Step #3 of the
Landscape Sprinkler System Design Tutorial
This is important, read it carefully!
The area watered by each sprinkler must overlap substantially the area watered by the adjacent sprinkler. This overlap may seem like a waste at first, but it is a very important necessity. Without this overlap it would be impossible to design sprinkler systems that provided uniform water coverage.
Have Doubts? See for yourself, it only takes a couple of minutes to prove! Grab a piece of paper and draw circles on it so that all areas of the paper are inside a circle, but no circles overlap. You can’t do it, can you?
That’s right, 100% overlap of watered areas is REQUIRED or you will get dry spots! This is known in the industry as “head-to-head coverage or head-to-head spacing”. A lot of those free design guides you find in stores and on the Internet get this wrong. They don’t show enough overlap! The writers of those brochures think you are going to look at the overlap and buy the brand of sprinkler that shows the least sprinkler heads. So they try to make it look like you can use less sprinklers with their brand. After you’ve bought the sprinklers if you have dry spots, well hey, it’s YOUR problem now! You’ll probably just buy a few more of their sprinklers to get rid of the dry spots. In fact, it will probably take more sprinklers to fix the dry spots than it would have to do it right the first time. $$$ Ching, ching!
Rule: Sprinkler Radius = distance between sprinklers
One more time: The water from any single sprinkler should actually get the sprinklers on each side of it wet!
Now that I’ve told you that you SHOULD use head to head spacing I’m going to backtrack a bit and tell you that you can space a few of the sprinklers slightly farther apart as needed to work around odd shaped areas. I still recommend that you keep at least 80% of the sprinklers at head-to-head spacing! Take the sprinkler head watering DIAMETER and multiply it by 0.6 to get the absolute maximum distance that should ever occur between any two adjacent sprinklers. (Remember most manufacturer’s give you the radius of the sprinkler, you need to multiply by 2 to get the diameter.) For example, 15′ radius spray heads should never be more than 18′ apart (30′ diameter x 0.6 = 18′). Note that we rounded to the nearest foot. If the sprinkler system is in a windy area I suggest the majority of the sprinklers be spaced at 45% of the diameter (that’s closer than head to head!), as winds over 10 mph really mess up the sprinkler patterns.
(Optional reading for those who need explanations.) Back when I designed my first sprinkler system in High School I wondered why they wanted so much overlap of the sprinklers. It seemed to me to be nothing more than a ploy to sell more sprinkler heads! I was smarter than that, so I stretched them out to save my folks some money! The result was big dry spots, and my parents wound up replacing the sprinkler system a few years later. (They never said anything about it to me, I just noticed the new sprinklers a few years later on a visit home from college.) Ouch! Not a good start for a future irrigation expert! Now that I’m a bit wiser and more knowledgeable I realize there is a good reason behind the head-to-head coverage. Unfortunately, it’s rather hard to explain. The perfect sprinkler would put out a pattern of water that is heaviest right next to the sprinkler, then uniformly declines out to the radius. So the farther you move away from the sprinkler, the less water falls on any given patch of ground. When we test sprinklers for water coverage we set up a series of cups between the sprinklers to collect the water that falls. That way we can see how much water falls at various distances from the sprinkler. In the diagram below you can see what happens when there are various distances between the sprinklers.
In example “A” the sprinklers are just barely overlapping and much more water is falling in the cups next to the sprinkler heads. But the middle 3 cups are only getting ½ the water of the cups next to the sprinkler. If you watered long enough to keep the middle green, the areas around the sprinklers would turn to mud! In example “B” we see that moving the sprinklers closer together has evened up the amount of water a bit more. However the areas near the heads are still getting 25% more water than the other areas. Not enough to cause mud, but you would definitely see rings of greener grass around the sprinklers! Example “C” shows almost head-to-head spacing. The cups are almost all uniformly full! So don’t stretch the distance between sprinklers.
What if you need a smaller radius than the sprinklers available?
Almost all sprinklers have a radius adjustment device on them so that you can reduce the radius of the water throw. This is one way you can adjust for narrower areas. Keep in mind that for most sprinklers you can’t reduce the radius by more than 50% without causing problems. The other solution for smaller areas is to use nozzles made to spray less far, or that spray a special pattern. An example of a special pattern would be the nozzles that spray a 4′ x 30′ rectangular pattern. These are commonly used in long, narrow areas.
Remember if you reduce the radius of the sprinkler you must reduce the distance between sprinklers by the same distance! Keep the coverage head-to-head!
