how to dig a trench for drainage

How to Dig a Trench for Drainage in 10 Steps

In Lawn & Garden by Jamie

Water is essential for life, and maintaining your lovely backyard and garden. It can also prove disastrous, flooding pavement, invading basements, and drowning plants. But it doesn’t have to be that way. If you have a problem with rainwater flooding, a trench drain can help.

What Is A Trench Drain?

A trench drain is essentially a gutter, a trench with a drain, that collects rainwater, slows it down, and diverts it to another area to prevent flooding and erosion. Depending on the amount of water it needs to divert, you can build a heavy duty trench drain with pipes and a large dry well, or as simple as a gravel-filled trench.

Trench drains are often confused with French drains, but a French drain deals with groundwater while a trench drain deals with heavy rain. A French drain is built underground, and it’s useful when groundwater seeps through foundations and into your basement.

A swale is another method for handling and redirecting heavy rains using broad and shallow ditches covered in vegetation. You can use them to direct water away from or toward certain areas. Got a garden patch that always dries out too quickly? A swale could help. Unlike the trench drain, which channels and increases the speed of the water, a swale spreads out the water so it slows down and soaks into the soil without flooding.

Small-scale farmers (especially those in permaculture) will use swales to redirect water if their fields don’t get enough water or dry too quickly. For homeowners, the beauty of the swale is that it blends into your landscape. It’s just another beautiful part of your garden. If your area doesn’t get a lot of rain, you can also use swales to conserve water.

For more options, check out these 9 ways to divert water in your yard.

3 Options For Trench Drains

Gravel

gravel trench

If water drains quickly through the soil and/or you need to handle a low amount of rainwater, then you can likely get away with just using a gravel trench. The gravel provides a space for the influx of water, then lets it slowly soak into the soil. Plant moisture-loving plants around it to take advantage.

Drain + Pipe

The next step up is to install a pipe and a drain. The water enters through the drain and the water rushes through the pipe to the exit. A perforated pipe will allow water to soak into the surrounding soil. The drain must be setat the lowest collection point or hooked up to a rain gutter.

Dry Well

Like its name suggests, a dry well is essentially a well that is filled up by rainwater. This barrel has holes in it that allow the collected water to slowly soak into the soil while avoiding flooding (and flooding storm sewers). A dry well can be handy in lowering water bills!

You can get dry wells in different sizes and which size you’ll want depends on the amount of water you could possibly get. (Think of what the heaviest possible rain is for your area, not the most common amount.) An overflow pipe is added to take the excess water over to a storm sewer or other spot. (So, nope, you don’t get out of digging the trench. But you do get to dig a big hole too!)

You can place the dry well at a low spot where water congregates, or hook it up to a drainpipe to bring water from one part of your yard to another where the water will do more good.


10 Steps On How to Install A Trench Drain

The following instructions will guide you in building a trench drain with a dry well, but offer divergent instructions if you want to build the two other types of trench drains.

If your yard is already soaked from a recent flood, check out these 10 ways to dry it up.

What Do You Need to Dig a Trench?

  • Spade shovel (or specialty tool listed in step 3)
  • Wheelbarrow
  • Level
  • Tape measure
  • Hammer
  • Drill
  • Reciprocating saw

1. Plan before you dig

A trench drain is a major change to the landscape, and if not thought through, can end up with disastrous consequences. It pays to plan it out thoroughly, or even hire a landscaper to consult with you even if you still DIY it yourself.

A few questions you need answered:

  • Are there any electrical wires, natural gas lines, or other underground obstructions? Call your local utilities so they can tell you where you shouldn’t dig.
  • Where does the surface water collect in your yard?
  • Where will the surface water go, and can the area handle the water? (Your neighbor will be mighty pissed if you flood their vegetable patch.)

It’s best you can direct the water to flow downhill and towards a storm sewer, waterway, or a rain garden.

2. Determine the slope

Water needs a slope of at 1 inch rise for every 10 feet of length run to flow properly — and to flow where you want it to flow. Check out our handy instructions for how to determine slope. Fortunately, that’s an angle of 0.5 degrees or 0.8% slope, which isn’t very much. If your lawn is totally flat, then you can easily create this slope by just digging the trench at an angle.

