Stormwater Management for House and Patio

Pool and patio trench drain

Having control of the stormwater drainage around your home is an important step in protecting the integrity of your house foundation, the safety and life of your outdoor living space, and the water quality of your in-ground pool or spa. It’s no fun dealing with wet basements, patio water pooling, garage flooding or heavy sediment flows in the pool. With proper planning and design, these events can be minimized or eliminated.

Two Part Problem

The whole home drainage concept should be addressed when a new home is built. That is not always possible, especially if you purchased a pre-owned home and inherited a set of conditions.

Outdoor living space with trench drain
Trench drains protect your outdoor living space.

The need for stormwater management seems to become more apparent when considering a patio or outdoor living space project. There are two characteristics that a pool deck, patio or outdoor living space have in common:

  1. They have a house or roofed structure attached to them, and
  2. They commonly have a surface that is impermeable to water (i.e. concrete or paver deck).

When considering the drainage of a given property, it is helpful to look at these two components independently and make design recommendations accordingly. This is not to say that their drainage plans can’t run together. When building from scratch, have the architect design a whole home drainage plan that is expandable. However, quite often a pool or living space is constructed after the home is built. If the home drainage isn’t good initially, the outdoor addition will just complicate matters.

Stormwater Runoff Measurements

The U.S. Department of Commerce has put together maps of rainfall measurements (from 1 year to 100 years) that anyone can access. See link above. Likewise, the U.S. Geological Survey has some data on rainfall and flooding.  Below is a general map of 100 year rainfall events for the 48 States. This data is in inches of rainfall per hour, and this map is a good starting point for designing the drainage plan for your home or outdoor living structure.

US Geological Survey Rainfall per Hour Map
US Geological Survey of Rainfall per Hour Over 100 Years

An easy way to calculate the storm water runoff of your house or patio (in gallons per minute) is by using the modified Rational Method equation.  This equation is:

Q = (C x I x A) / 96.23

where:
Q = Storm Water Runoff (in gallons per minute, gpm)
C = Coefficient of Runoff
I = Rainfall Intensity (in inches per hour)
A = Area of Drainage Zone (in square feet)

The Coefficient of Runoff, “C”, is a measure of how much water runoff your hardscape or yard will have.  Your roof, patio, concrete driveway will most likely have 100% (coefficient 1) of the rainwater run off the surface and require redirecting to a drainage area.  More permeable surface, such as sand or gravel, will have 60% runoff (coefficient 0.6) as water penetrates the ground and reduces total water flow.  Other ground coverings have been documented for this coefficient.  For hardscapes, we will use C=1.

Example: Dallas Driveway

For the person that lives in Dallas, Texas (I = 4) and has a 25 foot wide x 100 foot long (A=2500) concrete (C = 1) driveway that gentle slopes toward the garage, the 100 year storm water run off would be calculated as follows:

Q = (C x I x A) / 96.23
Q = (1 x 4 x 2500) /96.23
Q = (10,000) / 96.23
Q = 103.91 gallons per minute (running toward your garage door) in a 100 year rainfall event.

Don’t be frightened. When we look at a 25 year rainfall event, that volume of runoff water drops to about 65 gallons per minute. So, when designing a trench drain system for in front of your garage, you’ll need one that can remove 65 to 104 gallons of water each minute. Moreover, you’ll need a drainage pipe that can handle that volume, which would be minimum of 6” smooth walled pipe.

Secret Drainage Application

Let’s make this easy. NDS® is one of many drain manufactures which cater to the residential market. They have easy to use calculators that help determine Drainage of Runoff from of a given area and the Pipe Size needed for moving that water away. Use these links to find the values needed for assessing your drainage needs. They are much easier.

Part 1 – Managing Roof Drainage

We just did a little drainage exercise for a 2500 square foot driveway.  That Dallas Texas storm event of 2.5 inches per hour generated around 65 gallons per minute. If that is the square footage of your roof, you’ll be looking at the same volume of water. Except now, (and I assume you have gutters on your house, please!) you will be able to divide the runoff into 6 or 8 downspout locations. The runoff per downspout would be average around 8 – 10 gallons per minute.

