How to Design a Patio Drainage System

Many yards have areas that collect water, and a poorly designed patio can add to this problem. By designing a proper drainage system, you can ensure that water is carried away rather than remaining in the patio.

Drainage problems are more serious with patios than with porches and decks because these paved outdoor spaces adjoin the house. Proper patio drainage helps preserve the surface and protects your home.

Slope the Base

When preparing to install a new patio, use string lines and sight lines to map out the shape and ensure that it is level and properly sloped for drainage. Ideally, the gravel base and sand bed should be sloped at 2%.

Use a soil engineer to determine how much fill is needed and dig the area, making sure to preserve the 2% slope. Add the fill in stages, letting each layer settle before adding more. Lay a layer of landscape fabric over the excavated area to prevent weed growth.

Once the base is prepared, add a layer of gravel to a depth of four inches, ensuring that it is sloped at a consistent rate of one inch for every four feet. Follow the gravel layer with a layer of sand, using a plate compactor to ensure that it is evenly spread and level.

Install a French Drain

A French drain is a quick and effective way to prevent standing water that can damage outdoor patios, erode soil and harbor mosquito larvae. French drains also prevent water from seeping up through the foundation of a home and damaging exterior walls.

Start by identifying the problem area. This includes locating areas that are prone to saturation and ponding, as well as determining general pathways of water in storm events. On larger properties, this can be as simple as looking at the general topographic layout and identifying locations where water should be re-routed.

Next, dig a trench that’s at least 12″ wide and deep. Line the trench with a weed mat that allows for water to flow through but keeps dirt out of your gravel bed.

Install a Dry Well

When your lawn is unable to absorb storm water, a dry well can help you redirect excess rainwater. These underground structures range from simple gravel pits lined with landscape fabric and a PVC drain to more advanced perforated plastic or concrete tanks with larger interior storage capacity. It’s important to consult an expert who will take into account soil absorption, percolation tests, yard square footage and frequency of irrigation in calculating the volume needed for your dry well.

Installing a dry well requires some digging, so it’s best for homeowners who are comfortable working with power tools and have a strong understanding of landscaping basics. Before you dig, call 811 and have a professional mark out any buried pipes or wires. You can also buy pre-assembled dry wells at many garden centers.

Install a Gravel-Lined Trench

When a patio or walkway doesn’t soak through the soil, you can dig a French drain to collect water and channel it away. This is most common around foundations but it also works well in gardens or along driveways.

Dig a trench about 12-18 inches deep depending on your soil type. You’ll need a level to get the proper slope for drainage.

Fill the trench with a layer of gravel. This helps with grading and also allows the pipe to easily lie on top of it.

Add a drainpipe to the trench. This can be either flexible or rigid pipe with holes for drainage. Lay a bed of gravel over the pipe to prevent dirt from mixing with the gravel and promote water flow.

Install a Soakaway Crate

Soakaway crates are modular water storage cells that resemble old-fashioned plastic milk crates. They are much more efficient than the traditional pit-style soakaways filled with stones and hardcore.

Soakaway crates should always be lined with non-woven geotextile membrane (available from most drainage suppliers). This will stop soil particles washing into and clogging the crates, which can drastically reduce their longevity.

Soakaways should be sited in an area lower than the patio being drained and at least five metres away from buildings and foundations to avoid flood risk. It is important to work with a drainage expert to make sure that the correct size soakaway is installed and that all drainage pipes are connected correctly. Also, planning permission may be required if you are replacing an existing soakaway or extending the soakaway area.

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