Estimating activity durations

Logic tells the schedule what must come first; durations tell it how long everything takes. A beautifully linked network built on guessed durations is still a guess. This lesson shows how construction planners estimate durations from quantities and productivity, choose crew sizes, apply working calendars, allow for weather, and handle activities, such as curing and approvals, whose duration has nothing to do with crews. It builds directly on the quantity takeoff and labour pricing methods of CON-200.

The basic duration equation

For most construction activities, duration follows from three numbers: how much work there is, how fast one crew can do it, and how many crews will work at once.

An equivalent form uses labour-hours, which is how most historical productivity data is stored.

The quantity comes from the takeoff. The productivity comes from the firm's own cost records, trade contractors' experience, or published cost data, adjusted for the conditions of this job. The crew size and number of crews are planning decisions, limited by how many people can work productively in the space and by how many skilled workers the contractor or trade can supply. Durations are normally rounded up to whole working days; nobody plans to finish a wall at 2:15 in the afternoon.

Where productivity data comes from

The best productivity data is the contractor's own, collected from completed projects with similar work, crews and methods. A well-run firm records quantities installed and labour-hours spent by cost code on every job, and the estimating and scheduling departments use the results. Other sources include trade contractors, who know their own crews best and should be asked for durations on their work; published cost guides, which give national average productivity for many items; equipment manufacturers' production data; and the scheduler's experience.

Each source has limits. Company records reflect past conditions, which may not match a congested downtown site or a winter start. Published data are averages and need adjustment. Trade contractors may give optimistic durations to win the work or pessimistic ones to protect themselves. The scheduler's job is to compare sources, ask questions and choose a duration the people who will do the work believe in.

Crew size and the limits of adding people

It is tempting to shorten an activity by adding workers, and up to a point it works. Beyond that point, productivity per worker falls. Workers get in each other's way, waiting for materials or hoisting increases, supervision is stretched, and less experienced people are brought in. Every activity has an optimum crew size, and a maximum beyond which adding people adds cost without saving time. On the site, the limiting factor is often space: only so many drywall hangers can work productively on one floor area, and only one crew can work on a roof slope at a time.

Activities not driven by crews

Many schedule activities have durations that no crew can shorten. They must still be estimated carefully, because they often lie on the critical path.

• Curing and drying: concrete must reach a specified strength before forms are stripped or loads are applied; slabs must dry before moisture-sensitive flooring is installed, which can take weeks or months for a thick slab.
• Procurement: submittal preparation, review and approval, fabrication and delivery. Long-lead items such as switchgear, generators, elevators and custom curtain wall can take many months.
• Approvals and permits: building permit review, inspections, utility connections, and owner decisions.
• Testing and commissioning: system flushing, balancing, functional testing and training.

Procurement durations are best built as a chain of activities rather than one long bar: prepare submittal, consultant review, resubmit if required, approval, fabrication, delivery. Each link can then be tracked, and the review time the contract allows the consultant, often ten working days or so, can be shown.

Calendars

A calendar defines when work can happen: which days are working days, how many hours each day, and which holidays are excluded. Scheduling software assigns a calendar to every activity, and different activities can use different calendars.

• A 5-day calendar of 8 hours per day is typical for most building trades.
• A 6-day or extended-hour calendar may be used for critical work or by contract requirement.
• A 7-day calendar is used for curing, drying and delivery durations that continue over weekends.
• Seasonal calendars stop work that cannot be done in winter, such as asphalt paving or some landscaping, for months at a time.

Holidays vary by province and territory. Canada Day, Labour Day, Christmas Day and New Year's Day are observed everywhere, but Family Day, Saint-Jean-Baptiste Day, Civic Holiday and others vary. Construction collective agreements may add their own holidays, and Quebec's construction industry has two mandatory province-wide construction holiday periods, in summer and at the end of the year, that stop most construction work.

Weather

Weather is the most common cause of lost working days on outdoor construction, and contracts treat it carefully. The usual approach is to estimate the number of working days expected to be lost to adverse weather in each month, from historical climate records and experience, and to build that allowance into the schedule. Contracts often say that the contractor is entitled to more time only for weather that is worse than normal for the location and season.

Canadian contractors use Environment and Climate Change Canada's historical climate data for the nearest station to estimate typical days of heavy rain, snow, extreme cold and high wind by month. Winter construction methods, such as heating and hoarding, concrete accelerators and insulated blankets, let much work continue, but at a cost in money and productivity that the estimate and schedule must both reflect.

Deterministic and range estimates

Most schedules use a single duration for each activity, called a deterministic duration. It should be the planner's honest best estimate of the most likely time, not a target to motivate crews and not a padded figure to protect the contractor. Hidden padding in every activity makes the schedule long and is usually used up anyway, a pattern described by Parkinson's law, that work expands to fill the time available. Hidden optimism makes the schedule impossible from day one.

Where uncertainty is large, planners estimate a range: an optimistic duration, a most likely duration and a pessimistic duration. These three-point estimates are the basis of the PERT method and of Monte Carlo schedule risk analysis, covered in Lesson 6. Even when the schedule itself uses single durations, recording the ranges helps the team see which activities carry the most risk.

Documenting the basis of duration

Durations are assumptions, and assumptions should be written down. A basis of schedule document, often required by owners on larger projects, records for each major activity or group the quantity, productivity, crew size, calendar, weather allowance and source. When a duration is challenged in a review, or when the work goes slower than planned and the team must decide whether the plan or the execution was wrong, the documented basis answers the question.