How Crews Build Heat Tolerance

It’s Monday morning and two workers are assigned to the same roof. One is new to roofing. The other knows the job well but has spent the last week away from hot work. Before either heads up the ladder, the foreman has a decision to make. How much time should each worker spend in the heat today?

The new worker needs time to build heat tolerance, and the returning worker may need time to regain it. Even an experienced crew may need a gradual increase in exposure when the first heat wave of the season arrives.¹²

What happens as your body gets used to the heat?

Heat acclimatization is the process of adapting to repeated work in hot conditions. As workers adapt, they can start sweating earlier, produce more sweat with a lower salt concentration, and send more blood to the skin at the same core temperature. Over time, the same work can put less strain on the body, with a lower heart rate and core temperature.¹²

An experienced worker can begin losing those adaptations after a week or more away from hot work. A typical weekend away generally leaves heat tolerance intact. And taking a break in air conditioning during the shift won’t undo acclimatization.²

What could the first week look like?

The National Institute for Occupational Safety and Health (NIOSH) recommends building up heat exposure gradually over 7 to 14 days. For new workers, it recommends starting at no more than 20% of the usual time working in the heat, then adding no more than another 20 percentage points of that usual duration each day. Workers with previous experience doing the job can generally follow a faster progression.²³

Say the usual assignment involves five hours of work in the heat during an eight-hour shift. Here’s what the NIOSH percentages would look like for the two roofers.²³

Day	New worker	Worker with prior experience on the job
Monday	20% (1 hour)	50% (2 hours 30 minutes)
Tuesday	40% (2 hours)	60% (3 hours)
Wednesday	60% (3 hours)	80% (4 hours)
Thursday	80% (4 hours)	100% (5 hours)
Friday	100% (5 hours)	100% (5 hours)

The percentages apply to the usual time working in the heat, so Monday’s 20% is one hour in this example. The foreman could split that hour into shorter blocks with cool recovery between them and use the rest of the shift for ground-level tasks in a cooler area. Treat the table as an example of the fastest progression under this guidance. Some workers need more time, and everyone still needs a work and rest schedule suited to the heat, workload, and protective gear.¹²³⁴

How can a foreman put it into practice?

• Check who’s coming into the heat. At the morning huddle, identify new workers, people returning from time away, and crews facing a sudden jump in temperature.¹²

• Assign the cooler work in advance. Decide who will rotate off the roof and what they’ll do next. Set up cool drinking water and a nearby place for recovery before the shift begins.¹³

• Pair workers up and check in. Use a buddy system and closely supervise new workers during the first two weeks or until they’re fully acclimatized. Adjust the plan if conditions get hotter or the work gets more demanding.¹³⁴

Symptoms override the schedule. If a worker develops dizziness, nausea, or unusual weakness, stop work, move them to a cool area, and get help. Stay with them and arrange medical evaluation for suspected heat exhaustion. Confusion, slurred speech, or loss of consciousness can signal heat stroke. Call 911 and start rapid cooling immediately.⁵

Before Monday’s shift, write down who needs a gradual start, how long they’ll work in the heat, and where they’ll go to cool down. That gives the crew a plan they can follow from the first assignment.

Sources

1. Occupational Safety and Health Administration (OSHA). Heat: Protecting New Workers.

2. National Institute for Occupational Safety and Health (NIOSH). Acclimatization.

3. National Institute for Occupational Safety and Health (NIOSH). Workplace Recommendations.

4. Occupational Safety and Health Administration (OSHA). Heat Hazard Recognition.

5. National Institute for Occupational Safety and Health (NIOSH). Heat-related Illnesses.