I believe it is important to isolate and address your roof drainage prior to considering your patio drainage issues. Many times, patio drainage issues are due to not dealing with roof drainage first.  Below are a few good practices that will help keep your home basement dry and foundation intact. When possible, I always recommend:

  1. Foundation Drainage – Have a French Drain (not Trench Drain) at the base of your foundation footer. This keeps ground water pressure away from the basement walls and floor. A sump pump can be used in conjunction with the French drain in particularly soggy settings.
  2. Protection from Trees – Keep large trees and their roots away from the house. That little maple tree you planted in the front yard when the house was just built can start to do evil things to your foundation after 25 years, especially block foundations. Keep trees at a distance and help keep your gutters clean.
    Down Spout with Catch Basin Drainage
    Down spouts and catch basins protect your foundation.
  3. Downspout Management – Direct your gutter downspouts away from the house foundation. A great practice is to have each downspout terminate into a catch basin and direct the underground piping to a distant location. Consider a series of collection points connected by piping. Move this water to a natural drainage spot, to the storm sewer or even to an underground collection point where the water can be used for later irrigation or seep naturally back into the ground water. Avoid routing downspouts directly onto driveway or patio surfaces.
  4. Driveway Drainage – Let’s not forget your driveway. It came with your house. Hopefully, your house sits elevated above the street level, so your driveway doesn’t drain toward the house. If you are constantly fighting water infiltration into your garage, you will need to consider a trench drain across the garage door to help move that water away from your foundation. If possible, direct this water to the same drainage system that your downspout water is using. Make sure you have cleanouts in your plumbing so the occasional clog can be dealt with. Landscape or excavating contractors are a good source of information when needing to discuss details on this sort of drainage project.

Once you’ve studied and managed your house run-off water, you’ll probably have a better idea on how to handle a pool, patio or outdoor living space addition.

Part 2 – Managing Pool Deck, Patio and Outdoor Living Space Drainage

Swimming pool and shed drainage
Sufficient drainage prevents future problems for your investment.

Patios, pools and outdoor living spaces are a big investment and usually the second most expensive addition to your home. Getting these areas to drain correctly is important on so many levels. Of course, you’ll want to direct water away from your home or foundation. This helps protect your biggest investment. Improper drainage on your deck can also lead to:

  • Pooling of water leading to safety issues and deck discoloration,
  • Water leakage under the deck which could lead to deck washouts or frost heaving,
  • Storm water entering the pool and disrupting the pH and alkalinity and increasing maintenance.

And let’s not forget the cost and headache of having to deal with problems which could have been prevented with some good ol’ planning. You should be enjoying this outdoor space, not regretting it. So, if you are planning to invest in backyard living, consider speaking with a landscape architect and qualified pool contractor to help in the process. Don’t skimp on the drainage features.

Drainage Management Strategies

Though not to be the final word on drainage recommendations, I can offer a few strategies that I have seen work over the years. These include:

french drain at foundation of building
French drain at the foundation of a building or retaining wall helps to collect and drain water.
  1. Design your deck to be a high point – Make sure your patio or living space is elevated above the surrounding yard and landscaping slopes away from the decking. If your patio or hardscape is small enough, slope the surface so that water runs off the deck naturally into the yard. Use a French drain at the perimeter of the patio/landscaping interface to help collect and drain water, if necessary.
  2. Retaining walls and planters need space – Though beautiful and sometimes necessary, overflowing planters and retaining wall waterfalls can mean big problems for your pool and patio. Storm water can bring an assortment of organic matter into your living space to cause a real mess. If a planter or retaining wall is in your plan, place a swale or French drain between it and the patio. Have your in-patio flower garden well below the level of the deck and have an atrium style grating as part of your drainage plan for that area.
  3. Elevate your pool edge – This a a common place that most pool builders will incorporate to guard against flows of unwanted stormwater and debris.
Pool drainage systems
Sloping pool edges can guard your pool against runoff. Trench drain will create the best scenario to collect runoff and oversplash.

Don’t undersize your deck drain – Deck drains are not always necessary. But if one is needed, never, ever buy one of those “1 inch wide” deck drains and think it will work. It won’t. You won’t be happy. Go back to the beginning of this article, determine the drainage area and the rainfall for your location. Calculate the size of drain you will need. Then increase it. Leaves and other debris can constrict the grating in large downpours making your drain less efficient. A wide drain, though not in your initial deck concept, can help in the overall maintenance of the hardscape and can become a conversation piece at the same time.

After reading this article, you may still have questions about finding the best drainage solution for your project. Trench Drain Systems (TDS) has experts who can help you find the right drains to fit into your plan and budget. Contact us to request a quote by calling 610-638-1221. For further information on drainage visit www.TrenchDrain.com or www.DrainageKits.com.