Calculating the GPM for sprinklers when you reduce the radius is easy:
For spray heads you just use the manufacturer’s chart. When you use the radius adjustment on a spray you are simply reducing the water pressure by closing a small valve in the nozzle. As the pressure drops, so does the radius. Just look at the manufacturer’s chart for the radius you plan to reduce the sprinkler down to. Then read the GPM for that radius! For example, your designing for 30 PSI. The radius at 30 PSI of the sprinkler you selected is 15 feet with 1.85 GPM according to the manufacturer’s chart. But you want the radius to be 14 feet. Looking at the manufacturer’s chart you see that the radius of the same sprinkler is 14′ at 25 PSI with 1.65 GPM. So the GPM of that sprinkler if you reduce the radius to 14′ will be 1.65 GPM. That’s because when turn the radius adjustment screw to reduce the radius to 14′ what you REALLY did was reduce the pressure to 25 PSI!
For rotor heads the GPM stays the same no matter how much you reduce the radius! That’s because reducing the radius on a rotor doesn’t change the amount of water coming out of the nozzle. To change the radius a small screw extends into the stream of water coming out of the nozzle. The tip of the screw deflects the water which “screws it up” (pun intended) so it doesn’t go as far. This creates another problem, however, which is that it really messes up the uniformity of the water. So when you use the radius adjustment on rotors, you tend to get dry spots. This is one reason I strongly suggest that you use a smaller nozzle if possible rather than using the radius adjustment screw on the sprinkler. The other reason is that when you reduce the radius you really should also reduce the GPM of the sprinkler. Otherwise there will be a lot more water under the sprinkler with the reduced radius. Bottom line- use the radius adjustment screw on rotors only when nothing else will work.
Warning for rotors only:
When designing systems with rotors do NOT rely on the manufacturer’s stated radius for design. They get those distances by testing the rotors inside a building with no wind. The real world is harsher! If the gallonage of the rotor is less than 6 GPM the maximum spacing should never be more than 35′ between rotor type sprinklers.
Stryker’s Rule: the spacing in feet between rotors can never exceed the operating pressure in PSI at the sprinkler inlet (So a rotor with a 30 PSI operating pressure = 30 foot maximum spacing between rotors. Yes, I know the package says you can space them farther apart.)
Ignore the rule above and you will be very sorry!
Sprinkler Precipitation Rate and GPM
The precipitation rate is the amount of water the sprinkler throws onto the area it waters, measured in inches per hour. (Inches per hour is how deep, in inches, the water would be after one hour if it didn’t soak into the ground or run-off.) Precipitation rate must be considered when selecting your sprinkler heads to eliminate water application uniformity problems (dry spots).
Spray Heads: Almost all sprinkler manufacturers make their spray heads so that you can mix and match nozzle patterns and the precipitation rates will still match for all the heads. This is referred to as “matched precipitation rates”. Look for this feature when selecting your sprinklers. Important: do not mix different brands of spray heads and nozzles together on the same valve circuit without checking to see that they have the same performance specifications. Just because the nozzle will screw into the sprinkler body doesn’t mean it’s designed to work with that sprinkler!
Rotors: Rotor-type heads aren’t quite as easy. You must select the appropriate nozzle size for each rotor in order to match the precipitation rates. A simple illustration will help explain. Rotor heads move back and forth across the area to be watered. The rotation speed is the same regardless of whether the rotor is adjusted to water a 1/4 circle or a full circle. So the stream from a 1/4 circle head will pass over the same area 4 times in the same amount of time that it takes for a full circle head to make one pass over the area it waters. With the same size nozzle in both, a 1/4 circle rotor will put down 4 times as much water on the area under the pattern as a full circle rotor will. (Remember that after every quarter turn the 1/4 circle rotor reverses direction and covers the same area again!) To match the precipitation rates between these sprinklers, the quarter circle rotor must have a nozzle that puts out 1/4 the amount of water that the full circle nozzle puts out! A half circle rotor must have a nozzle that puts out 1/2 the water of a full circle. This is why when you buy a rotor-type sprinkler head they often include a handful of different size nozzles with it. Wait, there’s more (don’t panic yet, there is a simple solution forthcoming)!
If you have rotors that are adjusted for different radii you will need to adjust the nozzle size to compensate for the radius change also! For example if most of the rotors are set for a 30 foot radius, but one is adjusted down to 20 ft., the 20 ft. one will need a nozzle 1/2 the size. (Remember: when you reduce the RADIUS by 1/3 you reduce the AREA by a little more than half.)
