Why do houses have shingles and slanted roofs, but most other buildings have flat tops?
In: 8443
Ever see a large ice slide from your slanted roof? Imagine that happening from a larger slanted roof about ten stories up. There are certain risks to public safety inherent in the decision I’d imagine.
Just to exapand on what some others have said, a “flat” roof is rarely flat. They usually have a small enough pitch to seem flat but allow run off to other drainage.
The ones that are flat are designed to remove water in some way, like evaporation, which is partly why some roofs have stones on them, (the likes of portacabins etc)
Despite what you think, all roofs are angled.
Slopes and shingles are a style choice, and work well over short distances. However, to achieve the same effect over large distances would leave you with a roof nearly as tall as the building itself.
When you have a large area, it is MUCH better to have a slight slope with tarp and gravel. The structure needs to be stronger but that issue starts with holding up a building that large anyways. A little more near the top to support the weight of snow is hardly an issue when you consider how much you save on large cathedral roofs.
TL;DR. They are not flat, they are slightly angled and use material that does well at that angle.
From the engineering side, everyone is spot on. I didn’t think I would have anything to add, but I do.
Commercial buildings have the unique task of generating revenue for the occupants, whether they are owned or leased. Square shapes = maximum volume and usable area. A complicated roof structure (angles and what not) limit the available space inside the structure. Maximum ceiling heights without dead space ensure that you can fit in as much shit as the occupant needs.
In industrial buildings this means huge equipment, in commercial buildings this means maximum square footage with enough overhead (above ceiling) space for utilities. Next time you’re in a hospital, take a look at the ceiling, whether it’s a grid ceiling or drywall ceiling, just know that there’s anywhere from 4 to 8 feet or more of extra space above the ceiling and it is *jam packed* with wires, ducts, medical gas lines, air handling equipment, and sometimes tube systems. Every floor of the building is designed to hold as much *stuff* as possible. And when you get to the roof, that’s where a lot of the heavy mechanical equipment is, and at a hospital, the helipad. Flat roofs offer yet another way to maximize usable square footage.
Frank Gherry’s buildings are beautiful but so complex they have lots of water issues and result in a horrific waste of space. The flip side is that most commercial buildings are just sad looking yet efficient blocks.
As many have pointed out flat roofs allow for mech/plant on the roof. But in many commercial or residential tall buildings, developers will build to the absolute max height allowed by the land zoning. So they could either have an extra floor with 10 more sellable apartments (or offices) or use that space for an elaborate roof.
None of the top answers are anywhere near EL5. I am going to try:
Sloped roofs do not have to be water-tight to keep water out. People around the world use materials such as hay, leaves, wood shingles and clay tiles to keep water out.
The steeper a roof is, the faster water rolls off or it, but it is also harder to build a steep roof and it uses much more material.
Flat roofs are never fully flat since they have to provide a way for water to move off the roof. The slope of a “flat” roof can be much less stiff, but has to be completely waterproof to work. This means that modern materials or tar (which is an industrial product and is therefore “modern”) must be used.
The benefits of flat roofs are that you can enclose the most space for the least material, but this is also driven by what material your building is made of.
That’s the trick: Buildings *do have slanted roofs. They just have less of a slant because it’s cheaper and they don’t want an attic
A flat roof by name is misleading. Tapered insulation or framing is used on the roof to let it slope to the drain. A flat roof will hold ponding water and snow and extra weight on a roof is structurally a catastrophic situation. I was a commercial roofer in San Francisco at the foreman level for over 10 years.
Shingles are good because they are easy to install if done correctly and can last up to 50 years if you buy presidential shingles that are thicker and more heavy duty.
Architect here. The main reason is that people generally don’t like flat roofs on houses.
In western tradition, houses have had sloped roofs, though that varies by region and country. In some places, houses have flat roofs more often then not. But western culture tends to identify more with, say, European traditions, and those traditions include sloped roofs on houses. So when looking at a house, most people want the traditional “look” of a house.
Because sloped roofs are cheaper, but only to a point, Because buildings with sloped roofs get stupidly tall the larger they get. For smaller buildings, sloped roofs and shingles is the most cost effective way to do it. But as the building gets bigger, the slopes go on for longer, meaning they get taller until it’s completely unpractical.
Think about a 4/12 pitch, which is rule of thumb about as shallow as you want to go for a sloped, shingled roof. A 100 foot wide building would result in a peak that’s about 17 feet taller than the wall height. That means the gable (the triangle peak part) will be taller than the walls it’s sitting on somewhere thereabouts. Which looks goofy and results in additional cost in siding and paint.
On a Walmart that’s 250 feet across, your peak height will be *83 feet* higher than your top of wall height. Think about the extra siding, extra paint, etc. You can pay for a lot of low slope roof for that.
At a certain point, under a certain length, sloped and shingled is cheapest. Above that length, the additional cost associated with a low slope membrane roof is less than the cost of siding and painting the gable, as well as how goofy it would look for the majority of the facade being gable. I’d argue that threshold is around 80 to 100 feet, which coincidentally is around where you see sloped roofs give way to low slope roofs.
Latest Answers