What does it really mean when the weatherman forecasts a thirty percent (30%) chance of rain?
The following is an explanation as to why the roofing industry has accepted the “Thirty percent (30%)” rule -or chance of rain – as the norm and is subsequently recognized as the standard operation procedure for all legitimate contractors.
The National Weather Service (NOAA) determines the so-called Probability of Precipitation(PoP) as the chance of precipitation occurring at any point you select in the area.
Mathematically, NOAA defines PoP as:
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, if the forecaster knows precipitation will occur (confidence = 100%), he/she is expressing how much of the area will receive measurable rain. However most of the time, the forecaster is expressing a combination of degree of confidence and coverage area.
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 percent of the area, the PoP (chance of rain) is 40%. (PoP = .5 x .8 which equals .4 or 40%.)
So, using the formula above, a weatherman who has a sixty percent (60%) confidence that rain will occur over fifty percent (50%) of a given area will issue a thirty percent (30%) PoP forecast.
Any contractor, who works above this PoP percentage, is in essence, making a coin toss as to whether inclement weather will occur at their site. Adding to the contractor’s dilemma over whether to use this information for a work / no-work decision is that the confidence area (the “where”) of the above formula is not published. They are only provided the PoP.
Further complicating the matter is that a roofing contractor, unlike other specialty trades, cannot simply stop if the weather turns against them. Certain roofing products and often the building’s interior must be protected from moisture contamination and/or intrusion. 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 continuing to perform work after the onset of (sometimes) forecasted inclement weather in an attempt to minimize damage, thereby creating a dangerous working environment and a clear safety hazard and liability for the contactor and property owner.
So, while most people will forgo their umbrella on a day when the weatherman forecasts only a thirty percent chance of rain, your roofing contractor’s decision to delay work initially scheduled should be recognized as a savvy decision reached out of concern for the safety of his crew and your property.