Avoid using rotors with nozzle flows that are less than 2.5 GPM, except in corners (quarter circle patterns). Flows under 2.5 GPM give very poor coverage due to the tiny water stream. Even a slight breeze will distort the watering pattern and give you dry spots. I strongly suggest that you stick to using nozzles as close as possible to the GPM of those in the cheat chart below.
O.K. Now that you understand the principles, let’s simplify this a bit by using a cheat chart…
Unless you really know what you’re doing (in which case you wouldn’t be reading this tutorial), you should stick with the nozzles on this chart:
Quick & Dirty Guide for Rotor Nozzle Selection
1. Find the section of the chart with your desired spacing.
2. Find the pattern (1/2, full circle,etc.) of the sprinkler.
3. The chart tells you the GPM the nozzle must have.
4. Use a nozzle size that comes close to matching both the PSI – GPM combination.
5. Ignore the radius given by the manufacturer.
6. Be sure to read the notes below the chart!
For 20-29′ spacing between sprinklers-
1/4 circle . . . 30 PSI – 0.8 GPM
1/2 circle . . . 30 PSI – 1.6 GPM
3/4 circle . . . 30 PSI – 2.4 GPM
full circle . . 30 PSI at 3.2 GPM
Important: see notes below!
For 30-39′ spacing between sprinklers-
1/4 circle . . . 40 PSI – 1.5 GPM
1/2 circle . . . 40 PSI – 3.0 GPM
3/4 circle . . . 40 PSI – 4.5 GPM
full circle . . 40 PSI – 6.0 GPM
For 40-55′ spacing between sprinklers-
1/4 circle . . .55 PSI – 3.0 GPM
1/2 circle . . . 55 PSI – 5.5 GPM
3/4 circle . . . 55 PSI – 8.0 GPM
full circle . . 55 PSI – 11.0 GPM
It is critical that the water pressure (PSI) at the sprinkler be as high, or higher, than the distance between the sprinklers in feet (per Stryker’s Rule). For example, if you space the sprinklers 45′ apart, you must have at least 45 PSI of pressure at the sprinkler inlet. That’s the pressure at the sprinkler inlet, not the total pressure available. Remember, you will lose pressure in the pipes and valves, so the pressure at the sprinkler inlet will be lower than your available pressure! Go back to the tutorial pressure loss pages to figure out how much pressure will be lost in your sprinkler system.
Select the nozzle size closest to these GPMs without regard to the radius the manufacturer gives. For example, if you are looking at a 25′ radius, the chart above says to use a 1.6 GPM nozzle for a half-circle rotor. But you happen to notice that the rotor manufacturer’s literature says that at 25 PSI, a 1.6 GPM nozzle has a radius of 32 feet. So why am I telling you to space it at 25′? When the manufacturer tested the rotor on their test range (inside a large building with no wind) they measured a few drops of water 32′ from the rotor. When you install it out in your yard it will not perform as well. You may still get a few drops of water 30′ or even 32′ from the head, but not enough to grow anything. You need to trust me on this one! Remember, if the sprinkler sprays too far, most rotors have a radius reduction screw that will allow you to very easily reduce the radius. But, if the rotor does not spray far enough there is nothing you can do about it without a major expense! Best to play it safe.
You may want to make additional adjustments to nozzle sizes after installation to compensate for your specific conditions. Most rotors now come with a “nozzle tree” that contains most of the different nozzles for the rotor, so you can change the nozzle sizes if you need to. Some manufacturer’s don’t offer nozzles sizes larger than 3.0 GPM for their economy-priced heads (providing those extra nozzles would probably cost them at least another nickel in costs!). You may need to upgrade to the next better model line if you have a large yard! The larger size nozzles for 40′ spacing are not available with most of the “mini-rotor” models sold for residential use. You will need to upgrade to the next model. Also, sometimes other nozzle sizes are available separately from the manufacturer, for example low angle nozzles. You will probably need to get these from a store that specializes in irrigation sales, rather than a hardware or home store. Look in the yellow pages under “Irrigation” or “Sprinklers”, or try one of the online stores listed in the tutorial links pages.
There is a conflict between the nozzles recommended for the 20-29′ spacing range of the chart and my previous advice to “avoid using rotors with nozzle flows that are less than 2.5 GPM”. This is because the Nozzle Selection Guide assumes you will be mixing 20-29′ radius rotors together on the same valve with 30′ plus radius rotors. To keep from having enormous nozzles on the larger radius rotors I am recommending that you use smaller nozzles than I would otherwise consider for the smaller radius rotors. This is essentially a compromise. Sometimes it is not practical to obtain perfection! If all or a majority of your rotors will be spaced at 20-29′ apart, then you should probably use larger nozzles than I recommend in the chart. In other words, use those listed in the chart for 30-39′ spacing for the 20-29′ spacing. This will help avoid problems caused by the wind blowing the spray out of the irrigated area. However, if your sprinkler system will be located in an area with little or no wind you can go ahead and use the smaller nozzles in the chart. What is little or no wind? Go outside in the evening or early morning when you will likely be irrigating. If you can feel the wind blowing even gently against your face, I would consider that enough wind to need the larger nozzles.
