Let’s Talk About Pipe (Yay!)
There may come a time when you need a stormwater or drainage solution that requires sub-surface drainage i.e. pipes. Two kinds of pipe are commonly used in small scale drainage projects. Corrugated plastic pipe and rigid PVC pipe are comparable in material cost but differ significantly in installation cost and reliability.
Corrugated plastic pipe is available at any home improvement store. The ease of handling and installation has made it a favorite for the average landscaper. It is so ubiquitous that its use goes unquestioned, having achieved the status of ‘conventional wisdom’.
The advantage of corrugated plastic pipe (for the installer) is that it is flexible.
- The flexibility allows the pipe to be packaged and sold in long coils, making it economical to handle and ship.
- The continuous length requires fewer couplings and thus less labor to install.
- Less care and skill are required to excavate trenches when compared to rigid pipe, which requires trenches in straight lines with turns at specific angles to match elbow joints in the pipe run, thus less, and/or cheaper, labor is required to install it.
The disadvantage of corrugated plastic pipe (for the owner) is that it is flexible.
- The hose-like flexibility makes it difficult to lay the pipe flat along a trench to achieve a continuous slope. The resulting undulations in the profile create low spots along the pipe run.
- The corrugations that afford flexibility also function as speed bumps, reducing the velocity of the water flow and its corresponding capacity to flush sediment and debris out of the pipe.
- The cumulative effect is that sediment and debris will readily settle in the pipe, reducing its capacity over time.
- There is no way to clean out corrugated plastic drain pipe. The drain snake that is commonly used by plumbing services features a whirling blade that will readily tear through the soft plastic walls of the pipe.
If I had a dime for every linear foot of crushed, muck filled, corrugated plastic pipe I pulled out of the ground… OK, so it’s an old cliché. The ‘dime’ part is also dated, since it costs allot more than that, given the cost of the original installation, the replacement cost after it fails and the cost of the subsequent damage that may be encountered because of a failed drainage system.
Rigid PVC Sewer and Drain Pipe (SDR) on the other hand, while requiring more planning and time to install, can provide many more years of reliable service. Not to be confused with DWV (Drain, Waste and Vent) or Schedule 40 PVC, Sewer and Drain pipe has a thinner wall and thus requires different sized fittings (a tip for those DIYers). Because it is rigid, it is easier to ensure an accurate and continuous slope during installation. The smooth pipe walls have less friction loss than corrugated pipe. The resulting higher flow velocity facilitates self-cleaning and efficiently transports sediment and debris out of the pipe. Finally, if there ever is a need to clear the pipe, it can be snaked using conventional equipment.
Cost wise, the two products are comparable. However, rigid PVC must be cut and fitted with an elbow using PVC cement at each turn in the pipe run. Elbows are available in 22.5, 45 and 90 degree angles, so advance planning and layout are required to ensure that the required fittings are in hand and that the layout will fit around existing site features. It requires more time and effort to install rigid PVC, which is reflected in the project cost.
When comparing bids from installers, the bid that proposes rigid PVC pipe will most likely be higher than a bid proposing corrugated plastic pipe. However, the savings that are realized from choosing the corrugated plastic pipe may be outweighed by the shorter service life of the installation and a higher life-cycle cost. Life-cycle cost is calculated by taking the initial cost, adding anticipated maintenance costs, and dividing it by the number of years before the installation must be replaced. For example, a product that initially costs $3000, requires $100 in maintenance every other year, and lasts 10 years has a life-cycle cost of $350/year. A product that initially costs $4000, requires $100 in maintenance every other year, and lasts 20 years has a life-cycle cost of $250/year.
If a problem is significant enough to warrant the expense of installing a drainage system, then it is significant enough to invest in a solution that will remain reliable for as long as possible.