travelling in North America I noticed that a large proportion of buildings had flat roofs. ie thay did not have a pitch. Looking down at them from above, they had what appeared to be a bitumous membrane mixed with gravel.
Why not have sloped roofs, so that the water drains off?
It seems that this is just asking for a leaking roof or buildup of snow in winter. Does it have anything to do with ‘roof avalanches’?
Depends.
Apparently flat roofs do actually have a slight slope and drains. “Flat” roofs in places like the northeastern U.S. are designed to carry snow loads with a two or three times safety factor. For large buildings like shopping malls it often makes economic sense to build a very strong “falt” roof rather than a pitched or peak roof which will add a lot of volume and therefore air handling demand.
Granted, on small residential houses like mine you will see pitch increase as you move north, but that is an entirely different building scale.
The reason for making flat roofs is that you reduce the construction costs by not having to build a peak from which to extend the pitch. It is most frequently used on factories and shopping centers where they would need either an enormously high roof center to slope to the edges, or a series of “wave” like smaller pitched roofs with gutters at the base of each slope to carry off rain water. (We are often talking about acres of sloped roof, here.)
(Having never worked in a store or factory that did not experience a leak from puddles collecting on the flat roof, I think they should have been more imaginative in designing the buildings with some sort of slope, but they have never consulted me before designing any of them.)
Well. First off lets separate between residential and commercial buildings.
Residential buildings (I mean privately owned homes and not appartment buildings) will have a sloped or flat roof depending on the amount of percipitation in the region and the temperature variences. In Nevada you wouldn’t want to have a steeply sloped roof (or even sloped at all) because this costs more when there is no precipitation to accumulate up there.
For commercial buildings the above answers get most of it. I’ll just use an example to point out a new thing. Take Wal-Mart as an average commercial store. (They’re everywhere in the US). This store tends to be much larger than most counterparts in Europe. (Or at least so I hear). Think of the sheer amount of precipitation that would be falling off this slanted roof at any given time. Thats one massive wall of water that a gutter would have to take care of. It would be more expensive to build that gutter than it would to make separate drains in the roof.
tomndebb beat me too it, but I was going to mention that my current place of work and my previous place both had flat roofs, and both leaked (my current place still leaks). Tho most of the problem with my current workplace is a leak/overflow/something in the air conditioner drip pan.
Brian
it is not a comforting sight to see your cieling tiles dampen above your monitor
And how. Flat roofs are used on commercial buildings because they’re cheap to build, as others have pointed out. That is their only advantage. Although many are designed to have a very slight grade for water run-off, they virtually all sag, pool, and leak eventually. When that happens, they are difficult or impossible to repair properly, short of tarring the entire roof. That makes them expensive to own, since that has to be done regularly. This is all accepted as perfectly normal - the roof starts to leak every 3 or 4 years, and it has to be re-done.
There is a pretty well-known, and very interesting book called How Buildings Learn that contains, among many other things, something of a tirade against the short-sighted flat-roof design of most commercial buildings in the US.
I think the buildings that had the flat roofs were all commercial.
Exgineer. Wouldn’t a sloped roof reduce the heat load because there is a cavity between the ceiling and the outside?
tomndebb. I am talking about commercial buildings that are less than about 3000 square meters.
Marduk. Not sure what you mean. Where does the water on the flat roof go? It would still run into a gutter, wouldnt it? If it was flat you would end up with leaks.
It just seems weird having roofs that would build up with snow in winter, increase heat load in summer and more likely to leak all year round. I am sure there is an obvious explanation though.
OH, missed lagged2deaths post. Maybe it is just short sighted. I will check your link. Ta.
Well, the same rules of cost apply. The higher the roofpeak, the more material is required. 3,000 sq meters is still a fair sized building, and once they developed the techniques for laying asphalt roofs, there was no reason (other than common sense) to not use them on smaller buildings, as well. (It is also true that many smaller buildings that use asphalt roofs actually do have a pitch, but the pitch is fairly low and facades mask the fact that the roof is pitched.)
From my construction materials book:
Given the problem of leaks, I guess that must be a lot of $$ difference. 
One advantage that hasn’t been listed is that it also prevents snow and ice from coming off the roof all at once and causing damage to parked cars and people walking on the sidewalk. Laugh all you want but I have seen so many places around lately with snow fence and construction tape blocking sidewalks and parking spots where there is a danger of falling snow and ice.
-funneefarmer, whose dog will be cowering during the next thaw as the snow comes sliding off the roof of the barn, making sounds akin to thunder.