If you calculate the precipitation rates you will notice that the shorter spacings result in a higher precipitation rate than the larger spacings. This is because the smaller heads with lower GPM rates are more susceptible to wind and evaporation, and thus it is assumed less of the water is actually reaching the ground. The higher precipitation rate compensates for this.
If you are designing a sprinkler system for an area where the wind blows a lot you should look at the Irrigation and Wind FAQ.
Select Your Sprinklers
If you haven’t started shopping for sprinklers yet, now’s the time to start checking out what’s available. Check out which sprinklers are available and look them over. Write down a list of the heads you think will work well for your irrigation system on your Design Data Form. Be sure to list the PSI and GPM for each head as given in the manufacturer’s literature, along with the maximum spacing between heads.
Do not blow-off my advice on sprinkler spacing in order to save a few bucks on sprinkler heads! Right now you may be feeling pretty smug about how much money you saved by stretching the sprinkler spacing. But next summer you’re going to look pretty stupid to the neighbors, standing out there with a hose watering the yellow spots your new sprinklers don’t cover! I have a collection of “wish I’d listened to you” letters from people who didn’t take this advice. I get a few more of these every year, and these are just the brave folks willing to confess they messed up. They all say you should listen to me on this!
Later on you will need to know the flow rate for each sprinkler you use, so it might be helpful to make some notes on the back of your Design Data Form showing the nozzle size and GPM you will need for each different sprinkler you plan to use. Otherwise you’ll wind up having to look the information up over, and over, and over…
Draw the Sprinkler Heads on Your Plan
You’re now ready to pencil in the sprinkler head locations on your drawing. Hallelujah! I know it seems like it took a long time to get here, but to do a good job we needed to cover a lot of background information! Use a pencil to draw in the sprinkler heads so you can easily make adjustments to the locations later. Many people find it helpful to use a compass to draw a light pencil line showing the radius of water throw for each head.
Remember these tips:
- Keep the distance as uniform as possible between heads. To the extent possible a sprinkler should be equal distance from the adjacent sprinkler in each direction (forming a triangle if possible). Changes in spacing between adjacent sprinklers should be made as a gradual transition when possible.
- Try to position heads so that if you were to draw a straight line between adjacent heads they would form an equilateral triangle (each side of triangle is same length). This is called “triangular spacing” and creates more even water coverage than “square spacing” (ie; lines between 4 heads form a square). That said, you will often be unable to form a triangle so don’t panic if you can’t.
- Don’t stretch the spacings, use “head to head” spacing. Using too many sprinkler heads is seldom a problem, using too few sprinklers heads is ALWAYS a disaster!
- Start by drawing a sprinkler in each corner. Next, draw sprinklers around the perimeter of the irrigated area, watching that they are not too far apart (one more time, better too many than too few!). Adjust the locations to make the spacing between sprinklers as even as possible. After the perimeters are done, then draw the sprinklers in the interior area.
- If the sprinklers need to overlap so that the spray from one head goes over and beyond the next that’s OK. While you don’t want to over-water, it is always easier to correct an over watered area than a under watered one. For example, you can use the radius adjustments on the sprinklers to cut down the water in the over-irrigated areas. If need be you can even remove or relocate a sprinkler later. It’s much easier to remove one than to add one!
- Sprinklers that are placed closer than 6 feet apart need some special consideration. Standard spray-type sprinklers don’t work well if the radius is adjusted below 6 feet. (The opening the water goes through is so tiny that the normal expansion of the plastic or metal on a warm evening can close off the water flow!) If the area is long and narrow (4′ wide or less), use the strip pattern nozzles. I prefer the so called “side-strip” type that you place along the edge of the area, they have better patterns than the center strip nozzles. End-strip nozzles have notoriously bad patterns, they shouldn’t be more than 10′ from the next head! When using standard spray sprinklers (like quarter, half, and full circles) in areas where the radius must be adjusted to less than 6 feet use a “pressure compensating device” to reduce the radius. The pressure compensating device is normally installed under the nozzle where it reduces the flow and pressure through the nozzle. The good news is that by using a under sized pressure compensating device you can also reduce the nozzle radius! Unlike the adjustment screw on the nozzle these devices work well regardless of the temperature. However, you will need to select the proper size pressure compensating device for each nozzle. It is possible that every nozzle will need a different size! To select the right device you use a special chart provided by the pressure compensating device’s manufacturer. The chart will tell you exactly which device you must use with each different nozzle in order to get the radius you want. Most major sprinkler manufacturer’s make pressure compensating devices for their spray sprinklers, and the charts you need can be found in their catalogs. You may need to go to a commercial sprinkler supplier to find them.
