Why a flat roof?

Why do a lot of industrial and retail businesses have flat roofs? Think of Walmart, flat roof even in northern states with heavy snow. Stumped thanks.

i ask my self that too. from what i can tell its probably just easier when doing the iron work frame, and cost also.

I think it would take too much material to build it up to a pitch, plus it would result it a colossal liability if people happened to walk under the eaves when the snow slid off and got flattened. They make al least some flat roofs with rubber roofing which is sort of melted together into a continuous sheet, so it is pretty waterproof. A couple of Wal Marts did have partial collapses around here in 2007, I think, when we had an exceptional year for snow. The building trusses have to be built to bear the load of whatever snow may fall on them, I guess.
Zach

how do you put a sloped roof on a building that is 500'x800'+?

All prvious answers are correct. The roofs have multiple drains plumbed down thru the roof inside then to an outlet thru the wall. some also have scuppers(a notch thru the parapet wall to let the water off of the roof. With the drains the insulation is thinner near the drain and thicker as it get further away creating pitch. So the roof is not truly flat. It also allows them to utilize the roof top for their HVAC units instead of taking up space inside. gobble

Helped redo a roof on a flat roofed bungalow of a buddies about a lifetime ago... Had about 6 inches of rocks over roll roofing that was heat sealed and drains in the corners... Said that was the way to go because the rocks held moisture and the evaporation helped cool..

The walmarts i've been in were all steel beam,no trusses.

Thank you, I am not up on these matters.
Zach

I asked a roofer that once. He said that flat roofs were a good thing, they kept roofers busy fixing them! I do think that when the building is too big, that what others have said applies.

A lot of "flat roofs" going up now with a slight slope, maybe 1:32 ?
Just enough to run water but not require high peaks.

On the school where I worked they took a pitched roof off the old part and put on a flat roof because of melting snow on the pitched roof created ice dams and major leaks. With the flat roof the melt ran to drains and was gone. Usually the snow melted in a few days on the flat roof because the sun hit all of it. On the old pitched roof the north side was snow covered from November to March in most years.
The roofing company told us the strength of the support materials for the flat roof was greater than the old roof.

By code flat roofs are supposed to be pitched. I think the latest code requirement is 1/4:12. Either done structurally or by using tapered roof insulation.

As a experienced school employee of many years, I can make a few observations... Many school buildings throughout the Michigan area were built with "flat" roofs years ago. Have been leaky, costly to maintain, and generally a pain. Even with new rubber membrane technology they don't hold water. They've tried all kinds of insulation, everything. Now I'm seeing buildings being retrofitted with rafters and sloping roofs. Newest construction is going with steel roofing. Sloping roofs don't puddle water, leading to leaks.

One of the nastiest construction methods was the WINDOW buildings used for elemetary buildings years ago. Outside walls were particularly high with much glass. The roof line was inverted with V shaped roofline, water all ran inward toward middle hallway, and leaked all down the hallway!

As a experienced school employee of many years, I can make a few observations... Many school buildings throughout the Michigan area were built with "flat" roofs years ago. Have been leaky, costly to maintain, and generally a pain. Even with new rubber membrane technology they don't hold water. They've tried all kinds of insulation, everything. Now I'm seeing buildings being retrofitted with rafters and sloping roofs. Newest construction is going with steel roofing. Sloping roofs don't puddle water, leading to leaks.

One of the nastiest construction methods was the WINDOW buildings used for elemetary buildings years ago. Outside walls were particularly high with much glass. The roof line was inverted with V shaped roofline, water all ran inward toward middle hallway, and leaked all down the hallway!

I worked in the Hot tar roofing industry for 2 years and became a foreman. If all engineering and construction was done right the flat roof is far better but all archetectual blunders come back to haunt you. The most common problem and hardest to correct is that the decking is too light for the load. As the deck sags it can hold more water and as it holds more water the decking sags more. I doubt if bitumen is still available. The roof had to be flat and the bitumen would almost become liquid in the summer and reaeal itself. A lot of roofing crews were the less honorable than the other construction workers, All of our crews were made up of low level social rejects and many of ours were illeterate and without legal driver;s licenses. The worker turn over was terrible In 1966 we hired over 100 to maintain a crew of 10, I think the average tenure was about one month and several quit after one day. One fellow refused to climb a ladder and quit during the first hour.

The theory of a flat roof is good but all mistakes and omissions are always evudent LEAKS. Morning after a summertime shower the phone would ring. A promoise to get it as soon as the rain stopped would buy time until the next rain.
But I still like a flat roof but have seen few, Maybe thay do deserve their poor reputation.

You can blame Frank Lloyd Wright.

My FIL told me that when he was young,the gym at the high school was being built with a flat roof. It was to hold a few inches of water so the sun would not get to the material very often and it would last.

Cost is probably the biggest reason. I oversaw the maintenance on a building that had 1.25 million sq. ft. It was all flat. The size of this building made it very hard to install a sloped roofing system. Another thing with a large pitched roof the roof drain system would have (storm water)to be huge to handle a hard rain storm because of the speed the water would run off the roof. We installed a new roof on our building after a tornado tore part of the old one off. Took 2 years ( $ 7.5 Million) with as many as 50 people working around the clock 7 days a week when the weather permitted. We installed extra roof drains to eliminate ponding and had water coming out manhole covers in the building because the original piping was not large enough. A properly installed roof on a flat building will last a long time. Ours was designed by a roofing engineer and inspected weekly by him and daily by me. It was a 2 ply torch applied Siplast system.
Penetrations for equipment would be a lot harder on a pitched roof.

Last winter the new Cowboy's Stadium in Dallas had problems with sheets of ice sliding off the domed roof on people. Back to the drawing board. I bet they wish it had a flat roof.