I've got an irrigation system, but all I do is replace the heads when they wear out. You should be able to dig out the heads that are in at the wrong depth and put them back in correctly.

I found an irrigation forum, and people there may have better/more detailed advice.

Here is a link that might be useful: irrigation forum

Thanks bpgreen I posted my question on that forum. I must have missed that earlier. Sorry

What is the correct head depth?

Buy a ratcheting plastic pipe cutter. You can get them for $8-$100. Mine cost $8. If you need the cutters you'll also need PVC glue and some pipe cleaner fluid. It is often dark purple in color. The cleaner removes the outer surface of the pipe so the glue will melt deeper into the pipe forming a solid piece after it dries.

The rest of this I'm doing in steps to make it easier to sort out. I'm not trying to be pedantic.

Find a riser you want to adjust the height on.

Dig around that riser until you get to the cross connecting pipe underground. It will be 6-12 inches deep. Put all the soil onto a tarp or something that you can temporarily pull away. Measure the pipe diameter so you can speak intelligently with the people at the hardware store. Risers are usually 1/2-inch but rarely a 3/4 pipe is used.

Measure the height of the riser from the TOP of the lower fitting to the TOP of the spray nozzle.

Lay a piece of wood across the hole and measure from the top of the bottom fitting to the bottom of the wood across the hole. The bottom of the wood is the very top surface of the planet at the location of that hole. You want the TOP of the sprayer to be at that height.

At this point all I can say is you are going to get some experience. Adjusting the height of the riser might be as easy as adding a threaded "nipple" to the top of the riser or you might have to do some surgery with your pipe cutting tool. If you have to shorten the riser, you will need to cut it twice and remove a piece in the middle. Then you can rejoin the two pieces with an unthreaded nipple (need the glue and cleaner). Or you can cut off the top fitting and glue a new one on at a lower height. Use the measurements you made to determine the final length of the riser when you are finished. Make all your cuts leaving the riser as long as possible. It's much easier to work at the top of a hole than at the bottom.

There are many ways to approach a fix. Go to the hardware store and start memorizing the types of fittings you have available. There aren't that many but the combinations of threaded and unthreaded are important.

When you are finished you will have dirt left over that will not fit into the hole. This next part is very important.
ALL THE DIRT GOES BACK INTO THE HOLE.
That is why the tarp was important so you don't lose any of that dirt. If you do not put all the dirt back into the hole, you will soon enough have a depression around that riser. Even though you have a mound around the riser and the spray nozzle is below grade, all that dirt will eventually settle back to where the edge of the planet was when you started.

You will pay for the cutting tool the very first time you use it. The alternative is a hacksaw that usually ends up breaking pipe and fittings as well as delivering bloody hands. The cutter goes through pipe like a hot knife through butter. When I was doing this in the '60s all the pipe was galvanized steel. Hacksaws were required so the hole had to be big enough and deep enough to get a hacksaw in there. If the pipe was old enough you usually ended up replacing the horizontal pipe, too. What a MESS! Life is better with plastic!

Dchall ,Thanks alot for the instructions, I think I will start on this project this weekend. I am currently working on getting the leaves off the lawn. Hopefully I will be able to find all of the heads that are currently buried.

You really know your stuff, you had helped me about 2-3 years ago when I started an organic program on at my old house. I appreciate the advice.

Chances are that if the system was installed by the previous homeowner and you have heads that are too high, the pipe is buried too shallow to lower the without some plumbing modifications. Do yourself a favor and get some "funny pipe" and some funny pipe ells. "Funny pipe" is black flexible plastic which make the job of lowering heads alot easier. You might find "funny pipe" at your local big box store, but you will probably be better off finding a local irrigation supplier in your area. They are a great resource and they carry higher grade materials than the big boxes. What's more, you will find that the people at the supplier are far more knowlegable than what you would find elsewhere.