Roofing
- The highest heat exposure in construction. Dark membrane and torch work add radiant and process heat on top of full sun.
- No shade, no wind break, and the surface radiates upward into the body all day.
Construction leads U.S. industries in heat-related deaths, and the reasons are structural rather than incidental. The work is heavy and sustained. It happens on surfaces that absorb and re-radiate solar energy all day. Protective equipment blocks evaporation. Crews turn over constantly, so there are always unacclimatized workers on site. And schedule pressure runs directly against taking the breaks that prevent harm.
A WBGT reading is the starting point. On a construction site it is rarely the finishing point, because the site generates heat the ambient reading cannot see.
#1Industry for U.S. heat-related worker deaths
First weekWhen most construction heat fatalities occur
+30°FSurface temperature of dark roof membrane over air
1 cup / 20 minBaseline water intake target in heat
Trade by trade
The same site reading means very different things across the trades working on it.
Concrete
- Heavy sustained work, often starting early and running through the hottest part of the day because the pour cannot wait.
- Fresh concrete generates heat as it cures, and light finished surfaces reflect sunlight back up.
Steel erection and welding
- Metal absorbs and re-radiates solar heat; welding adds intense local radiant load.
- Leathers and welding gear block evaporation substantially.
Excavation and underground
- Trenches cut off wind and reflect radiant heat off the walls.
- Equipment exhaust adds heat in confined areas with no air movement.
Why the site beats the forecast
Surfaces
Dark roofing can run 30 to 50°F above air temperature; light concrete reflects solar radiation upward into workers from below.
No wind at the work face
Trenches, shafts, partially enclosed structures and the lee side of a building all lose the air movement the forecast assumes.
Equipment and process heat
Generators, compressors, torches, asphalt plants and curing concrete all add load no weather model knows about.
PPE
Hard hats, high-visibility layers, long sleeves, fall harnesses and respiratory protection each cut evaporative capacity.
Clothing adjustment factors
Clothing that blocks evaporation raises the effective WBGT. Add the adjustment to your measured reading before comparing it with a limit.
| Clothing | Add to WBGT |
|---|---|
| Work clothes, long sleeves and trousers | 0°C |
| Cloth coveralls | 0 to 2°C |
| Double-layer woven clothing | 3°C |
| SMS polypropylene coveralls | 0.5°C |
| Polyolefin coveralls | 1°C |
| Limited-use vapor-barrier coveralls | 11°C |
That last row is not a typo. An encapsulating vapor-barrier suit shuts down evaporative cooling almost completely, and WBGT screening stops being adequate at that point. Physiological monitoring takes over.
Scheduling around the heat
The controls that work on a construction site are mostly scheduling controls, because the environment itself is not negotiable.
- Move the heaviest work to the first hours of the shift. WBGT peaks in mid to late afternoon, later than air temperature.
- Consider shifting start times earlier during heat waves rather than shortening the day at the end.
- Rotate crews through the hottest tasks instead of leaving one crew on a roof all day.
- Mechanize the heaviest tasks where possible. Reducing metabolic load moves the threshold more than anything else available.
- Put shade and water at the work face, not at the gate. Ten minutes of walking means workers will not go.
- Build the acclimatization ramp into the schedule for new crews rather than treating it as something to catch up on.
What a heat illness prevention plan contains
Whether or not a specific standard applies to you, this is the structure regulators, insurers and plaintiffs' attorneys all expect to see.
- Measurement. Who takes the reading, where, how often, and how it is recorded.
- Trigger levels. What changes at each threshold, written down in advance rather than decided in the moment.
- Water. Cool potable water available at the work area, in quantity, with a target of roughly one cup every 15 to 20 minutes.
- Shade and rest. Shade available at or before the trigger, large enough for everyone on break at once, and rest that is actually taken.
- Acclimatization. A graded schedule for new workers and anyone returning after more than a week away, with closer supervision during it.
- Training. Symptoms, reporting, first aid and the emergency plan, for workers and supervisors, in a language they understand.
- Emergency response. Who calls, what the site address is, how responders reach the work area, and who starts cooling in the meantime.
Warning signs of heat illness
Heat stress shows up before it becomes an emergency. Anyone on site should speak up immediately on any of these:
Dizziness or lightheadedness
Nausea or vomiting
Chills or goosebumps in the heat
Severe or throbbing headache
Confusion or out-of-character behavior
Stumbling, cramping or collapse
When in doubt, cool first and call for help. Collapse or confusion in the heat should be treated as possible exertional heat stroke: begin cold-water immersion immediately and call 911. Cooling comes before transport.
WBGT for the jobsite
Check conditions for a specific site before mobilizing, and use the 14-day forecast to sequence the pour, the tear-off or the deck before the week starts rather than reacting to it on the day.
- OSHA mode for occupational thresholds
- Any jobsite location or city
- 14-day forecast for scheduling heavy work
- Hourly detail for planning the shift
WBGT is estimated from weather data with the peer-reviewed Liljegren model. Where a policy requires on-site measurement, use the app for planning and as a cross-check alongside your instrument.
Common questions
What WBGT is too hot for construction work?
For heavy work, the ACGIH limit for acclimatized workers is 27.5°C, or 81.5°F, even with 25 to 50% rest each hour. Clothing adjustments push the effective reading up, so a site measuring in the low 80s can already be over the line for heavy work in coveralls.
Why is construction the highest-risk industry for heat?
Heavy sustained work, surfaces that radiate heat, protective equipment that blocks evaporation, constant crew turnover meaning a steady supply of unacclimatized workers, and schedule pressure that discourages breaks. The factors compound.
How much hotter is a roof than the air temperature?
Dark roofing membrane commonly runs 30 to 50°F above air temperature in full sun and radiates that heat upward into anyone working on it. No weather-based WBGT estimate accounts for this, which is why roofing work needs on-site measurement.
What is the acclimatization rule for new construction workers?
No more than 20% of normal workload in heat on day one, increasing by up to 20% per day. For experienced workers returning after more than a week away, start at 50% and build to full over four days. Most construction heat deaths occur in the first week.
Does a WBGT app work on a construction site?
For planning, scheduling and awareness, yes. For the compliance reading on a roof, in a trench or near a hot process, use an on-site instrument, because those environments run well above ambient and no weather model can see them.
Related WBGT guides
Know the real heat number
Real-time WBGT, flag levels and forecasts. Free on iPhone.
