What’s the Minimum Roof Slope To Accommodate Rainwater Drainage?

Here’s the deal:

Builders have to consider many factors before construction.

And one of the most important decisions is deciding how much to slope the roof.

So how much roof slope is required to accommodate rainwater drainage?

According to the International Building Code, a minimum slope of ¼” for 1 foot of roof is necessary for proper rainwater drainage for a flat roof. However, areas that receive large amounts of rainfall should have a steeper slope like 1-inch for 1 foot of roof or more.

But there are still many factors that can determine what roof slope is ideal for you.

What are those factors? Read on below to find out.

Table of Contents

How Is A Roof Slope Determined?

Before we head on to discussing the minimum roof slope, we must first understand how to calculate it.

Roof slope, otherwise known as angle or pitch, is determined by the rise/decline in the height of the roof over a distance of 1 foot (12 inches). So, for example, let’s say that a roof has a slope of 4:12, which means that over the distance of 1 foot, the roof rises/declines by 4 inches in height.

Considering this ratio of 4:12, the angle of the roof would be around 15 degrees. But this is not always the case. So let’s move on to finding the minimum roof slope requirement for rainwater drainage.

Minimum Roof Slope Requirement For Drainage

For flat roofs, the minimum recommended roof slope is 1/8” per 1 foot to accommodate rainwater drainage. As a ratio, it would be 0.125:12, or 0.125 inches slope per 12 inches.

However, the minimum slope as per the International Building Code for standard flat roofs made from asphalt or tar is 1/4″ per 1 foot. Or in ratio terms, 0.25:12, or 0.25 inches of slope per 12 inches.

Despite how it may sound, both of these slopes are quite low and are usually not enough for proper rainwater drainage. The minimum recommended roof slope as per the International Building Code is only 1.19-degrees steep.

Generally, a standard flat roof made from common roof materials should have a slope of around 2:12, which means a slope of 2 inches per 12 inches (1 foot). But there are still many factors that play a role in determining the ideal roof slope for your flat roof.

Factors Affecting Minimum Roof Slope Requirement

You might hear many different recommendations regarding the minimum roof slope for your flat roof. But choosing the ideal slope includes many factors which you must consider before making the final decision. Here are some of them:

Climate and Location

Our motive here is to achieve the best roof slope to accommodate rainwater drainage. But have you considered how much rain your location receives?

The climate of your location significantly impacts how effective a roof slope will be at draining rainwater. Some areas that receive multiple episodes of heavy rainfall and snow need a steeper roof slope for proper drainage.

However, in areas with more moderate rainfall, a steep slope would be unnecessary. Therefore, the buildings in these areas would be better off with a flatter roof slope.

Roofing Material

The material that your roof is made out of can impact the minimum recommended roof slope for a flat roof. An important objective when creating a roof slope on a flat roof is to prevent rainwater from accumulating on the roof for more than 48 hours. After this time, water starts to damage the roof material, and over time can erode it, causing the roof to leak.

So a sturdier, more durable roofing material doesn’t require as much slope as a lesser material because it is less likely to corrode or leak, even if rainwater pools for more than 48 hours.

If you’re using a less durable material, you would be better off creating a steeper roof slope so that the rainwater will more easily drain from it.

Another vital factor to consider here is the roof membrane. A roof has two parts, the base and the membrane. A membrane is a coat on top of the plywood. Membranes are typically made out of very sturdy and waterproof materials such as thermoplastics, synthetic rubbers, and modified bitumen.

These membranes create a watertight surface on the roof to prevent water from seeping into the base material and causing leaks or damage to the roof base. Through a roof membrane, you can significantly enhance the life of your roof, as well as create a high appeal through custom membranes or vinyl floorings if you want to create a roof garden or recreational area.

Drainage Systems

While selecting the roof slope, you must also consider which type of drainage system you are going to be installing in your roof. Different drainage systems have different requirements, both before installation and after regarding maintenance.

If you’re selecting a more efficient drainage system, building a steeply sloping roof wouldn’t be necessary. Since most of the water will drain on its own, you won’t need to depend on the slope to remove all the water from the roof for you.

But if you’re deciding not to spend too much on drainage systems or maintaining them, you would be better off having a steeper roof slope to remove all the water through the help of gravity.

Types Of Drainage Systems For Roofs

A sound drainage system is just as crucial in accommodating proper rainwater drainage from your roof as a sloped roof. So while you’re choosing how much of a slope you’re going to keep, it’s good that you decide what kind of drainage system you want on your roof.

Choosing the right drainage system can significantly impact your roof’s life, as well as the value of your building. Most of the factors which affect your choice of a drainage system are the same as those mentioned above for roof slopes.

Here are some standard drainage systems used throughout the world:


A gutter is a long, open water collection pipe installed at the edge of the roof. Gutters are the most common type of drainage systems used throughout the world and can easily handle large amounts of rainwater.

However, the downside to gutters is that they’re high maintenance and not very long-lasting. Debris from rainwater can quickly get stuck in those gutters, which reduces their efficiency. Gutters also tend to crack and break, causing them to become completely ineffective.

Inner Drains

Inner drains are used mostly in industrial and commercial buildings with large roofs. They collect water from holes in the roof and lead the water through a long chain of pipes built into the roof. They’re very effective at draining large quantities of water, require less maintenance, effectively filter out debris, and aren’t affected by weather conditions.

However, inner drains tend to be expensive to install because they are built into the roof during construction. Also, if some debris does get stuck in the pipes of inner drains, only a technician can unblock it. You also need to install enough internal drains if you want to drain out all the water from heavy rainfalls effectively.


Scuppers are perhaps the most affordable yet effective draining methods of all. A scupper is just an opening in the roof connected to a small metal pipe that allows rainwater to run through. They’re low cost and easy to install, don’t require frequent or extensive maintenance, and works just as well as other drainage methods.

The only downside to scuppers is that when connected to another drainage system like gutters, their efficiency can be affected if the gutter becomes clogged or breaks.


How do you stop a roof from leaking inside?

To stop leakage, find the spot where water is seeping inside. Next, cover that patch of roofing with a shingle, plywood, or tar entirely over the patch and its adjoining areas.

How long does a flat roof last?

A typical flat roof lasts for around 25 years if properly installed before needing repair or complete reroofing.

How does rainwater drain out from a flat roof?

A flat roof slopes towards the installed drains, which carry rainwater to the ground.

What is the best roofing material for flat roofs?

Some suitable materials for the base of a roof include asphalt, metal, and modified bitumen roofing. Membrane materials include PVC and synthetic rubbers.

Are flat roofs more likely to leak?

Flat roofs are more prone to leak because, if they lack a slope, they cannot effectively remove all the water. When this water accumulates, it slowly seeps into the roof and causes leaks.

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