Calculating the Ideal Sewer/Drain Pipe Slope Easily

How Much Slope Should a Drain Pipe Have?

Unlike water which is delivered to your drains under pressure, waste has to flow out of your house via gravity. The only way to ensure that happens without drainage problems is by ensuring that the drain/sewer line has a proper slope.

Before going further, let me start by saying that most people confuse the words, drain pipe, drain line and main line. Basically a drain line is a line which carries waste from a fixture (toilet, sink, tub etc.); while a main line is the sewer line, which carries waste from your house to the city’s sewer lines or septic tank.

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The proper slope on a drain/sewer line is ¼ inch per foot of pipe. This means that for every 1 foot the pipe travels horizontally, it should dip by ¼ inch towards the city’s sewer lines or septic tank.

If the drain line is too flat, it will fail to generate the required force to push out the waste. As a result the drain line will clog frequently and you will experience sewage backups especially in your basement.

You would think that a steeper slope is the best thing that could happen to a drain line but it turns out that it is actually as bad as lack of slope at all. Too much slope on a drain line means that water will flow out faster than solids, which accumulates resulting in clogs.

In simpler terms, a drain/sewer line should have a slope of 2%. This means that if a drain line is 100 feet long it should have 2 feet vertical drop from start to finish. In other words, this translates into ¼ inch slope for every 1 foot of pipe.

The calculations are easy if you are installing the drain pipe in a straight line. They are however a little challenging if the line has to take corners through your yard.

Drain pipe slope is not dependent on the size (diameter) of the pipe. A 4-inch drain pipe will need the same slope as a 6-inch drainpipe.

It can be very hard to get the exact slope you want on a drain pipe. The minimum slope you should have on your drain pipe however should be 1%.

A 1% slope on your drain line means that for every 1 foot that the pipe travels horizontally it drops vertically by 1/8 inch. In other words, your drain line drops down by 1 foot for every 100 feet of pipe.

On the other hand, the maximum slope you should have on your drain pipe is 25%.  This translates to 3 inches vertical drop for every 1 foot of pipe.

How to Calculate a Drain Pipe Slope

The following are the steps to take when calculating the ideal slope on your sewer/drain line:

1. Plan the Drain Route

A sewer line drains into the city sewer lines near the street or a septic tank if you are on a septic system. From where the main drain line exits your house, you need to plan the most ideal path for the sewer line.

It is best if the sewer line travels in a straight line. That makes your calculations easy and also reduces the likelihood of clogs.

If there are a lot of obstacles in your yards, the sewer line will need to curve around to avoid them. This is why planning is important.

You will need to figure out the sewer line route with the least resistance. By least resistance I mean the route with few turns or not turns at all.

It goes without saying that turns reduce the speed (flow rate) of waste as it flows through the line. Most sewer lines also tend to clog and leak around corners.

Another thing you have to do is to try as much as much as possible to make sure that the sewer line takes the shortest route possible to the city sewer lines or septic tank. The longer the sewer line is the more likely it is to clog.

2. Take Measurements

Taking measurements goes hand in hand with planning. After planning the sewer line route, use stakes and strings to physically mark the route in your yard then measure the entire length from start to finish.

The measurements you have taken above will help you calculate the slope you need on your sewer line but not the size of drain pipe needed.

You can only know the proper size of drain pipe required later after calculating the slope. Since the sewer line will be sloping, the required pipe will be longer than the actual route measured above.

3. Calculate the Drain Slope

After marking out the preferred drainage route and measuring it out, it is time to calculate the slope required. Let us see how you do that.

If after measuring the drain route you found 20 feet and you need to slope the drain pipe by ¼ inch for every foot of pipe, you will need to multiply 0.25 inches by 20 feet.

In this case, the required slope will be 5 inches. This means that the drainpipe will need to slope by 5 inches from where it exits your house to where it is connected to the city’s sewer lines or septic tank.

Example 2: Assuming you got the 20 feet but you need to slope the drainpipe by ½ inch for every foot of pipe, multiply 0.5 inches by 20 feet. You will find that you need to slope your drain pipe by 10 inches from start to finish.

In longer drain pipes which have are more susceptible to frequent clogs, you can increase the slope to the maximum required one.

If say you have a drain pipe route of 100 feet, you can opt to slope the pipe by 3 inches for every foot of pipe. In this case, multiplying 3 inches by 100 feet will give you a slope of 300 inches (25 feet) for the entire 100 feet of drainpipe.

Calculating the slope of a straight drainpipe is easy. If however the drainpipe will have to take corners through your yard, you will need to calculate the slope of each section separately.

For instance, if the drain pipe goes through 2 corners, those are 3 sections of straight line. Measure the length of each and calculate its slope independently before going to the next one.

Due to the reduced flow rates caused by corners, I would recommend a steeper slope if your drainpipe has to go through a corner. A minimum of ½ inch for every foot of pipe would be ideal.

4. Dig and Shop

Now that you know what slope and size of pipe you need, the only thing remaining for you to do is dig and lay the pipes. Before that though, you will need to buy the pipes.

In the past, clay and cast iron pipes were used by now PVC drain pipes are the most preferred, and for good reasons. They are cheap, easy to install and most importantly are not attacked by tree roots. The biggest headache for old sewer lines is clogs as a result of tree roots.

As you shop for the pipes, be sure to buy pipe fittings and pipe hangers as well. Your plumber should however advise you accordingly.

Wrap Up

Installing or replacing a sewer line is not an easy job although it may look like it is. In my opinion it is best left to professionals who are well conversant with plumbing codes and best industry practices.

In plumbing, many things can go wrong and I don’t think there is anything worse than a sewage backup. It is even worse when you realize that the sewer line was not properly installed and you have to dig it out and install it again properly.

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