# Yard Drain Slope: How to Get It Right (Your Questions Answered!)

A beautifully landscaped entrance improves your home’s appeal in more than one way. That means preventing the yard from turning into a muddy bog every time it rains. To flood-proof that dreamy garden in your backyard, you need proper drainage.

We have already discussed how a backyard drainage system helps to keep your yard dry and green. And the most vital — note the stress on the word “vital” — aspect of drainage system design is calculating the slope to get it just right.

Imagine that the drain is installed but water is still pooling in the yard and drowning your pet plants. Frustrating, to say the least.

Then it’s time to take a deep breath and delve deeper into the drain’s design. Chances are, the issue is with the yard drainage slope.

## Why Drain Slope Is Important: Here’s Why You Should Get It Right

Consider it this way: gravity can only act as the cleaning agent when the drain slope or drainage fall ratio is properly calculated. Even if the slope is incorrect in a small section of the drain system, there can be problems.

So, what happens if the slope is not correctly calculated?

It is obvious that providing less slope than needed will prevent the water from flowing freely, leading to overflows. If the water is stagnant, it will lead to bacterial growth and foul odours. Plus, the accumulated water can cause flooding or damage the infrastructure.

But does that make maximising the drain slope a good idea?

Hardly!

A surplus slope on a drain pipe will lead to the water flowing too fast. **This can cause gurgling of the pipes and the solid waste will be left behind.** In time this debris will block or clog the drain pipes. In other words, you will need to clean the drains frequently.

At times, the slope needs to be increased due to the lack of space — that’s totally understandable. In such scenarios, a licensed plumber can help you determine the maximum allowable slope.

For drainage pipes, a uniform slope or gradient is required across the entire length. If the drain can travel in a straight line, great.

Not always, though.

There might be trees or structures in the yard that you need to avoid. With bends or turns in the drain route, things get more complicated. Then the slope of each segment should be separately calculated.

If you are a maths whiz, great. If not, good luck with that. Common question — can I DIY slope grading?

You can, if you are a DIY champ and familiar with measuring ground levels and calculating slopes. Add to that the activities like digging the trench and laying the pipes.

If this is not something you are used to, consult a licensed plumber. Keep in mind that an error in the slope is not just about the drain clogging. It can result in backflow, damaging your home’s foundations.

Not to forget, redirecting the excess water through correctly sloped yard drainage will also help prevent yard soil erosion. Besides, experienced plumbers will be familiar with the codes and standards that you need to follow in your area.

Sure, a DIY approach will reduce costs. It is up to you to decide if that is worth the risk.

## 3-Step Slope Calculation for Yard Drainage

Ever seen a piping isometric drawing? There, right-angled triangles adjacent to a pipe indicate the slope. **The numbers on the triangle mention the rate of slope change.**

This rate of change of slope is the “Rise over Run”. A ratio of 1:100 (1 in 100) means that for every 100 units of horizontal pipe run, it will drop vertically by one unit. The unit can be metres or millimetres. Likewise, it can be 1:50 or 1:200.

For instance, a 1:150 slope means that for every 150 metres run along the ground, the vertical height decreases by 1 metre. Take care not to mix up the units while calculating the slope.

Other than a ratio, this can also be expressed in percentage — a 1% or 2% slope. So technically, it is not calculated as a 10-degree slope yard or a 20-degree slope yard.

Note that pipes of smaller diameter require more slope. The reason is that smaller pipes have less internal surface area and are more prone to clogging. In contrast, larger pipes can carry higher water volumes readily and need less slope.

The details about the design of surface drainage systems can be found in the standard AS/NZS 3500.3:2021.

Here, we’ll outline the basic methods of slope calculation.

### 1. Study Your Yard

The primary idea is to create a slope that carries the water away from the house. The first step is to check if the yard has a natural slope. Aligning your drain with that slope is the best idea.

For a yard with no slope, the trench pit should be dug to match the required drain slope. If the yard is not uniform or has a negative slope, **levelling it by adding soil to some areas** might be needed.

### 2. Plan and Measure

The next step is to measure the distance that the water needs to travel. Ideally, you should keep the drain length as small as possible. **Long pipelines will increase the chances of clogging.**

The route of the drain will also tell you how much pipe will be needed.

### 3. Determine the Slope

To calculate the slope, **divide the vertical drop of the pipe (rise length) by the horizontal run of the pipe (run length)**. For example, if the run length is 25 metres and the rise length is 0.5 metres, the gradient is 0.5/25= 0.02 or 2%.

Considering the pipe diameter used and the flow rate through the pipe, the plumber will suggest the ideal slope required. The pipe diameter will depend on the volume of water flowing through the drain. This in turn depends on factors like the annual rainfall intensity and the surface area of your roof.

The slope will vary according to the site conditions. It can be between one and three per cent, or steeper. However, some consider that the minimum slope for yard drainage should be two per cent.

Note that the yard level may vary along the drain route. Now that the slope is finalized, compare it with the yard levels. This will determine how much you need to dig down to maintain it uniform throughout the drain route.

Generally, after the drainage system is installed, a pipe slope test is done to check if the slope is correct.

## Slope From Land Drainage: FAQs

### How much slope do you need for yard drainage?

The slope needed for yard drainage will depend on the yard conditions. The site measurements will give you the exact number. Generally, it can vary between one and three per cent.

### What is a good drain slope?

A good drainage slope ensures optimum drainage of your yard. It should be neither too low nor too steep.

### How to calculate drain slope?

To calculate the drain slope, determine the horizontal length of the drain (x) and the elevation difference between the start and end points (y). Dividing y by x will give you the slope. Multiply the result by 100 to get the slope per cent.