Introduction to Irrigation Material Take-offs
Take-Off Major Items
The following is adapted from an article I wrote for a contractor's magazine. Although oriented toward the professional irrigation installer, the principle shared here apply to anyone installing an irrigation system.
The first thing you need before you can install your new irrigation system is a list of the materials you will need. The construction industry term for creating this list is to "prepare a take-off" of the materials. The list itself is also referred to as the "take-off".
The material take-off for an irrigation system is likely to be the most time-consuming and disliked part of preparing a bid for most contractors. The job is tedious, strains the eyes, and is often downright boring. Yet it is demanding, because it must be very accurate or the project will be lost, or worse, under-priced. Often the job of preparing the take-off is passed on to the staff member with the least seniority and experience- probably the worst possible person for the job! The ideal take-off person needs to have a broad knowledge of irrigation equipment and installation methods.
Each company should have a standard system for doing take-offs in order to promote uniformity, reduce errors, and speed the work along. This system should incorporate "double checks" wherever possible to help catch omissions and errors. The little extra time used to perform these double checks is cheap insurance when compared to the cost of furnishing 10 or 15 sprinkler heads not accounted for in the bid! For you do-it-yourselfers the savings comes from reducing those unplanned trips to the store for a couple of extra parts!
The last ingredient that every system for performing take-offs should have is a checklist of items normally found in irrigation systems. The alternative to a checklist is to use a take-off form that incorporates the checklist. That is the method that will be used here. The take-off form is located on the next page, you may want to print out a copy of it at this time so you can refer to it as you read this tutorial. In order to give a better understanding of how such a take-off system would work, the following is a description of how my take-off system works. (Note: at the time this article was written I operated a take-off preparation service that prepared take-offs for 15 irrigation supply houses. I've since moved on to other, less stressful and more interesting pursuits!)
The procedure starts by noting on my take-off form the project name, location, bid date, and the name and phone number of the irrigation system designer. This information can save valuable time later, should questions arise. Next the project specifications are carefully read, and all important information relating to materials are noted on the back of my take-off form. (Specifications are detailed written requirements used to define the scope of work on commercial and government construction plans. Just the irrigation portion is often 20-40 pages long!) I review my notes on the back of the form at the completion of the take-off process to assure compliance. As I read the "specs" I especially look for items which conflict with the plans and will have to be resolved with the irrigation system designer. Often there are many conflicts. Another item I look for is what I refer to as "extras"-- items called for in the specs but not shown on the plans, such as check valves and drain valves. These items can eat up profit margins fast if left out. Although properly prepared specifications should have material requirements listed in a separate section for easy reference, most specifications are not properly written. So its necessary to read the whole thing.
Next I tackle the plans or drawings. Using my take-off form, I make sure that each item listed in the plan legend is on my form. I note any details on the form, such as model numbers, and optional features specified on the legend. If the item is not on the form, I add it at the end. At this time I do not worry about quantities, I just want to insure that every item on the legend gets on my take-off form. After everything on the legend is noted on my form I'm ready to start counting quantities of items. I cover the plan with a sheet of transparent tracing tissue so that I can make marks and notes without defacing the actual plan. If you have an extra copy of the plan you can write directly on it. Don't worry now about things that need to be measured (like pipe lengths) just concentrate on items that can be counted. When I count quantities I start at the top of my take-off form and work down, line by line. As each item is counted I mark it on the plan using a yellow, felt-tip underlining pen. When counting large quantities I also use a mechanical push-button counter, like those used by bus drivers, to keep track of the quantity. It's really easy to get mixed up when counting, especially if someone interrupts you!
After all the items on the take-off form are counted I make a brief check of the plans for any items not highlighted with the yellow pen. This allows me to catch items I missed. Often I will also find items that the designer left off of the legend. These are noted so they can be identified later by contacting the system designer. I also check the standard items listed on my take-off form that I didn't find on the plans. Are any of these items going to be needed?
