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Critical Path Method

 

Here's more details on how the Critical Path Method works and how you can develop one for your own home building project.

First let's review why you would even want to do this! Because it's going to reveal to you the sequence of home building events - which activities follow which other activities - and which activities can be going on at the same time.

Then you put some time durations on each home building activity and you'll have a timeline for your project and a scheduling tool par excellence! Follow along. You're going to enjoy this. And you're going to be really surprised at what a valuable tool it will be when construction begins and you are managing the construction on a day-to-day basis.

 

A CPM EXAMPLE


In a CPM diagram, each activity is represented by an arrow. Each activity must have a beginning (either at the beginning of the project or at the end of some preceding activity) and an end (either just before the beginning of some succeeding activity, or at the end of the project).

The following figure illustrates this principal. Note: It's just an example! We know it's not all the activities you'll need to build your new home.

The activities shown are:

  • Clear Lot
  • Pour Footings
  • Frame House
  • Insulate
  • Drywall
  • Paint
  • Exterior Siding/Roofing

 

ACTIVITIES AND EVENTS

 

The activities are represented by the arrows which begin and end with the circles 0, 10, 20,30, 40, 50, and 60. The circles are called “events”, and simply designate the point in time at which one activity has ended and another is about to begin. The numbers in the event circles are arbitrarily assigned and have no meaning other than their use to name the arrows (activities). For example, the “Clear Lot” activity is called “Activity 0-10.” The project starts at the circle labeled “0” and proceeds to the right.

POSITION AND DEPENDENCY
By their position in the diagram, the dependency of the activities is shown. For example, it is obvious that the footings can not be poured until the trees have been cleared from the area of construction.

Similarly, the house can not be framed until a footing has been poured. Exterior siding and roofing can take place at the same time as insulating, hanging drywall, and painting, but not until framing is completed.

Activities that happen at the same time (roofing and drywall) are shown as parallel arrows. Assuming the project is completed at “60”, all activities must be completed by then.

ACTIVITY DURATION
Notice the number under each arrow. This represents the number of days it will take to complete each activity. You will see that insulating, hanging the drywall, and painting will take a total of 5 days, while exterior siding and roofing will only take 2 days.

It would be possible to wait three days after the framing is completed before starting on the siding and roofing and still complete the project on time.

FLOAT TIME
There is said to be three days of “Float Time” in activity 30-60. That is because while activity 30-60 will only take two days to complete, it is scheduled to be done sometime during the five day period when insulation, drywall, and painting are being completed.

Activity 30-60 doesn’t have to be started immediately after the framing is completed. You could wait until after the first day of drywall work before starting 30-60 without holding up the job.

 

CRITICAL ACTIVITIES
Any activity which has float time is not on the critical path. All other activities in our example are “critical.” A delay in completing any of them will delay the completion of the job. Notice that if the float time in activity 30-60 is used up, then that activity also becomes critical.

The Critical Path is that path through the Critical Path Diagram which links critical activities sequentially. A delay in any activity on the Critical Path will delay completion of the job.


A TYPICAL CPM PRE-CONSTRUCTION
ACTIVITIES




The following activities take place prior to construction:

  • Purchase Lot
  • Plans and Specifications
  • Estimate
  • Secure Construction Loan
  • Secure Building Permit(s)
  • Line Up Subs and Suppliers

 

The time required to accomplish these things could vary considerably from situation to situation, so we have shown their relationship to the process in their own diagram below. No time frame has been assigned to these activities. You may take six days or six months to find a lot.

It really doesn’t matter. But once you start construction, it should proceed in a consistent, well organized manner.

Note that activities 0-10 and 10-15 could be reversed. In other words, you could start with a lot and design a home to fit it, or you could design your home and then find a lot that will accommodate it

 

A Real Live Critical Path Method Diagram


The following is a list of the more important activities you would encounter in building a typical home. We have assumed that this home will be a framed home on a crawl space, with a prefab fireplace, and connections to municipal water and sewer.

For the most part, the tasks are self-explanatory. That is, when we say “pouring the footings” or “framing the house,” you may not know everything that is involved in those tasks at this point, but you generally know what is being talked about.

That’s all that is necessary for now - knowing that pouring the footings and framing are part of the process, discovering about how long these tasks should take, and finding how they fit into the overall process of construction.

ACTIVITIES

 

  • Lot Preparation (clearing and rough grading)
  • Footings
  • Foundation
  • Soil Treatment (spraying to prevent termites)
  • Framing (wood structure of the home plus sheathing)
  • Roofing
  • Set Windows and Exterior Doors
  • Cornice, Veneer, and Exterior Trim
  • Exterior Painting
  • Gutters and Downspouts
  • Set Fireplace
  • Rough Plumbing (pipes in slab, pipes in walls)
  • Rough Electrical (wiring in walls, switch and outlet boxes installed)
  • Rough HVAC (ducts in walls)
  • Telephone Pre-wire (telephone wire in walls)
  • Framing Inspection
  • Insulation
  • Drywall
  • Interior Doors and Trim
  • Appliances
  • Cabinets
  • Ceramic Tile
  • Painting
  • Hang Wall Paper
  • Finish Plumbing (water heater, showers, tubs, sinks, faucets)
  • Finish Electrical (light fixtures, switches, outlets, fans, smoke detECTOR)
  • Finish HVAC (furnace, a.c., air handler, registers, thermostat)
  • Glass and Hardware
  • Floor Coverings
  • Walks, Drives, and Patios
  • Deck
  • Finish Grading (final elevations set, trash removed)
  • Landscaping
  • Final Inspection
  • Punch Out (all remaining uncompleted and defective work finished)
  • Turn Utilities On

 

The list itself more or less shows the order in which activities occur. But it falls short of showing which activities are dependent on each other and which activities can occur simultaneously. To discover these relationships, we can put them into a CPM diagram.

