Why do houses have shingles and slanted roofs, but most other buildings have flat tops?

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Why do houses have shingles and slanted roofs, but most other buildings have flat tops?

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Rain moves down off the slanted roof to prevent pooling and leaks, a larger roof is difficult/expensive to build with a slant so may have a generally flattish roof with a slight slope with drainage system removing water from the middle of the roof.

A shingle roof is a very cheap, very efficient, easy and reliable method of waterproofing a building. Tile, slate and terracotta are also similarly effective but more expensive. The sloped roofs also keep things like snow to lower levels.

A flat roof normally requires a more thoroughly waterproof system, as it can actually hold water, and more robust maintenance. They’re more complex to install, and if the waterproofing part is exposed to the elements they have more maintenance issues than a shingle roof. None of this is great for residential use.

But for larger commercial buildings it makes sense. The buildings are so large that a sloped roof would add significant height and construction expense while adding loads (cost) to the rest of the building. These types of structures also normally like to have mechanical systems on the roof (AC, Chillers, Air handling units, etc) as this doesn’t take away from leasable space inside the building. To build these on the roof you want a flat roof.

With a slanted roof, the larger (wider) the building, the taller the roof would have to be. a large building would have a very tall roof. Imagine a walmart with a slanted roof. The roof would be 100 feet tall.

Also, shingles are more expensive than the waterproof membrane they use for a flat roof. If nobody cares what the roof looks like, (or if nobody can see it) a flat roof a much cheaper option.

Shingles are surprisingly durable and their multiple layers give some redundancy

Flat roofs are common on commercial buildings because it gives you a place to put your equipment like large HVAC gear or elevator equipment that needs to protrude out the top. A trade off for being “flat” is that they actually need to slope inwards towards a drain pipe that will transport rain/snowmelt off the roof, and they need to be a lot stronger to deal with the added weight of snow on top

Buildings further from the equator tend to have more slanted roofs to keep snow from building up. Slight slants on a roof can shed snow, but if you’re at risk of getting 36 inches of snow overnight then you need a roof that will physically shed snow regardless of the suns input and that why houses in the US North East have much steeper slants on their roof than houses in Texas and the like where snow is less common and less severe

Snow, mostly. A flat roof will pile on snow until it collapses while a slanted roof will eventually have the snow slide off. However, the slanted roof is more expensive as there’s more roof surface to build. Thus we see flatter roofs in hotter climates and pointed roofs in colder climates.

That being said, home owners often prefer one style or another and modern building techniques means that you don’t necessarily need a pointed roof to keep the ceiling from collapsing.

Biggest 2 reasons is for:

* Significant added cost to make sloped roofs on commercial buildings
* Imagine the cost of making a sloped roof on something like the new Giga Factory in Texas. Also, the slope space is practically useless for something like a warehouse or factory.
* Mechanical Equipment (ACs, chillers, heaters, etc…)
* Easy installation / maintenance – with the size of most of this equipment, it would be a nightmare trying to install / service the huge AC units on commercial buildings with a sloped roof on it…

Before the modern rubber membrane flat roof or the asphalt, tarpaper roofs, buildings that would have a flat roof like a factory had “saw tooth roofs, a succession of small sloped roof with vertical glass window walls. This had the advantage of also being able to add skylights to bring light into the space. https://en.wikipedia.org/wiki/Saw-tooth_roof

ive been told that flat roofs in high population area let you control the flow of water going to the drains. if all roof were slanted there could be a overflow.

r/montreal meetup at the open house? lol

I watched this [video](https://youtu.be/QW0ydAMVQ2w) that delves exactly in to this topic but it’s not an ELI5 video per se

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.

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.

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.

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

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.

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.

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.

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.

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)

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.