Executive Summary
- Homeowners can have a dry well tank installed for on-site stormwater management—they can prevent yard flooding, water-fueled foundation damage, and municipal runoff issues.
- These systems work by collecting roof runoff through gutters and pipes, and temporarily storing it in an underground perforated tank that gradually disperses it into the soil after it passes through gravel.
- Before installing a dry well tank system, it is essential to assess soil drainage, calculate the runoff volume, follow local setback rules, and ensure compliance with city permitting and stormwater regulations.
- Dry well tank installation costs throughout Portland can range from $3,000 to $20,000 or more, depending on the system size, material, soil conditions, and local rates for permits and inspections.
What Are Residential Dry Well Tank Systems?
Dry well tank systems are predominantly underground structures designed to collect, retain, and then carefully disperse water that runs off of rooftops or impervious surfaces, such as driveways or patios. So, if your residential property is vulnerable to flooding or foundation damage after rainfall, consider using a dry well for stormwater management.
These systems typically involve a catchment area, like your roof, piping that redirects the water, and a perforated tank submerged in a gravel pit. These components work together to slowly release excess water into the surrounding soil, rather than having it pool on your property, penetrate your home’s foundation, erode land, or overwhelm municipal storm drain networks.
Due to Portland, Oregon’s above-average rainfall, many homeowners could benefit from having a dry well tank installed on their property. However, many steps need to be taken to ensure efficiency and regulatory compliance, including scheduling soil percolation tests and educating yourself on setback requirements.
The guide below covers the most important aspects of residential dry well tank systems, including their core components, key cost considerations, and upkeep requirements.
System Components & How They Work
Most dry well systems in Portland, Oregon, consist of the following four components. However, some systems may include additional components, such as sediment filters, which can help prevent debris from entering the tank and well.
Catchment & Conveyance
Effective stormwater management begins at the point of contact with your property: your rooftop. Gutters and downspouts are used to collect rain from your roof. Then, the water is directed into distribution pipes that slope into the underground system.
It is essential to note that dry wells are specifically designed to handle stormwater, not wastewater, such as water drained from sinks or toilets, which makes dry wells different from septic tanks.
Perforated Tank
After being transported underground, the excess water is directed to a temporary, perforated holding chamber, where it will gradually filter out through the perforations. The size of the tank should be determined by the amount of runoff the property generates.
The perforated tank is typically made of plastic or concrete, with the former being more lightweight and easier to install, and the latter being more durable but more expensive.
Gravel Pit
In most cases, the perforated tank mentioned above is submerged in a stone-filled pit that helps slow the rate at which the water filters into the surrounding soil, minimizing the risk of oversaturation.
Overflow Lines
If the dry well is not large enough to handle extreme weather conditions, the tank may become overwhelmed. To prevent flooding, the system may include overflow lines that redirect excess water away from your home.
Design & Site Evaluation
Ensuring your tank is the correct size and installed in the most suitable location is crucial for effective and compliant stormwater management. Therefore, before installation, the following assessments and regulations must be prioritized.
Soil Percolation Test
A percolation test measures the rate at which the soil absorbs water at a specific site. To perform it, a professional will dig a small hole and fill it with water before timing the drainage rate. The results will help determine the necessary size and depth of a dry well system.
Generally, sandy soils are best suited for installing dry wells. Clay-heavy or compacted soils may still be suitable, but dry wells should be deeper or larger to accommodate the slower absorption rate. Systems with an insufficient depth or size may be unsuitable without certain amendments.
Runoff Volume Calculation
The size of your roof and any nearby impervious surfaces will play a significant role in the amount of water fed to the dry well system. To ensure the size and depth of your dry well are suitable, an experienced professional should calculate the expected runoff volume.
The professional should measure the square footage of your roof and other impervious surfaces that drain into the system before following the equation below—keep in mind that one inch of rain over 1 square foot equals 0.623 gallons of water.
Runoff Volume (in gallons) = Area of surfaces (sq ft) × Rainfall Depth (inches) × 0.623
Setback Requirements
In Portland, dry well tank systems must comply with specific setback guidelines established by the Bureau of Environmental Services. For one, systems typically need to be placed at least 5 feet from building foundations and 10 feet from property lines. Additionally, the system should be located well away from septic systems or private wells to prevent contamination. Nevertheless, exact distances can vary depending on soil conditions and site topography, making it important to get a site evaluation from a qualified professional for accuracy.
Permit & Code Compliance
In Portland, a city permit is not needed for drywells on single-family residential properties. However, a plumbing permit from the Bureau of Development Services may be needed to connect the dry well to downspouts when running pipes underground.
Depending on your situation, installing a drywell could also require registration with the Oregon Department of Environmental Quality’s Underground Injection Control Program.
Installation Process Overview
After selecting the ideal location for a dry well installation, an environmental professional usually follows the four steps below.
1. Excavation & Base Preparation
The team of trained professionals will dig trenches for the dry well tank. Additionally, the team will compact and level the subgrades to support the tank structures.
