I know you will be surprised to know that technically there is no such thing as a flat roof. All roofs are slightly pitched which helps to drain water. So for draining water properly, sloping a flat roof is essential. And in this article, we will show you how to slope a flat roof for drainage?

To slope a flat roof for efficient drainage, firstly you must consult your local building codes. For sloping, you’ll also need authorization from your local government. Then you should figure out how much slope you’ll need for your flat roof and then get to work.

Let’s take a closer look at the process of sloping a flat roof for drainage.

How To Slope A Flat Roof For Drainage

Every flat roof should have a drainage system to prevent stocking water and other damages. Before starting the sloping, you should know about some common sorts of drainage systems for a flat roof that may help you to decide which type of system you need.

Let’s check some of them.

Guttering drainage

Guttering drainage roof

It is the most common and cost-effective drainage system. In this system, vinyl, aluminum, and galvanized steel are used as the most prevalent gutter materials.

This kind of drainage is great for managing the flow of water from the flat roof. You won’t have to worry about soil degradation with guttering drainage.

But it has a disadvantage. And that is, it is prone to clogging as it is an open drainage system. As a result, in order to get the most out of your investment, you’ll need to maintain it on a regular basis.

Siphonic Drainage

Siphonic Drainage roof

Water is flashed automatically in this drainage. Some outlets are connected with each other and drain into a single large pipe.

In this drainage system, slanted pipes are not necessary and water drains more quickly than any other system. It is very easy to install.

Inner Drainage

Inner Drainage roof

Inner drainage is widely used commercially. Here all pipes are put under the flat roof in such a way that pipes will not be seen.

The most benefit of it is that it will not cause any damage to your property if any pipe leaks. But as the pipes are installed under the roof, repairing is costly.

Now let’s have a look into some detailed processes that you need to follow for making flat roof drainage.

Step 1: Check the local building codes.

This is a crucial step. Before starting your work, make sure you have known all the construction codes of your area. Your whole work will fail if you don’t get permission from your local authority.  

Step 2: Measure how much slope height you need.

The correct measurement of the slope is important to make a successful drainage system. For building slope correctly, taking a one-centimeter slope per sixty-centimeter horizontal width is recommended for perfect drainage. 

If you use firings on constructing your flat roof drainage, this measurement will be enough. These are basically timber battens used to construct a slope in the flat roof.

There are also several determinants that should be required before calculating slope height. They are:

  • Precipitation amount
  • Speed requirements for draining
  • The design of the roof
  • Snow and ice chances in the winter 
  • The framework’s structural strength 

The quantity of water/ice which can possibly gather on the roof during intense rainfall or severe snowfall is the most important factor to consider. . Many of the houses in the region have more complicated roof designs made of specialty materials like metal and tile.

Each of these criteria aid roof designers in determining the right slope angle while building the roof.


Have you found your answer yet? I truly hope so. You now know how to slope a flat roof for drainage. So, what are you wasting your time on? Get down to your business.

Ponding on a flat roof can lead to a variety of issues, including leaks, vegetation development, and structural damage. As a result, the flat roof’s estimated life gets lowered. As a result, appropriate draining is necessary to improve the lifetime. 

Three different types of drainage systems are discussed here. You can select any of them based on your construction codes and requirements. Don’t be concerned; simply begin your job by following the procedures outlined in this article.

Lastly, be careful when working.


As a civil engineer and roofer, I love to share the experience that I have gained through the last couple of years. In the roofing industry, practical experience is a very crucial fact that can help you a lot. Hence, I want to help you with my blog.

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