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Roo Roofing Blog

DIY & HOME OWNER TIPS

19
Jul

What goes into a flat roof design?

Flat roof designs date back to the 1920s, and have proved popular in recent years as many new properties embrace this particular look.

In today's blog, we look at how flat roofs are constructed to account for factors such as rainfall and drainage, and provide a summary of three of the most popular flat roof designs curently on the market.

 

They're flat... to a certain degree

One common concern many have when it comes to a flat roof is how exactly rain water is supposed to run off an entirely flat, presumably horizontal surface. Well, the truth is, flat roofs aren't 100% flat. The parapet roof, for example, is actually very much like a traditional sloped roof - the illusion of it being flat is created by the surrounding parapet walls.

For roof terraces, there is a very subtle slant to it. So while it feels and looks entirely horizontal to the naked eye, the flat surface is just elevated enough for rain water to run off it. There can also be flat green roofs, but ensuring rain water doesn't build up on these roofs is often achieved through a drainage cell/pipe situated in the soil.

 

Roof types

As mentioned, three common types of flat roofing are the parapet roof, roof terrace, and flat green roof. Of these three, parapet roofs are the most popular.

Parapet roof

We noted that this type of roof is actually sloped, and its sense of flatness is more achieved by the surrounding parapet walls. Below is a photo of the King's College Chapel in Cambridge, England. While your own home won't have the same architectural splendour, this provides an idea of how the roof is sloped but is then surrounded by walling that, from ground level, would make the roof appear flat.

kings-chapel-roof

For a modern day roof utilising parapet walling, the roof is usually shaped in a slight pyramid-like style. To account for rain, most of these roofs are surrounded by box gutters. They're larger and stronger than normal gutters and can hold more water (or the weight of an average person). Because, unlike normal roofs, the surrounding walls mean the rain water can't fall over the side if the gutters become blocked or overwhelmed. This is why box gutters must be used with parapet roofs/walls.

modern-flat-roof-parapet

Roof terrace

Roof terraces aren't as popular as parapet roofs, but they're certainly a design that appeals to many architects and people in general. After all, what makes for a better view of your surrounds than a roof terrace? The problem is that this type of roof is complicated, both in terms of execution and adhering to construction codes.

concrete-roof-terraceSource

For the terraces that are built, they're typically comprised of a concrete slab, which is slightly sloped to allow for rain to naturally flow off and into the guttering below, and this can then be reinforced. Given its porous nature, the concrete needs to be sealed so it won't sustain water damage. Framed terraces are also an option (often comprised of wooden/timber planks). While they are lighter than concrete-based terraces, they're also exceedingly more difficult to water proof.

Flat green roofs

For those who want to make their roof more environmentally friendly, there is the ability to create a green roof. Now, in the theory, green roofs can exist on flat or sloped roofs, but some of the best examples of green roofs always tend to crop up on those with a flat design.

chicago-green-roof-flatSource: "20080708 Chicago City Hall Green Roof" by TonyTheTiger - I created this work entirely by myself. --TonyTheTiger. Licensed under CC BY-SA 3.0 via Wikimedia Commons.

The issue of pooling water is what matters when it comes to a roof covered in vegetation. This is why almost any green roof you come across includes a waterproof membrane. This allows rainwater to absorb into the vegetation and soil, but it won't break through into the actual roof/sub-flooring. As the point of a green roof is for the water to be absorbed, these flat roofs don't have to worry as much about having a subtle slope. What is important, however, is that the roof is reinforced to account for the additonal weight of not only the greenery, but the variable weight/pressure that occurs when there is heavy rainfall.