2. Tank & Well Assembly
Depending on your system’s design, the professional will either lower prefabricated tanks or install modular components before filling the empty spaces in the dry well pits with crushed stone. Then, they will likely place geotextile fabric to prevent sediment clogging.
3. Pipe & Inlet Connections
When the downspouts, French drains, or other storm drains are connected to the tank inlet by a professional, they must ensure there is a proper pitch to promote sufficient gravity-fed flow.
4. Backfill & Restoration
Since the area has already been backfilled with gravel and native soil, restorative landscaping can be performed, making the area look more aesthetically pleasing and reconfiguring slopes to accommodate water flow if necessary.
Typical Cost Ranges
Although several variables can influence the price of a residential dry well tank system installation, most projects fall into the following three categories, as depicted in the table below.
| System Scope | Typical Price Range |
| Small, basic installation | $2,000 to $6,000 |
| Mid-size setup | $6,000 to $12,000 |
| Complex configuration | $12,000 to $50,000 or more |
The estimated pricing in the table above accounts for materials, labor, excavation, and permits. However, restoration and landscaping are often extra.
Key Cost Drivers
Some examples of factors that can impact the cost of a dry well are:
- Soil conditions – Poor drainage requires a greater excavation depth and increased gravel requirements, especially when the system is installed in clay or compacted soils.
- Tank material and size – Plastic modular tanks are widely considered more affordable and easier to install; however, concrete tanks last longer and resist shifting in exchange for their higher price.
- Excavation difficulty – Properties with sloped terrain, rock layers, invasive tree roots, or limited access to the system can significantly raise labor and machinery costs.
- Permit & inspection fees – Depending on your site’s location, you may need to pay permit fees that typically range from $100 to $ 1,000 or more. Some areas also require soil engineering reports or post-installation inspections.
Maintenance & Longevity
Although the systems are relatively undemanding, staying on top of your dry well tank’s maintenance needs can increase its longevity. With regular upkeep, a dry well tank system can last 20 to 30 years or more, especially if the system is made from concrete.
Ensure you prioritize the following dry well services to maximize the benefits of your residential system.
Annual Inspection
Some examples of things that a trained dry well professional should check yearly are:
- Inlet filters and screens to remove leaves and debris
- Overflow pipes to look for potential blockages
- Signs of erosion and the slope of the ground around the system outlets
Sediment Flushing
Since sediment can progressively build up in the tank, it typically requires pumping every 3–5 years. When flushing the system, contractors usually use vacuum trucks to remove silt and restore capacity.
Vegetation Control
It is important to keep roots from trees, bamboo, or deep-rooted shrubs away from the tank and well area. If you decide to plant anything in the area, be sure to use shallow-root grasses or native species.
DIY vs. Professional Installation
Although installing a dry well tank system yourself may be possible if you have sandy soil and a small, flat, and accessible site, it is still best to hire a professional to ensure the system’s effectiveness and longevity.
Historically, DIY miscalculations can have led to issues like overflows and foundation intrusion that may require water damage restoration, and noncompliance with local codes. On the other hand, relying on professionals can guarantee accuracy, thanks to their highly specialized training, invaluable experience, and access to top-of-the-line digging equipment.
When to Hire a Pro
Some examples of dry well-related scenarios that would warrant hiring a licensed contractor are:
- Your site features compacted or poorly draining soil
- Your system needs to handle runoff from multiple structures
- Your site is near a water body or septic system
- You want a long-term, warranty-backed solution
Finding Expertise
Some examples of things you should look for in a prospective dry well contractor are:
- Experience with stormwater systems
- Familiarity with local regulations and permitting
- Implementation of excavation equipment
- Strong references and adequate insurance
Choosing the Right System for Your Home
The suitability of a dry well tank setup depends on your lot size, runoff volume, and any plans for future property development. Some examples of things you should focus on when attempting to choose the right system for your home are:
- Matching capacity to runoff – Accurate drainage calculations are key because undersized systems may fail quickly, and oversized setups can be wasteful.
- Balancing cost and durability – Plastic tanks may be ideal for budget-conscious homeowners who want simple systems. However, the more expensive concrete tanks may offer ideal long-term reliability.
- Planning for future growth – Consider sizing your system to handle future runoff from other structures you want to build, such as patios or driveways.
Turn to Alpha Environmental for Your Dry Well Needs
Whether you’re dealing with yard flooding, trying to protect your home’s foundation from water damage, or looking for a sustainable drainage solution, you should consider installing a dry well tank system to help your property handle stormwater more efficiently.
For decades, Alpha Environmental Services has helped property owners in the greater Portland, Oregon, area with stormwater management by installing and maintaining dry wells, bioswales, and bioretention areas. Our well-trained and compassionate team is always ready to help fellow Oregonians explore stormwater solutions that fit their specific needs. We can help you protect your home and the surrounding environment from excess water for years to come.
Contact Alpha Environmental to schedule a site visit and receive customized estimates for any stormwater management services you may need.