THE CPM DIAGRAM
To see the CPM diagram for the construction of the home described by the construction activities listed above, click here.

The following notes will help you see how the diagram was constructed and how it can be used to help you schedule activities, deliveries, etc.

1. In a real CPM, like the one you just opened, the arrows are drawn to a length that is proportional to the time involved in the activity. In other words, an arrow representing an activity that takes four days to complete is drawn twice as long as the arrow for an activity lasting two days.

The vertical lines on the grid behind the CPM represent days, and are numbered at the top of the page. We’ll use these later to keep track of where we are in the construction process, and for planning future activities.

Arrow 10-15 (Foundations) is five units long, since it is scheduled to last five days. Similarly, 20-25 (Framing) is drawn nine units long. Arrow 0-20 (Temporary Electric Service) is drawn thirteen units long even though it only takes one day to do the job. There are twelve Float Days in this activity.

In other words, getting the temporary electric service to the job site can take place any time during that thirteen day period preceding framing. Note that one day is the least amount of time shown for any activity. In reality, treating the soil for termites (15-20) will only take an hour or so!


2. Notice that the construction diagram shows the home being built in a total of 59 working days (about twelve weeks). There are no idle days shown - that is, days when the home sits with no work being done on it. You’ll probably have some of these days! You may also experience some delays because of materials shipments, inspectors, subcontractors that don't show up, and bad weather.

So don’t be surprised if you take longer than 59 working days. This is an “ideal.” If you take twice as long, it’s OK. Obviously this is a small house. Probably one this builder and his regular subs have built before. Your house will probably take six months to a year to complete. We just chose an example that would fit in the illustration!

Take a look at this builder. He built homes on a 12-Day schedule!


3. The gray arrows (40-45) are called “dummy” arrows. They do not represent any activity or any passage of time. They either show dependence or are used to prevent two activities from having the same number. Arrow 50-55 is a dependency arrow.

It shows that the exterior doors and windows should be set before you insulate (so insulation can be stuffed into those narrow spaces between the door and window frames and the 2x4 studs). Therefore it is said that activity 55-60 (insulation) is dependent on activity 25-50 (Exterior Doors and Windows).

It would be more accurate to say that the starting of activity 55-60 is dependent on the completion of activity 25-50. Arrow 40-45 is an example of the other type of dummy. Without 40-45, “Rough Elec” and “Telephone Pre-wire” would have the same number (40-55).


4. The usual schedule for inspections and surveys is indicated. The footing inspection is usually performed after the trench has been excavated and any form materials and steel are in place, but prior to pouring the concrete.


UNDERSTANDING THE CPM DIAGRAM
Careful study of the CPM Diagram will help you develop an understanding of the steps necessary to build your home, and how they all fit together. Most of the “activities” listed above are actually several activities grouped together under one heading. For example, “Footings” is actually comprised of:

1. Lay out the house.
2. Dig the trenches.
3. Place form materials (if required).
4. Place steel (if required).
5. Footing inspection.
6. Pour concrete.

As the contractor you will only need to schedule the beginning of the task. The subcontractor who actually does the work will take care of scheduling the sub-tasks.

 

DIAGRAMS FOR DIFFERENT CONSTRUCTION METHODS


The CPM diagram shown above would differ slightly from one involving say masonry construction, or a masonry fireplace, or a well and septic tank, or a slab instead of a crawl space. An example of an alternate diagram to cover a slab floor system is shown here.

Notice that only five days are shown here for framing, whereas on the first one there were nine days allotted for framing. That’s because with slab construction, there is no first floor to frame. So you will save three or four days.

 

DEVELOPING YOUR OWN CPM DIAGRAM


O.K. You understand how the Critical Path Method Diagram works. Sort of! The best way to really understand it deep down is to develop one for your own project. But how do you do that? Well, to start with here's a blank CPM grid sheet you can use.

But how do you discover which activities will be required to build your house? And how long will each activity take? You get all this good stuff from your subcontractors and suppliers as you are interviewing them and preparing your cost estimate!

You simply ask questions, like, "Exactly what is involved in your part of the project?" And, "How many days should I plan to schedule for you to be on the job?" And, "What trades must be finished with their work before you start. Which ones can be there at the same time you are on the job? Which one will come in after you finish your work?" And, "What's the lead time for ordering my windows?"

Believe me, your subs and suppliers will know the answers to these questions. They can give you all the information you need to complete your own Critical Path Method diagram.

If you want to try your hand at developing a Critical Path Method Diagram, just make several photocopies of the blank grid sheet and tape them together. Take your time with this. If you get stuck, call the sub or supplier involved. You don't need to tell them you're doing a CPM diagram. They probably wouldn't know what you were talking about! Just tell them you're doing some "schedule planning."

Try to get it as accurate as possible. You'll be using this plan all through the home building process to see where you are going, where you have been, and to schedule materials and subcontractors.

Good luck!


Whew! Some of that was kind of complicated, wasn’t it?
Well, the good news is that you don’t really have to understand it all or use it to build your home. The Bar Chart is an easier concept to understand and use, although it doesn’t give you as much information as the CPM.

For the first time, you were able to see all of the tasks involved in building your home, and how they all fit together. By plugging this information into a chart, we were able to develop a workable schedule that let us put some dates to the project.


For additional information on the Critical Path Method,
see Lesson Nine of our online course
Successful Home Contracting
.



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