Diverting rainwater from a roof is an important task for any homeowner. Properly managing rainwater runoff can prevent water damage to your home’s foundation, basement, and landscaping. It can also provide a supplemental water source for irrigating lawns and gardens. There are several methods for diverting roof rainwater, ranging from simple gutter and downspout systems to more complex rainwater harvesting setups. Let’s look at the most common options for managing roof rainwater drainage and diversion.
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Using Gutters and Downspouts
The most basic way to divert rainwater from your roof is by installing a gutter and downspout system. Gutters are open troughs installed along the edge of the roofline. They collect the rainwater running off the roof and channel it into downspouts. Downspouts are vertical pipes that carry the water from the gutters down the side of the building.
Gutters and downspouts divert roof water away from the house foundation and discharge it somewhere on the property where it can soak harmlessly into the ground. This prevents water from pooling around the foundation, seeping into the basement, or eroding the soil. Downspouts should discharge at least 5 feet away from the house.
When installing gutters and downspouts:
- Choose gutters made from galvanized steel, aluminum, copper, or vinyl. Galvanized steel and copper offer the most durability.
- Install gutter hangers spaced every 2 feet along the gutter to provide proper slope and support.
- The gutters should slope slightly towards the downspouts, around 1/4 inch for every 10 feet of gutter.
- Use downspout elbows and extensions to direct water away from the house.
- Make sure gutters, downspouts, and extensions are clear of debris. Clogs can lead to overflow and water damage.
Properly installed and maintained gutters and downspouts are an inexpensive and effective first line of defense against roof rainwater damage.
Rainwater Collection Systems
For a more active rainwater diversion system, consider a rainwater harvesting setup. These systems collect and store roof rainwater for later use in irrigation or other applications.
A typical residential rainwater harvesting system consists of:
- Catchment area – The roof surface that captures rainwater.
- Gutters and downspouts – Channel water from the roof to the storage tank.
- Leaf screens – Filter out debris before water enters the tank.
- Storage tank – Stores the collected rainwater.
- Pump and pipes – Delivers water to its end use (gardens, etc.).
- Overflow outlet – Safely channels excess rainwater runoff away from the tank.
Storage tanks are above or below ground and range from small 55 gallon barrels to large 5,000+ gallon underground cisterns. The tank size determines how much rainwater can be collected and stored. You can start small with just a rain barrel at each downspout and add more capacity later if desired.
Using a rainwater harvesting system for irrigation helps conserve municipal water and saves on utility bills. Collected rainwater contains no chlorine, calcium, or lime which can harm plants when used long term. And because rainwater is soft, it prevents mineral deposits from forming on irrigation equipment.
How Much Rainwater Can You Collect?
The amount of rainwater you can collect depends on:
- Collection area – Larger roofs collect more water.
- Rainfall – Wetter climates produce more rain.
- Storage capacity – Bigger tanks store more water.
A general estimate is that for every 1 inch of rain on a catchment area of 1 square foot, you can collect around 0.6 gallons of water.
So on a 2,000 square foot roof, 1 inch of rain yields around 1,200 gallons of rainwater. Over the course of a year in a region with 50 inches of annual rainfall, that equates to 60,000 gallons of potential rainwater for collection.
This table provides more rainwater collection estimates based on roof size and rainfall:
| Roof Size (sq. ft.) | Annual Rainfall (in) | Annual Rainwater Potential (gal) |
|---|---|---|
| 1,000 | 30 | 18,000 |
| 1,500 | 40 | 36,000 |
| 2,000 | 50 | 60,000 |
| 2,500 | 60 | 90,000 |
Keep in mind that real world collection is always less than the theoretical maximum. Collection efficiency for a well-designed system is typically around 70%.
French Drains and Dry Wells
French drains and dry wells provide another means of diverting roof runoff away from the house foundation. A french drain is a trench lined with gravel or stone that redirects surface water away from a problem area. A dry well is a subsurface storage pit filled with gravel that allows water to slowly infiltrate into the surrounding soil.
For roof runoff management, french drains or dry wells are installed near the roof downspouts. The drains provide an outlet for the downspouts to feed into, moving water away from the house. Water fills the void spaces in the gravel and stone lining and gradually seeps into the soil. This prevents ponding or flooding issues caused by large volumes of roof runoff.
Key tips for installing drains or dry wells:
- Locate the trench or pit at least 10 feet away from the house.
- Line the trench or dry well with landscape fabric to prevent clogging.
- Fill the trench or pit with uniformly-sized gravel. Larger stone goes at the bottom.
- Install a grate over the inlet to filter debris.
- Route downspout extensions or underground drain pipes into the trench or dry well.
Proper drainage design is critical for these systems to work effectively. Have a professional engineer or landscaper design the drainage system if you anticipate handling large volumes of roof runoff.
Controlling Roof Rainwater Onsite
Sometimes it’s not possible (or ideal) to divert roof rainwater completely offsite. In these cases, you can use landscaping features and erosion control measures to manage the runoff within your property.
Examples include:
- Rain gardens – Shallow, dug-out areas planted with water-tolerant plants. Designed to pool small amounts of rainwater runoff.
- Bioswales – Landscape elements that convey rainwater along gently sloped vegetation-lined channels.
- Permeable patios – Allows rainwater to soak through into reservoir base materials.
- Vegetated filter strips – Bands of dense vegetation that causes rainwater runoff to pond and infiltrate.
Proper grading across your property also encourages the spread and infiltration of rainwater. Swales and berms can be used to reroute water away from buildings and impervious surfaces. Simple downspout extenders help spread roof runoff over landscaped areas.
Onsite control features are typically used in conjunction with other drainage methods like French drains or rain barrels. Managing roof rainwater discharge throughout your property helps restore the natural movement of water and prevent concentrated flows that lead to erosion and flooding.
Conclusion
Managing roof rainwater runoff is an important consideration for any home. While gutters and downspouts provide the basic first step, integrating other diversion, collection, or onsite control measures can further protect your home from water damage due to excessive roof runoff. Careful planning and professional drainage design help ensure your system adequately handles the roof runoff from even the largest storm events.