How to install drainage before installing flagstone patios

Updated April 17, 2017

Flagstone patios in enclosed areas, like those nestled into a notch in the exterior perimeter of a home, can become problem areas. Rain may accumulate on these patios when they are not well drained. Heavy rains may temporarily flood the patio and soak the lower portions of the house walls. Installing drainage for flagstone patios will move the water away from the patio and the home.

Find the elevation of the patio site, using the sight level. Add the thickness of the base material and patio stones you will be using to that measurement. Find the elevation of the nearest place lower than the patio that is suitable for a drain to discharge. Ideally this would be a grassy or plant-covered area that is not near a stream or creek.

Dig a trench from the centre of the patio to the lower area. For single-level patios of 200 square feet or less, a single drain will be adequate. For larger patios, continue the trench to each side of the centre point so you can add additional drains. You will have to increase the depth of the trench from the centre point to the discharge area when using more than one drain. Make the bottom of the trench about 8 inches lower than the finished level of the patio. Make sure the bottoms of the trenches drop at least a quarter inch for every 10 feet of length.

Drive the stakes into the ground at each side of the patio, in line with the centre of the trench that runs below the drain/s. Attach the string to the stakes and set it at the finished height of the patio.

Make the drain assembly by cutting a 6-inch piece of PVC pipe and cleaning its outside surfaces and the inside flanges of the grate and elbow with the cleaner. Coat the inside flanges of the grate and elbow and the exterior of the PVC pipe with solvent. Slide the grate on one end of the pipe and the elbow onto the other end. Hold for 20 seconds. Repeat this for each drain on multiple drain installations, substituting a PVC T fitting for each drain that is not at the end of the line.

Connect the drain assembly to the drain pipe by cleaning the inside of the elbow flange and the outside of the pipe, then applying cement to both and inserting the pipe into the elbow. For multiple drains, connect the elbows at the farthest ends to the tees at each subsequent drain with pieces of PVC pipe until reaching the centre drain. Install a drain T fitting for connection to the drain line just to the right, or left of the centre drain before making the centre drain connection. Connect the drain T fitting to the drain line.

Cement drain pipe sections together, using couplings, until the pipe reaches the drain discharge area.

Set the final drain heights using the sight level and packing dirt below and around them to hold them at the right elevations so the system slopes at least one-quarter inch per 10 feet over its entire length.

Back fill dirt below all pipes and drains to ensure the lines do not sink when soil is placed on top of them.

Back fill the trenches.

Things You'll Need

  • Sight level
  • Shovel
  • Pick axe
  • Trencher (optional)
  • Hammer
  • Wooden stakes
  • String
  • 3-inch PVC drain grate
  • 3-inch PVC elbow
  • 3-inch PVC T fittings (optional)
  • 3-inch PVC coupling/s (optional)
  • 3-inch PVC pipe
  • PVC cleaner
  • PVC solvent
  • Rag
  • Saw
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