Why do we need roofs on houses? Why can’t we just build it flat on the top?
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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
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