The next step is to measure the linear footage of just the main-line pipe. I use a precision measuring device with a little wheel on it. As I roll it across the plans it measures the distance covered. There are some really fancy measuring tools that contain small calculators which automatically convert the scale distance on the plan to actual distance in the field! For the do-it-yourselfer a ruler will work just fine for measuring the pipe lengths. When measuring distances on blueprints it is important to remember that most blueprints are slightly larger or smaller than the original drawings, and site conditions often vary from the plans. In addition, each person has natural tendencies to measure either higher or lower than the actual quantity. I tend to naturally measure about 2% high so I just round the quantities off to the next highest 20' increment. If you don't have any experience to guide you I would suggest adding at least 2% to help offset any variances. Most irrigation material suppliers will allow you to return a reasonable amount of unused materials for credit, so it's not going to cost much more for the convenience of extra materials on hand during the installation. When measuring the mainline it is important to double check for small off-shoots of the mainline that might not be obvious until you look closely at the plans. A couple of examples might be short mainlines leading to hose bibs near a trash enclosure, or quick coupler valves for hand watering baseball diamond infields.
Mainline fittings are the next item to receive my attention. This is where a good understanding of installation methods and an ability to visualize the system on the ground are useful. As each fitting is counted, I circle its location on the plan with a black pen. Again this helps me keep track of what has been counted and what hasn't. For most situations there are any number of fitting combinations that can be used. For example a tee fitting may be needed where a 1" pipe comes in from one end, a 3/4" goes out the opposite, and another 3/4" goes out from the side. In this case there is a tee made with this exact configuration that you could use (1"x3/4"x3/4"). But you could also use a 1"x1"x3/4" tee with a 1"x3/4" reducer in the end outlet. Or you could use a 1"x1"x1" tee with two, 1"x3/4" reducers. This can get pretty confusing when you get out in the field and start trying to remember which combination you planned to use at each location. If you plan to install the system yourself, it's a good idea to note down the fittings in pencil on the plan so that later when you start installing you will use the same fitting combinations. How do you know which fitting size combinations are available? I've listed all the common ones on another take-off form for you! It's available later in the tutorial. Don't want to wait? Click here. What a deal! ;-)
Next I count the lateral line fittings, if needed. Don't forget to circle each fitting location on the tracing paper as you add the fittings to your take-off form so you don't forget what you've counted! For contractors bidding on work I recommend that you NOT make a lateral fitting count. Chances are the system will not be installed exactly as designed due to minor variations in the field. A reasonably accurate estimate of the fitting costs can be made by using a percentage of the lateral pipe costs, usually something in the range of 30-45 percent. Experience will allow you to fine tune this percentage and many estimators become very accurate with time.
In order to accurately measure the quantities of each size lateral pipe, I use a color coding method. For small residential systems this is unlikely to be necessary, but on larger systems it can help preserve your sanity. Using inexpensive children's coloring pens, I trace the entire piping system onto the tracing paper. For each pipe size I use a different color of pen. Although this is time-consuming, I feel that the increased accuracy justifies the effort. (An added benefit is that when people see your plan with all those pretty colors they complement you on your coloring ability. Over the years I've had any number of people suggest I might have a future career in finger-painting.) In the process of tracing the pipes I often come across sprinklers that I missed earlier (I can tell I missed them because they don't have the yellow mark on them). Sometimes I will notice a design error, such as a missing sprinkler head, which I make note of . This way I can point it out to the designer which relieves me of liability for any dry spots and often leads to additional work and payments. Thus tracing the pipe is one of the double checks that I use for accuracy. Now I have a color coded drawing which makes measuring the quantity of each pipe size much easier. On really large plans I use a black pen to divide the plan into smaller sections, and measure the pipe one section at a time. This helps me keep track of what I have already measured so I don't get lost.
