If your driveway crosses a ditch, there is probably a culvert under it, even if you have never noticed it. That pipe quietly carries water from one side of the driveway to the other. When it works, you never think about it. When it fails, water backs up, runs over the top and can wash out your driveway in a single storm. Here is what a driveway culvert does, how to tell if yours is in trouble and what keeps it working.
What does a driveway culvert do?
A culvert is a pipe set under a driveway, road or path where it crosses a ditch or a natural low spot. Water flowing along the ditch passes through the pipe instead of being blocked by the driveway. Without one, the driveway acts like a dam: water either pools on the uphill side or spills across the gravel, carrying rock with it.
Most driveway culverts are either corrugated metal or plastic pipe, with a layer of rock and compacted material over the top so vehicles can drive across.
What are the signs a culvert is failing?
- Water pooling on the uphill side of the driveway after rain instead of flowing through.
- Water running over the driveway during storms.
- A dip, sinkhole or soft spot in the driveway right over the pipe, which can mean the pipe is collapsing or water is washing material away around it.
- Crushed or bent ends from vehicles or equipment.
- Rust holes in older metal pipe, especially along the bottom where water sits.
- A pipe full of sediment so only a small opening is left.
- Erosion at the outlet, where water exiting the pipe is cutting a hole or washing away the ditch.
Why do culverts fail?
Most culvert problems come down to a few causes:
- Debris. Leaves, branches and sediment plug the opening, so water cannot get through.
- Too small a pipe. A pipe that handles normal rain can be overwhelmed by a big winter storm.
- Not enough cover. If there is too little material over the pipe, vehicle weight can crush it.
- Age. Older metal pipes rust out over time.
- Poor installation. A pipe that is not set on the right grade, or that is not bedded and backfilled properly, can sag, shift or let water wash around the outside.
How do you maintain a driveway culvert?
- Check both ends in early fall and clear out leaves, branches and sediment before the rain starts.
- Look again after big storms. One heavy rain can carry in enough debris to block the opening.
- Keep the ditch clean on both sides so water reaches the pipe and has somewhere to go when it comes out.
- Protect the outlet. If water is cutting into the ditch where it exits the pipe, a layer of larger rock at the outlet helps slow it down and stop erosion.
- Watch the driveway surface above the pipe for dips or soft spots, which are early signs of trouble underneath.
How big should a driveway culvert be?
Culvert size depends on how much water reaches it, which depends on the size of the area draining toward it, the slope and the ground cover. A pipe for a small roadside ditch is very different from one carrying a seasonal creek.
Many counties set a minimum culvert size for driveways that connect to a county road, and a new driveway connection often needs a permit from the county road department. If your driveway meets a public road, check with your county before installing or replacing a culvert.
Repair or replace?
| Situation | Usually means |
|---|---|
| Ends are plugged with debris | Clean it out |
| Outlet is eroding | Add rock at the outlet |
| Pipe is partly full of sediment | Clean out, check the grade |
| Small dent at one end | Often repairable |
| Rusted through, crushed or collapsing | Replace |
| Water regularly overtops the driveway | Likely undersized, consider a larger pipe |
Replacing a culvert means digging out the driveway over the old pipe, setting a new pipe on the correct grade with proper bedding, backfilling and compacting in layers, and rebuilding the driveway surface on top so it does not leave a soft spot.
Frequently asked questions
It depends on where the culvert is. A culvert where your driveway connects to a county road often requires a permit from the county road department, and culverts in streams can involve other agencies. Check with your county before starting.
It depends on the material, the soil and the water it carries. Plastic pipe resists rust, while older metal pipes often rust through along the bottom over time. Keeping it clean and well covered helps any culvert last longer.
A sinkhole or dip over the pipe usually means the pipe is damaged or water is flowing around the outside of it and carrying material away. It is worth checking soon, since it tends to get worse quickly.
Enough compacted material over the top to spread vehicle weight so the pipe is not crushed. The right amount depends on the pipe type and size and on the traffic it will carry.
For more on keeping water off your driveway, see our fall driveway checklist and our overview of drainage solutions.