An automatically controlled underground lawn sprinkling system is a convenient timesaver that eliminates the need for wrestling with hoses and dragging portable sprinklers from place to place. It is also one of the best ways to help ensure a healthy lawn, by automatically providing the required amount of water at regular intervals -- whether or not anyone is at home.

The ready availability of plactic pipe and fittings, pop-up sprinkler heads, valves and all other accessories, has made installation of an underground sprinkling system a practical project for any do-it-yourself homeowner willing to tackle the job.

Either semi-rigid PVC pipe or the more flexible polyethylene pipe can be used for the underground plumbing. PVC pipe is joined with fittings that are permanently cemented or solvent-welded together; polyethylene pipe is joined with slip-in serrated fittings that are held with stainless steel hose clamps on the surface.

The first step is to draw a plot on graph paper, making everything to scale. Show the house and all walks, driveways and other structures, and indicate all large trees and shrubs that could interfere with spray patterns from individual sprinkler heads. Before you can indicate where the sprinklers will go, you must know the number of gallons per minute (GPM) available from the faucet you will be using. You will also want to know the static water pressure.

Sprinkler heads are rated according to the GPM they will require to cover a circle (or part of a circle) at the usual rated pressure. Only by knowing the total GPM available will you know how many separate sprinkler heads can be hooked up in each circuit or section. For example, if the water line to be used provides a maximum of only 15 GPM, and if each sprinkler head draws 2 GPM, you cannot hook up more than seven of them in each section. Thus, if your entire system will require 20 sprinkler heads, you will have to split them up into at least three sections, each controlled by a separate valve so they can be turned on in sequence until a full cycle is completed.

To determine pressure you can use a pressure guage hooked to a faucet located where the connection will be made, assuming it is fairly close to the water meter and near where the main water line enters the house. The pressure should be measured with the faucet wide open, and with all others in the house turned off. You can get this information from your local water company in many communities, or you can compute the flow by consulting charts available from most dealers and manufacturers of sprinkling equipment.

Locate the sprinkler positions on your plan by using a compass to draw circles and part circles. The center of each circle is separated by the amount specified as the diameter of the circle covered by that particular model sprinkler. Use half-circle heads along walks and driveways, and use quarter-circle or three-quarter-circle heads in corners to minimize overspray and waste. All spray patterns should overlap to ensure there will be no missed spots. Add sprinklers where needed to reach behind large trees or other obstructions. Add "bubblers" or shrub sprays where needed to water foundation plantings and shrubbery beds.

When all sprinkler heads have been located, divide the total system into separate sections so none will exceed the total number of GPM available. All the heads in any one section should be of a similar type so the demand will be balanced. For example, do not mix pop-up lawn sprinklers with shrub sprays in one section.

Next, draw the paths the pipelines will follow from one head to another, as well as from the supply valve manifold, (the place where all control valves are hooked into the main supply line). As a rule, all supply valves are grouped together (as illustrated), so that one main valve will shut down the entire system. Each control valve is then connected to its separate group of sprinklers.

In each section, plan for pipe runs that are no longer than necessary to reach all heads. Where possible, branch off to reach the heads rather than run the pipe from one head to another in such a way that water to the last head has a much longer distance to cover before it gets there.

To start actual installation, drive stakes into the ground where the sprinkler will go, then stretch strings to indicate the paths the pipes will follow to each sprinkler head. When all locations have been checked, shallow trenches can be dug for the pipe. Dig the trenches deep enough so that when the risers and sprinklers are installed, the top of each sprinkler head will be flush with the grass, (usualy 6 to 8 inches deep is about right).

To minimize damage to the lawn, two methods are commonly used. One is dug out V-shaped wedges of sod and put these carefully next-to the trenches so they can be put back into the three-sided "flaps" and fold them along the trench so they can be flipped back into the same place to cover the scars later.

In both cases, extra soil that has to be removed if you must go deeper is placed on the opposite side of the trench. When you have finished installing the pipelines, the soil is replaced first, tamped down with your foot, then the sod is replaced and again tamped. Any scars will disappear after one or two mowings.

Connect all pipes, using a tee and a short riser pipe at the point where each sprinkler head will go. When everything is connected, hook up to the water source and flush the system before actually installing the sprinkler heads. Hook each section to its own valve, then follow the manufacturer's instructions about how to connect the automatic valves to the clock-controller so it will turn each valve on in sequence for the length of time needed, and at the hour you want.

After the entire system has been hooked up, and the automatic valves installed as indicated, test the system before burying the pipes and closing the trenches. Then replace the loose soil and the sod and tamp into place.

Where pipes must go under walks, you can use water pressure to create a small tunnel. Hook your garden hose to a length of metal pipe long enough to pass all the way under the walk, then turn the water on full force and start pushing the pipe under the concrete from a trench dug on one side. The water pressure will dig its own trench as you push the pipe forward.