The best pipe for underground downspout drainage is 4-inch solid PVC (SDR-35). It provides smooth, uninhibited water flow, is highly resistant to root intrusion, and—unlike black corrugated tubing—can handle the weight of heavy topsoil.
Top Underground Downspout Drain Pipe Options:
Dig-up test results in the U.S. and around the world indicate that PVC pipe can be expected to provide reliable service in excess of 100 years. The average water main is failing at 4 7 years. Corrosion is the major cause.
Schedule 40 PVC Pipe (or SDR 35/26)
This is your main line for carrying water away from your foundation. Schedule 40 is thick-walled and durable—great for under driveways or high-traffic areas.
So when this downspout fills up during an extreme downpour (and they do and they will) it no longer can vent. It's going to be just full of water. So we want to move this water as fast as we possibly can. By venting the downspout, you're going to pick up a lot more speed.
Buried downspouts effectively move roof runoff away from your foundation, but they frequently suffer from hidden clogs (caused by roof shingle gravel, leaves, or tree roots), freezing in winter, and standing water. These issues can cause gutters to overflow or force water directly into your basement or crawl space.
Air admittance valve vent. An air admittance valve (AAV) is an alternative option to fitting a vent in a soil pipe – a soil pipe vent if you will.
Burying downspouts improves the aesthetics of your home by hiding the pipes, enhances safety by reducing tripping hazards, and provides effective drainage that prevents water from pooling around your foundation, thus reducing the risk of basement flooding and soil erosion.
For most underground applications, Schedule 40 PVC is considered the best for its strength, durability, and resistance to corrosion, with a lifespan often exceeding 100 years. It is ideal for heavy-duty, high-traffic, or pressurized projects. For non-pressure, gravity-fed drainage, SDR 35 is a common, cost-effective choice, while Schedule 80 is best for high-pressure industrial jobs.
For direct underground main water lines, High-Density Polyethylene (HDPE) is the industry standard and the best choice. It is incredibly tough, handles shifting soil well, and can be joined seamlessly. PEX is better suited for indoor plumbing, short repairs, or tight spaces, but it risks splitting or being punctured by rocks if buried directly underground.
Mistakes During Underground Installation
One of the most common mistakes is not following the required burial depth. According to the NEC (National Electrical Code), the minimum burial depth for rigid nonmetallic conduit (PVC) in most residential applications is 18 inches.
The "135-degree rule" in plumbing states that the total (aggregate) horizontal change in direction for a drainage pipe—specifically trap arms—cannot exceed 135 degrees between the P-trap and the vent. Exceeding 135 degrees (such as using three 45-degree bends or a 90 and two 45s) requires a cleanout because sharp turns hinder flow and cause clogs.
Yes, you can drive over buried PVC pipe, but it depends heavily on the burial depth, the weight of the vehicle, and the type of PVC. Properly buried, sturdy PVC can withstand heavy traffic, but shallow or uncompacted installations can be easily crushed.
Bury your gutter drainage pipes at least 12-24 inches deep, with northern regions requiring depths of up to 4 feet to prevent freeze damage. Your local frost line should determine minimum depth, while soil composition and traffic patterns may necessitate deeper installation.
While 4-inch corrugated is the industry standard, there are several reasons why a 3-inch downspout pipe could be the right choice for particular drainage application.
To recap, PVC is best for rigid, low-cost installations, such as residential/commercial plumbing, drainage, and irrigation. HDPE is best for high-durability and flexible needs in industrial applications, as it can handle higher pressure, higher temperatures, and greater impacts.
Comparison between PEX pipes and PVC pipes for water supply systems. 1. Durability: Both PEX pipes and PVC pipes are known for their durability and long-lasting nature. However, PEX pipes tend to have better resistance to extreme temperatures and thermal expansion/contraction, making them less prone to leaks or bursts.
Standard schedule 40 PVC will work fine, the size depends on how much water you expect to pass through the pipe over a period of time, but commonly it's around 4" for buried underground drains. II think the 4" size has a lot to do with resisting getting clogged up.
Yes, Schedule 80 PVC is stronger than Schedule 40 PVC. Schedule 80 is designed with a thicker wall, making it highly resistant to physical impact, buckling from external pressure, and higher internal burst pressure.
Proper Drainage Prevents Erosion Around Stone or Landscaping
In most situations, the best drainage downspouts are made from PCV or corrugated pipe for durability and ease of installation.
Buried downspouts effectively move roof runoff away from your foundation, but they frequently suffer from hidden clogs (caused by roof shingle gravel, leaves, or tree roots), freezing in winter, and standing water. These issues can cause gutters to overflow or force water directly into your basement or crawl space.
Every time it rains, more water flows through the pipe and the roots grow larger until the underground pipe becomes clogged and potentially bursts. If an underground drain becomes bound by roots, it's nearly impossible to fix.
Leaks and Fittings Issues
PEX plumbing uses fittings and connectors that are typically made of plastic or metal. Over time, these fittings can loosen, leading to leaks. Problems can arise with crimp rings, as improper use of a crimping tool can weaken the joints.
If a soil pipe isn't vented, gases can build up and create foul odours. This can also cause poor drainage and noises in your plumbing system.
Depending on your specific application, PVC alternatives range from flexible water supply tubing to rigid, heavy-duty metals. The best substitute depends on your needs: