Takt time planning and line of balance

Buildings with repeated floors, suites or zones offer a chance that one-off work does not: the same sequence of trades passes through the same kind of space again and again. If each trade takes the same time in each space and moves on in a steady rhythm, work flows like a production line in which the product stays still and the crews move. Takt time planning, developed for construction by researchers such as Iris Tommelein and Hylton Olivieri and widely used on residential, hotel, hospital and office fit-outs, plans this rhythm explicitly. Line of balance, which CON-210 lesson 11 introduced as a way of drawing repetitive work, shows whether trades flow smoothly or collide. This lesson develops both methods using the residential floors of Willowmere Residences.

Takt time

Takt is a German word for the beat or pulse in music. In manufacturing, takt time is the available production time divided by customer demand: if a plant runs 450 minutes a day and customers buy 90 units a day, it must finish one unit every five minutes. In construction, takt time is the fixed amount of time each trade spends in each area before moving to the next. Every trade works to the same takt, so the trades move through the building in step, like the wagons of a train.

Takt planning works in the other direction too. Instead of deriving takt from a deadline, the planner can choose zone sizes and crew sizes so that every trade can finish one zone in the same takt, typically one to five working days. The overall duration then follows from the number of zones and the number of trade steps.

Zones and the takt plan

Takt planning begins by dividing the building into takt areas or zones of roughly equal work content for each trade. On Willowmere, each typical residential floor of twelve suites is divided into four zones of three suites each, giving 7 × 4 = 28 zones. The trades that pass through each zone are grouped into a sequence of steps, called wagons, each of which must be finished in one takt. Where a trade's work in a zone would take longer than one takt, the planner either adds crew members, splits the work into two wagons or makes the zones smaller. Where a trade would take much less than a takt, it can be combined with another trade's work in the same wagon or work across two zones per takt.

With nine wagons and a takt of two working days, the first zone is finished after 9 × 2 = 18 working days, and each later zone finishes two days after the one before. The last of the 28 zones starts its first wagon at day (28 − 1) × 2 = 54 and finishes 18 days later.

# Takt plan for Willowmere residential fit-out: 28 zones, 9 wagons, 2-day takt
zones, wagons, takt = 28, 9, 2
duration = (zones + wagons - 1) * takt
print(f"Fit-out duration: ({zones} + {wagons} - 1) x {takt} = {duration} working days")

# Which wagon is in which zone on a given day (zones numbered 1..28, floor = (z-1)//4 + 2)
def in_zone(day, wagon):
    z = (day // takt) - (wagon - 1) + 1
    return z if 1 <= z <= zones else None
for day in (0, 10, 20, 40):
    row = []
    for w in range(1, wagons + 1):
        z = in_zone(day, w)
        row.append(f"W{w}:{'L' + str((z - 1) // 4 + 2) + 'ABCD'[(z - 1) % 4] if z else '--'}")
    print(f"day {day:>2}: " + "  ".join(row))

# Compare with a conventional trade-by-trade CPM sequence (each trade does a whole floor before moving on)
cpm_floor_days = [6, 8, 8, 4, 8, 8, 6, 8, 10]      # days per floor per trade, with typical start-to-start lags
cpm = sum(cpm_floor_days) + (7 - 1) * max(cpm_floor_days)
print(f"Floor-by-floor schedule with the slowest trade pacing: {cpm} working days")

Fit-out duration: (28 + 9 - 1) x 2 = 72 working days
day  0: W1:L2A  W2:--  W3:--  W4:--  W5:--  W6:--  W7:--  W8:--  W9:--
day 10: W1:L3B  W2:L3A  W3:L2D  W4:L2C  W5:L2B  W6:L2A  W7:--  W8:--  W9:--
day 20: W1:L4C  W2:L4B  W3:L4A  W4:L3D  W5:L3C  W6:L3B  W7:L3A  W8:L2D  W9:L2C
day 40: W1:L7A  W2:L6D  W3:L6C  W4:L6B  W5:L6A  W6:L5D  W7:L5C  W8:L5B  W9:L5A
Floor-by-floor schedule with the slowest trade pacing: 126 working days

The takt plan finishes the residential fit-out in 72 working days. A conventional schedule in which each trade does a whole floor before moving on, with the slowest trade setting the pace between floors, takes 126 working days in this example. The difference does not come from working faster in any zone; it comes from smaller batches (zones rather than floors), a common rhythm that eliminates waiting between trades and balanced crews so that no single trade sets a slow pace. Real comparisons are rarely this dramatic, because good conventional schedules overlap trades too, but takt plans on residential and hospital fit-outs have often reported duration reductions of 10 to 30 % along with steadier crew sizes.

Balancing crews to the takt

The key engineering step in takt planning is balancing each wagon's work to the takt. The planner estimates the worker-hours each trade needs per zone, divides by the takt and the hours in a working day, and sets the crew size accordingly. Where the numbers do not divide evenly, the takt or zone size is adjusted, or work is moved between wagons. Crews sized this way stay constant for weeks, which trades value: they can plan their labor, avoid the boom and bust of conventional schedules and keep the same people on the job, which improves learning and quality.