At this point, I usually do a quick check of the plan for anything I missed counting and then remove the tracing paper. Then I measure the control wire needed for the automatic valves. The first control wire to measure is the common wire. This wire goes from the controller to the first valve, then on to the next valve, the one after that, etc., until it has reached the last valve. In other words, all the valves are connected to the same wire, which is why it is called the "common" wire. The common wire is a white color for easy identification (this is a universal standard), and each controller requires a separate common wire. When using multiple common wires I use white wires each with a different color stripe to identify the wires. Never connect the same common wire to more than one controller, the power feedback can burn out the circuitry in some controllers! If a spare wire is required it will be the same length as the common wire, so you can save a little measuring. I always install at least one spare wire just in case something happens to one of the wires during installation. That way I don't have to waste lots of time trying to find the break in the original wire, I just use the spare wire.
The next wires to measure are the lead wires. One lead wire goes from each valve to the controller. In other words, there is one lead wire for each valve. Lead wires can be any color other than white. Red and black are the most commonly used colors. As with measuring the pipe I use a small measuring wheel to measure the wire lengths. Be sure to include extra wire for making the connections at the valve and controller! Also make sure there are no stub-outs for future valves shown on the plan. If there are, you will need to include wires for each of the future valves also! For smaller systems a different color wire can be used for each valve. Often residential systems use a multi-wire cable. This cable contains several wires in a single jacket. If you use cable you need a cable with one wire for each valve plus the common. So if you have 5 valves you will need a 6 wire cable. I suggest that you also leave at least one wire in the cable unused for a spare, so you would then want a 7 wire cable.
Next I measure any sleeves needed for wires or pipes under paving. The sleeve size for pipes needs to be twice the diameter of the pipe inside it. Remember that the couplings between pipe sections need to fit into the sleeve also, and they are much larger in diameter than the pipe! For wire sleeves the size varies with the size of the wire and the number of wires in the sleeve. I like to leave lots of extra room to make it easier to feed the wires into the sleeve.
Now I make a quick review of all the notes on the plans to make sure nothing is missed and then move on to the detail drawings. The detail drawings show how various small assemblies are to be constructed. For example a valve detail is often provided on the plans showing exactly how the valves are to be installed. I've provided a number of sample details for you do-it-yourselfers, see the list in the Details section of the Installation Tutorial Table of Contents. Click here. You should find an appropriate detail there to guide your installation. Using these details as my guide I calculate the various fittings, valve boxes, wire connectors, etc., that will be required to install the valves, backflow preventers, sprinklers, etc. Almost every piece of equipment needs related materials to be installed. For example, a typical valve needs short lengths of pipe leading to the valve, inlet and outlet fittings, a valve box, some gravel to line the bottom of the box, and wire connectors for making waterproof wire splices. Again, as a double check I start at the top of the take-off form and work down it one item at a time, checking to make sure I have included all the related items needed to install each piece of equipment.
The last items to go onto my take-off are the many small but necessary miscellaneous materials that can really add up to big costs if left off. These include items like PVC cement, wire splice waterproofing kits, thread sealant, etc. Most of these are listed on the standard take-off form. A final review of the take-off form is then made and the take-off is complete! Of course, I've provided a checklist. Click here. Would you expect any less from me?
When contractors are bidding on large projects I recommend that they use two take-offs, each made independently by a different person, and compare the take-offs to find errors. I have found that simply reviewing someone else's take-off doesn't work, the power of suggestion seems to result in the reviewer obtaining the same errors as the original take-off preparer. I have seen this happen many, many times. Often contractors can get their irrigation material suppliers to provide at least a minimal take-off to use as a double check. There are also outside consultants who prepare take-offs for a fee, as I used to do. They often advertise their services in the classified sections of trade publications.
Finally, when preparing a take-off remember to be careful. Don't hurry. Use a checklist or take-off form to guide you. Double check your figures, and last but not least take a tip from professional accountants and always use a calculator for all of your math (a printing calculator where you can save the tapes is even better)!
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