Tuesday, December 29, 2015

Putting in A Lawn Sprinkler System

In this week's video I landscaped a small front yard. Part of the job involved replacing the sprinkler system. Although the sprinklers were a small part of that project, lawn sprinklers are a popular DIY project, so I thought some tips and information would be in order. Large sprinkler irrigation systems with multiple zones and elevation changes can be pretty complicated to design (not nearly as complicated as fire sprinkler systems, but that's a topic for a different day). If you have that much square footage though you are either 1) rich enough to hire a sprinkler contractor or 2) a farmer, and you probably know more about irrigation than the contractor does. Accordingly, most of what I say here will be applicable to a small single-zone system such as you might see in a suburban front yard. Putting in this sort of system is well within the scope of a handyman or weekend DIYer. If you need more extensive information, you should check out Jess Stryker's Irrigation Tutorials website, which is probably the most complete online resource I've seen.


In the old days most sprinkler systems were made out of galvanized steel pipe, but plastic pipe is now universally used because it is cheaper, has better hydraulic characteristics, and never rusts out. There are several kinds of plastic pipe available, each with its own advantages. I think that 95% of weekend handymen will probably use ordinary white PVC, however. It works fine, requires no special tools, and is available at any hardware store. Nearly all lawn systems use pop-up style heads because they are less prone to damage and tripping. Shrub heads (the kind that screw on top of a fixed pipe) are used only in planters, especially with certain wet-climate plants that like to be watered from above. For liability reasons they should never be used where people could trip over them.

Steps to Put in the System

  1. Draw the area. Your drawing doesn't need to be beautiful, but it does need to be to scale and show planters and things like tree trunks that might block sprinkler spray. Graph paper comes in handy. 1/4"=1' is a handy scale for many front yards.

    Example lawn sprinkler layout, part 1
  2. Locate the sprinklers. The goal is to use the minimum number of heads that will still provide complete coverage of the area. Common sprinkler heads are sold by their spray pattern (360°, 180°, 90°, straight line, or adjustable) and how far they throw water. For instance a head might throw water in a 90° pattern in a 20' radius. You can use a compass to draw the radii of each spray pattern and make sure you have coverage. You might need to erase a couple of times before you get a pattern with which you are happy.

    Example lawn sprinkler layout, part 2
  3. Draw in the laterals and mains. The design that uses the least running feet of pipe is nearly always the cheapest--both in materials and labor--regardless of pipe size. Secondary to this consideration, try to minimize the number of fittings and tees needed and the number of times you need to get pipe across sidewalks or other walkways.

    Example lawn sprinkler layout, part 3
  4. Size the pipe. This can be a complicated mathematical procedure in larger systems. For a small system with good local water pressure (over 45 psi static), however, you can usually get away with simple rules of thumb. In fact, for the sort of system shown here you could just make everything 3/4" and be assured that you will have enough water. Pros usually use something called a "PSI/100" table to size pipe. For your convenience, I have boiled this process down into an easy to use spreadsheet which you can get right here:

    Screenshot of the pipe sizing spreadsheet

    Example lawn sprinkler layout, part 4
  5. Buy your materials. PVC pipe is cheap, and home centers accept returns, so be sure to pick up plenty. You can always return unused lengths later. Also pick up a can each of PVC glue and PVC primer.
  6. Dig your trenches. If you have a nice lawn, try to cut the sod away in chunks that you can put back later. If you put it in the shade and keep it moist it will be safe for several days. I like to put in plenty of stakes or flags that I can look at for reference while I'm digging. Lateral pipes should always be buried at least 6" deep, and 12"-18" is better.  Mainline pipe is required by code to be at least 18" deep in most areas.  Additionally, if you live in a cold climate make sure to put the pipe well below the frost line.
  7. Crossing Sidewalks presents special problems. Luckily, ordinary sidewalks are only a few inches deep and are laid on a bed of sand or gravel. If you excavate enough on either side you can usually just hammer the pipe through. On wider sidewalks or driveways you might need to saw the concrete away and patch it later.
  8. Lay the pipe. It is usually easiest to put the mains in then work out to the ends of the laterals. Make sure the outlets point up. If you haven't worked with PVC pipe before, my video has a quick demonstration of how to cut and glue it at about 2:26. I like to put a 1/2" plug in each outlet before I install the tee, which keeps dirt out and lets you pressure-test the system.
  9. Install the backflow/control valve and make the system connection. This will usually involve shutting off your water supply at the curb and cutting a tee into your main water line (upstream of the house shut-off valve and the pressure deducing valve, if present).

  10. Turn the Water Back on and Check for Leaks. You shouldn't have any if you followed the directions when gluing your pipe. If you do find a leaking fitting, just cut out the whole fitting and replace it.
  11. Remove Plugs from and Flush the Pipes. Let the water run for several minutes to wash out any dirt and plastic bits that got in during installation. This will keep them from clogging your heads later.
  12. Install the Sprinklers
  13. Cover the Pipe. Tamp and smooth the dirt and replace the sod, if applicable.
  14. Install the Timer. Follow the instructions in the package. These days you can get a simple battery powered-timer for around $15. Typically, installation is a simple matter of slipping it over the top of the control valve and setting the times.
  15. Put your drawing away in a safe place. If you ever need to dig up part of the system to repair it, it will be handy to know the pipe sizes and locations.
The above seems like a lot of instructions, but the work usually goes pretty fast. Even a fairly inexperienced handyman can usually put in a simple system like the one in the drawings in a weekend, and most pros could do it in a few hours.

Dealing with Low Water Pressure

The most common problem that people encounter, especially if they live in older, heavily built-up neighborhoods, is not having enough water pressure to run the system. If your water pressure is extremely low (in the 20s or below) you might be stuck upgrading your water service or installing a booster pump, and you should probably call in a pro. If it isn't as bad as that, some of the tips below might help.
  • Use rotary sprinklers instead of fixed heads. While rotaries use the same total amount of water, they don't need it all at once, because they only water one sliver of the circle at a time.
  • Use bigger pipe. Larger diameter pipe has less friction loss.
  • Break your system into smaller zones. With fewer heads on each circuit, each circuit will require less water.
  • Consider a Looped or Gridded System. Conventional irrigation systems are shaped like a tree with the mains as a trunk and the laterals as branches. Systems in which the heads are set on a continuous loop of pipe or in which the laterals run in a grid between two mains have much better hydraulics (which is why they are very popular in fire sprinkler systems). Unfortunately, actually calculating the pressure losses and sizing the pipe requires some fairly serious mathematics and is usually done with specialized computer programs. Nonetheless, I think loops and grids are underutilized in irrigation applications. In the video I replaced a conventional "tree" system with a 1/2" loop and the new sprinklers work much better than the old ones ever did.
  • Consider a Drip Irrigation System It takes much less pressure to drip water than to spray it, and you have more control over where it goes. I don't have the space in this post to go into drip systems, but there are plenty of other good resources online.

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