Study the example drawing below.
Again, notice that the radius of each sprinkler’s spray goes all the way to the next sprinkler! This is critical.
Note that in the example above only the lawn area outlined with a green curving edge is being watered. The area between the lawn (green line) and the edge of the property (brown line) would most likely be planted with shrubs and irrigated separately from the lawn. In most cases a drip system would be considered for watering the shrubs as it is less expensive and more efficient. See the separate guidelines for designing drip irrigation systems.
Bonus landscape design tip: Creating a border of shrubs around the perimeter of your yard is, in most cases, a good landscape design practice. A shrub border helps to reduce the visual impact of the fence (assuming that like most residential properties you have a fence.) Shrubs also typically use less than half the water of lawn areas of the same size, saving money spent for water. Once a month you need to weed and trim shrub areas, as opposed to the lawn that needs to be mowed every other week at the least in summer. A border using shrubs of various sizes, textures and colors can add greatly to the attractiveness of your yard. Place smaller shrubs near the lawn, with larger growing varieties behind them next to the fence.
Sprinkler Layout for Narrow Planters:
This example shows the typical placement for sprinkler heads in a narrow planter. In this example, special spray sprinkler nozzles called “end-strips” and “side-strips” are used. These nozzles spray a long, but narrow, pattern. A typical pattern is 4′ x 30′ (4′ out and 15′ in either direction from the head). There are also spray nozzles called “center-strips” which don’t work as well. Be careful when using end-strips. They tend to have a weak coverage area on either side of the nozzle (the yellow area in the drawing above). Avoid using 2 end-strips facing each other in a lawn area. If possible always install a side-strip in the middle between 2 end-strips. The sprinkler layout above is for lawn. In a shrub area you can eliminate the sprinklers on one side as long as the width of the planter is 4 feet or less- so you can install the sprinklers on one side only. Shrubs don’t need as even a watering pattern. Lawns require heads on both sides. Note the triangular arrangement of the sprinklers, which gives more even coverage. Yes, it takes an extra head to create the triangle pattern, and you need to space the heads a little closer together than the normal maximum on one side to create the “triangle pattern”, but it’s worth the cost.
For narrow strips wider than 5′ you would use regular half circle heads on both sides. The distance between the sprinkler heads should not be more than 1 foot greater than the width of the planter. In other words, if the planter is 8 feet wide you would install half circle heads on both sides of the planter, not more than 9 feet apart from each other. As with the example above, it is best if you arrange the sprinklers in a triangular pattern.
As we saw previously, the flow rate in gallons per minute (GPM) of each sprinkler head is determined by the nozzle installed in the head. It is necessary to know the GPM for each head in order to determine which heads will be connected to each valve and in order to determine the size of each pipe in the sprinkler system.
You will probably need to dig up the sprinkler manufacturer’s literature again. In the literature the manufacturer shows different GPM and radius information for each sprinkler nozzle based on the operating pressure (PSI). Now we can use that information to find the GPM for each sprinkler head. First, determine what the SPACING is between each head and the others around it. Next, look for the radius closest to that spacing and use the corresponding GPM as the flow for the head.
Write down on your plan the GPM for each sprinkler next to the sprinkler symbol.
Hint: You will find the GPM and radius data for many of the popular sprinklers in the product reviews .
Example: You note that a spray type head on your plan is a 1/2 circle pattern and the distance to the 3 closest adjacent heads are 13 feet, 12 ft., and 14 ft.. So the spacing for this head is 14 ft. (the highest of the 3). Looking at the manufacturer’s literature you note that a radius of 14 ft. for the 1/2 circle nozzle in this sprinkler requires a pressure of 25 PSI and a flow of 1.65 GPM. Write down the flow of 1.65 GPM next to the sprinkler head on your drawing. You then repeat this procedure for each sprinkler head on your drawing.
Add sprinkler GPMs on your plan.
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Text and Images by Jess Stryker unless noted. Copyright © Jess Stryker, 1997-2012. All rights reserved.