Why do we need roofs on houses? Why can’t we just build it flat on the top?
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You can in some places. But a flat roof in areas where snow is common is dangerous, as the snow will accumulate on top of it and can collapse the roof. Pitching the roof also helps rainwater run off; rainwater that runs off quickly doesn’t have as much time to soak into or through the roof, causing damage to the attic/house.
Flatter roofs are most useful in places that don’t see a lot of precipitation, like deserts.
Some areas do have flat (or at least flatter) roofs, but those tend to be in hotter, drier areas (deserts, etc.). A roof helps keep out the elements, especially rain and snow. A sloped roof sheds rain quickly, and a steeply sloped roof sloughs off heavy snowfall. Water sitting on top of a house can quickly find its way inside, destroying contents and weakening the structure, so the faster it’s removed, the better. Snow can be heavy, and a deep snow can collapse a roof if it’s allowed to build up.
And once you have a pointed roof, you have additional room to place things like heat exchangers/HVAC systems, cooling fans, etc. Plus the ability to better insulate the lower living areas by having an attic or crawlspace.
In some places you can but flat roofs hold precipitation, which can cause problems. If you live somewhere it snows a lot, snow can build up enough on the roof to cause it to collapse; you can mitigate this by periodically shoveling off the roof but… well, it should be obvious why that’s not preferred. Rain is less dramatic but can still be a problem: it’ll run off a sloped roof easily but will pool up on a flat roof, which can cause water damage and leaks over time.
I’m going to guess you’ve never lived in Snow Country. Imagine 6+ feet of snow on top of a flat roof. At approx. 20lbs per cubic foot, that’s 120lbs per sqft of roof. Say the house has 1000 sqft of roof (our 3/2 has about 1800 sqft), that would mean your flat roof is holding up 120,000 lbs. That might be a bit past its specified loading capacity.
Where I live there is lot of snow. Flat roofed commercial buildings have been known to have roof collapses from the weight of snow on the roof. Peaked roofs avoid that.
Slanted roofs move rainwater and snowfall much better, helping prevent leaks. Even flat-roofed buildings have a little bit of slope to help move water. Also, having a vented attic helps have a buffer zone for the heat that builds under a roof on sunny hot days, helping with energy costs. This doesn’t mean flat roofs on single family houses don’t exist.
Roofs need to shed precipitation. The shallower the angle of a roof, the less incentive rainwater has to flow off it, and the more time it has to wick its way underneath shingles or through seams. In areas that get below freezing, flat roofs can accumulate enough ice and snow to cave them in. The more snow an area gets, the steeper the roofs tend to be.
Water falls from the sky sometimes. Having a pond of water standing on your flat roof is a really great way to give it a chance to seep through and ruin things. Sloping the roof is an easy-peasy way to make it flow away.
Flat roofs are a thing, but they’re actually a lot more trouble to build and repair compare to a pitch roof.
First off, a house with a smooth flat top roof will just have water flow over the edge and run down the walls. This is not the best, because that water can wear down or stain your walls (a lot of dirty debris can end up on your roof, be it via wind or flying/climbing critters), among other things. So with most flat roofs, the edge border of the flat roof are raised up to keep water from flowing over the sides, and instead there is a drain on the roof where the water can flow down neatly to the ground or sewers through a pipe.
But this creates other problems. Since the sides are raised up, your flat roof basically becomes a pool where water can collect. If there is a lot of rain or your drain is clogged, you can literally have a several inches of water being held by the flat roof. If your flat roof surface is not completely sealed up, that water can leak though the tiniest gap or hole. Which is why most flat roofs for homes and buildings are usually built by melting a layer of rubber or tar over everything, (hopefully) creating a leak-free barrier.
On a pitch roof, that water will just flow down the angled slope of the roof (usually into a gutter and drain so again it doesn’t run down your walls). So pitch roofs can get away with not having a completely watertight surface. Shingles and tiles you see on most pitched roofs have gaps where they overlap each other -but since the gaps “points” downwards – water flowing down the roof won’t go in that way, unless there’s ice damming. Which is easily prevented by putting a layer of ice-water protector membrane – aka sheet of adhesive rubber – several feet up from the edge under the shingles or tiles.
With a flat roof, there’s usually a lot more work and more things can go wrong – which is why generally it costs about 3 times more to replace a flat roof vs pitched roof for the same top area. And of course, there’s visual appeal – for residential homes, a pitched roof generally consider more appealing than a flat roof.
But too steep (greater than 45 degrees) of a pitch roof (so-call cathedral roofs which are popular nowadays) can also lead to problems, namely presenting a more direct face against strong winds that result in shingles being blown off more often.
Source: Did roofing for a couple of years
Regardless of the shape, it is still a roof, if the inclination is less than 10 degrees it is called a flat roof. You do not what a roof totally flat because you what rain to drain away, if it starts out flat you can get a low spot so you want some inclination to avoid standing water.
The amount of inflation you need will depend on the type of roof material. Roof tiles for example have a minimum inclination requirement to be watertight.
Another factor is cost. The roof needs to be strong enough with for example wood you need a lot less material if you make a triangle and then lost a flat beam. That is especially the case if it needs to be able to handle snow loads.
The result is that quite often a roof with a triangular crosssection is a cheap and efficient way to build a roof on a small building. This is especially the case with traditional building materials.
Quite flat roofs are very common in commercial buildings. Concrete and metal beams can be used. For a building with a very large footprint flat roof might be the only practical option because an inclined roof would be very high
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