# Crew sizes needed for each wagon at a 2-day takt (8-hour days), with utilization
hours_per_zone = {"framing": 92, "mechanical": 110, "plumb + elec": 120, "inspect/firestop/insul": 54,
                  "board": 128, "tape": 96, "paint + tile": 118, "millwork": 104, "finish trades": 140}
takt_days, day_h = 2, 8
import math
for wagon, h in hours_per_zone.items():
    crew = math.ceil(h / (takt_days * day_h))
    util = h / (crew * takt_days * day_h)
    print(f"{wagon:<24}{h:>4} h/zone  crew {crew:>2}  utilization {util:5.1%}")

framing                   92 h/zone  crew  6  utilization 95.8%
mechanical               110 h/zone  crew  7  utilization 98.2%
plumb + elec             120 h/zone  crew  8  utilization 93.8%
inspect/firestop/insul    54 h/zone  crew  4  utilization 84.4%
board                    128 h/zone  crew  8  utilization 100.0%
tape                      96 h/zone  crew  6  utilization 100.0%
paint + tile             118 h/zone  crew  8  utilization 92.2%
millwork                 104 h/zone  crew  7  utilization 92.9%
finish trades            140 h/zone  crew  9  utilization 97.2%

Some wagons will have spare capacity at the chosen takt, shown by utilization below 100 %. That spare time is a capacity buffer: crews that finish early in a zone can complete deficiencies, help with preparation for the next zone or work on off-takt tasks. Lean practitioners generally prefer a small amount of spare capacity in each wagon to crews planned at 100 %, for the queueing reasons discussed in lesson 3.

Takt control

A takt plan needs daily control to hold its rhythm. Teams typically hold a short takt control meeting each day at the takt board, where each wagon reports whether it will finish its zone on time. If a wagon is at risk, the team intervenes immediately: shifting workers, clearing a constraint or allowing a short catch-up period. Because every trade depends on the one before it releasing the zone on time, a single late wagon can stop the whole train. Some plans include a buffer zone or a buffer takt between wagons where hand-offs are unreliable, at the cost of a longer duration. Lesson 8 discusses buffers in more detail.

Line of balance

Line of balance (LOB) is an older method, developed for repetitive manufacturing and housing in the mid-twentieth century, that plots each trade's progress as a line on a chart with units or locations on one axis and time on the other. The slope of each line is the trade's production rate: steep lines are fast trades, shallow lines slow ones. When lines are parallel, trades flow at the same rate. When a fast trade follows a slow one, its line converges toward the slow trade's line and they would collide; the fast trade must start later or slow down. When a fast trade precedes a slow one, the gap between them grows, creating work in progress and space between them.

# Line of balance: start times so that no trade overtakes the one before it
# floors 2..8 (7 units); each trade: rate in floors per week; minimum buffer of 0.5 week between trades
trades = [("framing", 1.75), ("rough-in", 1.0), ("drywall", 1.4), ("finishing", 1.0), ("final", 1.6)]
units, buffer = 7, 0.5
start = 0.0
prev = None
for name, rate in trades:
    dur = units / rate
    if prev:
        p_start, p_rate = prev
        # earliest start so the gap at the first unit and at the last unit is at least the buffer
        s_first = p_start + 1 / p_rate + buffer - 1 / rate
        s_last = p_start + units / p_rate + buffer - units / rate
        start = max(s_first, s_last)
    finish = start + dur
    print(f"{name:<10} rate {rate:.2f} floors/wk  start wk {start:5.2f}  finish wk {finish:5.2f}")
    prev = (start, rate)

framing    rate 1.75 floors/wk  start wk  0.00  finish wk  4.00
rough-in   rate 1.00 floors/wk  start wk  0.07  finish wk  7.07
drywall    rate 1.40 floors/wk  start wk  2.57  finish wk  7.57
finishing  rate 1.00 floors/wk  start wk  2.79  finish wk  9.79
final      rate 1.60 floors/wk  start wk  5.91  finish wk 10.29

The calculation shows a basic rule of line of balance. When a trade is faster than the trade before it, the critical point is the last unit: it must start late enough that it does not catch up at the top floor. When it is slower, the critical point is the first unit. The total duration depends heavily on the slowest trades, so balancing rates, by adding crews to slow trades or slowing fast ones to match, shortens the overall schedule and keeps crews working continuously. Takt planning can be seen as a disciplined form of line of balance in which every line has the same slope by design.

Where takt works and where it does not

Takt planning is most effective where work repeats: multi-story residential and hotel buildings, hospital patient floors, office fit-outs, schools with repeated classroom wings, and linear projects such as pipelines and roads. It is harder to apply to unique spaces such as lobbies, mechanical rooms, kitchens and operating theaters, which are usually planned separately as off-takt work. It requires reliable hand-offs, which is why takt and the Last Planner System are often used together: takt sets the rhythm and the Last Planner System makes the work in each zone ready.