3. Dig the trench

shovel and dig trench

Now for the fun part — digging the trench. It needs to be wide enough that it can fit a good-sized drainage pipe and gravel, as well as incorporating the slope. The heavier the rain, the bigger the pipe. The trench should be at least 18 inches deep to accommodate the gravel base and pipe.

If you’re including a dry well, dig a hole bigger than the dry well where the water congregates.

While all you need is a shovel and a will to work, there are a few tools that can help make trench digging easier:

  • Trench shovel or drain spade. These shovels are designed with a thinner head that’s about the same size as most trenches.
  • A pick mattock. Just like miners used! The pick end can break up hard soil while the mattock end can chop through roots.
  • Like the mattock, a sharp hoe can break through roots and move a lot of soil.
  • Walk-behind trencher. If the soil is compacted and impossible to dig through, then you may want to step up to a walk-behind trencher. While a walk-behind trencher is eye-wateringly pricey, you can save a ton of money by renting one of these from a hardware store.

4. Lay landscaping fabric in the trench

Landscaping fabric will keep the gravel base contained and prevent it from mixing into the surrounding soil, and will keep roots from infiltrating.

5. Pour 3 inches of gravel into the trench

The gravel will help support the pipe and improve drainage. If you’re using a perforated pipe, the water will drain out of the pipe, into the gravel, then slowly soak into the soil. If you’re building a trench under cement or by a foundation, this isn’t that ideal.

For a simple drain that doesn’t need to move a ton of water or for soil that allows water to drain quickly, gravel is all you need.

6. Line the hole with landscape fabric

Line the hole with landscape fabric. The landscape fabric will keep soil from clogging up the dry well.

7. Lay the pipe leading to and from the dry well

If you’re using the dry well to move water from a downspout, then attach the downspout adapter and elbow to the downspout. Fit the pipes together and lay the pipe into the trench.

Check that the slope of the pipes is steep enough – ¼-inch down for every foot of run. Adjust the slope by removing gravel or soil. Trim the end of the pipe if it’s too long.

Once they fit well, glue the pieces together with PVC cement and trim the end of the pipe.

If you use perforated pipes, cover the tube with a fabric sock filter. The fabric sock filter will keep out small particles so the pipe won’t clog.

8. Assemble and install the dry well

Remove the perforations from the dry well and screw the sides of the dry well together. Shovel the landscape fabric-lined hole with 6 inches of gravel. Set the dry well in the hole and slide the pipe into the large hole, then centre the dry well. Fill the hole up with gravel until you reach the top of the dry well. Cover with the lid.

9. Cover and bury the dry well

Cover the top of the hole with landscape fabric (water will go through, but silt will not), then bury the whole thing (dry well and pipes) with soil and sod.

10. Attach a cover to the end of the tube

If you used an overflow pipe leading to a storm drain, then the end of the pipe can’t be buried. Lay down a bed of gravel under the end of the tube and where the water will go to prevent erosion. Then cover the end of the pipe with a drain cover to keep things (silt, insects, small creatures) from getting inside.

That’s it! You’ve completed your very own trench drain.


Digging a Trench FAQs

How much will it cost to dig a trench?

According to Home Advisor, digging a trench costs about $8 per linear foot, including equipment, labour, and cleanup. The price may vary depending on your location and the materials you use in your trench. If it’s a drainage trench, you can save money by digging it yourself, although hiring a professional can save you from costly mistakes. (For other uses, like utilities, whoever is installing the system will include trench digging.)

How Deep Should a Trench Drain Be?

A trench drain should be dug at least 18 inches deep, but the depth will vary if you need to dig at an angle to run water over a flat lawn.

What size pipe for yard drainage?

3-inch, 4-inch, and 5-inch pipes are suited for yard drainage. The bigger the pipe, the less prone it is to being clogged by rocks and organic material. 4-inch pipes are most used.

What type of pipe should I use for yard drainage?

PVC solid pipes are the best option for yard drainage, as it’s great at draining, less likely to clog and easier to clog when it does, more resistant to root intrusion, and more durable and long-lasting. Solid pipes are more challenging to work with and more expensive than corrugated pipe, but solid pipes make up for it in the decades after installation.

However, PVC just isn’t very eco-friendly or sustainable. [Manufacturers like Aquatherm are designing polypropylene pipes that are more easily recycled.