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What a 30% chance of rain means to your contractor...

Your contractor is ever mindful of an unspoken “30% RULE.” The “30 % RULE” is how a contractor determines whether to mobilize the work crews.

To get there, let me explain how weather forecasters determine and assign a probability, and what that probability means.. The following sub-section is a direct copy from the National Weather Service. (Link: )


The “Probability of Precipitation” (PoP) describes the chance of precipitation occurring at any point you select in the area.

Mathematically, PoP is defined as follows:

PoP = C x A, where “C” = the Confidence that precipitation will occur somewhere in the forecast area, and where “A” = the percent of the Area that will receive measurable precipitation, if it occurs at all.

So… in the case of the forecast above, if the forecaster knows precipitation is sure to occur (100% confidence), he/she is expressing how much of the area will receive measurable rain.


But, most of the time, the forecaster is expressing a combination of degree of confidence and areal coverage. If the forecaster is only 50% sure that precipitation will occur, and expects that, if it does occur, it will produce measurable rain over about 80% of the area, the PoP (chance of rain) is 40%. (PoP = .5 x .8, which equals .4 or 40%)

Given the above formula:

· A weather forecaster

  • who has a 60% confidence

  • that rain will occur over 50% of a given area

· Will issue a 30% PoP forecast.

A contractor who sees this PoP covering a work site is, in essence, making a guess as to whether inclement weather will occur at his or her site. Adding to the contractor’s dilemma, the reports are generally silent as to the critical components:

  • Confidence; and

  • Area.

Unfortunately, they only have access to the resulting PoP.

Roofing contractors, unlike other specialty trades, cannot simply stop if the weather turns against them. Certain roofing products and often the building’s interior has to be protected from moisture. It can, at any given point during the construction day, take several hours to temporarily “dry in” a roof. This can result in crew personnel working in inclement weather in an attempt to minimize damage. This creates a dangerous working environment and a clear safety hazard.

Lastly, to address the issue with weather patterns changing from inclement to clear within the course of the construction day. The simple fact is that no contractor can see the future. Roofing contractors generally have to make a work/no-work decision very early in the morning, typically by 6 a.m. or earlier. Due to current labor laws, they cannot instruct employees to “stand-by” to see if the weather clears. This stand-by instruction creates a “lock” on the employee’s time and the employee has to be paid as though they were onsite and working.

So, when your contractor calls it a day, before it’s even started, know there’s some science behind the decision…not just “gone fishing.”

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