Digging a trench for a new sewer line is a big project that requires careful planning and execution. A properly installed sewer line is crucial for ensuring waste water flows away from your home without backups or blockages. Follow this step-by-step guide to learn how to dig and install a sewer line trench correctly.
Gather Tools and Materials
Before starting any digging, assemble the necessary tools and materials:
- Shovels – Need both flat and rounded tip shovels.
- Pickaxe – Helpful for breaking up compacted soil.
- Measuring tape
- String and stakes – Mark the trench location.
- Gloves & safety glasses
- Wheelbarrow
- Sewer pipe – PVC or concrete pipe, sized according to your plan.
- Gravel or crushed rock – For the trench base.
- Landscaping fabric – Lines the trench to prevent soil ingress.
- Sand – Fills space around pipe to support and drain.
- Sewer fittings – Elbows, tees, cleanouts, etc.
- Pipe primer & cement – Joins and seals the pipe.
- Backfill soil – Use leftover soil unless unsuitable.
Plan the Sewer Line Route
Determine where your new sewer line will connect to the existing sewer stub or septic tank inlet. Mark the connection point and map out the most direct route to where the new line will exit your home.
Consider these factors when planning the route:
- Avoid crossing under structures or concrete paths when possible.
- Minimize turns and elbows.
- Maintain proper slope downhill, usually 1/4 inch per foot.
- Keep pipe runs under 100 feet if possible.
- Allow room for cleanouts every 50 feet or after turns.
- Have utility lines marked to avoid conflicts.
Mark the Trench Location
Once you’ve determined the best sewer line route, mark the path above ground. Use stakes and string to trace the trench location across the yard. Mark the starting point, ending point and any changes in direction. The trench should follow the string line.
Dig a starting hole at the connection point to expose the existing sewer stub or inlet. Then mark the trench exit location on your home’s exterior. Use spray paint or flour to mark the final trench route for easy reference while digging.
Dig the Trench
Digging the trench is the most labor intensive phase. Follow these tips for efficient digging:
- Remove sod and topsoil first and stockpile separately. They will be used later for backfill.
- Use a flat shovel for the initial depth, then switch to a rounded tip shovel at deeper depths.
- Use a pickaxe to break up compacted soil.
- Dig a narrow trench about 2 feet wide. This preserves more topsoil for backfill.
- Maintain a constant downward slope in the direction of sewage flow. Use a line level often to check slope.
- Make the trench deep enough to allow 6 inches of gravel base below the pipe. Add more depth to account for the pipe diameter.
- Pile excavated soil alongside the trench, leaving room to work.
Add Gravel and Fabric
Once you’ve dug to the appropriate depth, it’s time to construct the trench base:
- Add a 6 inch layer of gravel in the trench bottom. Compact it thoroughly.
- Line the gravel with landscaping fabric to prevent soil from migrating into the gravel.
- Place more gravel over the fabric up to the planned pipe starting point.
Install the Sewer Piping
With the trench prepped, you’re ready to add the sewer line:
- Begin at the existing sewer connection point.
- Lay pipe lengths from lowest to highest point.
- Maintain a downward slope in the direction of flow.
- Join pipe with proper primer and cement. Don’t skimp on cement – two coats outside, one inside.
- Use elbows or bends for any change in direction. Avoid 90 degree turns.
- Install cleanouts at the end of the pipe run and every 50 feet or after turns.
- Backfill over each section of pipe with a few inches of sand to support the pipe before adding more.
Backfill the Trench
After all sewer piping is laid, tested and connected, finish by backfilling the trench:
- Partially backfill over sand layer around pipe with leftover soil, free of large rocks and debris.
- Compact soil lightly every 6 inches while backfilling. Don’t crush pipe.
- When 6 inches below ground level, add leftover topsoil to avoid settling.
- Mound leftover soil over the trench. It will settle gradually.
- Replace any sod or plants displaced by digging.
- Slope soil away from the house for proper drainage.
Sewer Line Trench FAQs
Digging your own sewer line trench is a big project. Here are answers to some frequently asked questions:
How deep should my sewer line be buried?
Bury sewer lines at least 18 inches deep, preferably 24 inches. Local codes may require a minimum depth. The deeper the better to prevent freezing.
Can I run sewer pipe under a driveway or concrete slab?
It’s best to avoid this if possible due to difficulty accessing for future repairs. Use solid PVC or sleeved pipe if unavoidable.
What slope should the sewer line have?
Maintain a downward slope of at least 1/4 inch per linear foot. A 2% slope is recommended.
How wide does my trench need to be?
For 4-inch sewer pipe, a trench width of 18-24 inches is typical. Size accordingly for larger diameter pipe.
When do I need a cleanout for my sewer line?
Install cleanouts at least every 50 feet as well as after any change in direction or connection point.
Can I use gravel or crushed rock from my yard as trench base?
Only use clean, washed gravel or crushed rock without fines. The angular rock pieces provide better drainage.
How soon can I use my new sewer line?
Allow the PVC cement joints to fully cure for at least 24 hours before using the system. Test for any leaks first.
Conclusion
Installing an underground sewer line is an ambitious DIY project that requires planning, proper materials and methodical execution. If dug and installed correctly, a properly sloped and buried PVC sewer line can provide trouble-free waste water removal from your home for decades. Pay close attention to slope, depth, bedding material and backfill to ensure success. And don’t be afraid to call in a professional if the project scope seems beyond your skill or physical abilities. Taking the time to install your sewer line trench correctly will pay off for years to come.