How the weather here wears a roof

A roof near Conshohocken goes through about 64 freeze-thaw days in a typical winter and takes about 44 inches of precipitation a year, on NOAA’s 1991 to 2020 records for Philadelphia International Airport. From 1996 through 2025 the airport also logged about four days a year with wind gusts of 50 mph or more. Hail is less frequent. NOAA’s Storm Events Database lists 116 hail reports across all of Montgomery County from 1996 through 2025.

Where the numbers come from

The longest daily weather record near Conshohocken comes from Philadelphia International Airport, NOAA station USW00013739, which goes back to 1940. The station sits at an elevation of 7 feet, about 15 miles southeast of Conshohocken by the station coordinates in ACIS. NOAA’s Cooperative Network station in Norristown, USC00366370, is under 3 miles from Conshohocken and has kept daily temperatures since 1951. Its record has gaps, including no complete winters from 1985-86 through 2000-01. This guide uses the airport for long-run figures and Norristown as a check on how the area closer to Conshohocken compares.

The daily temperature and rain figures come from the Applied Climate Information System (ACIS), and the wind gusts from NOAA’s National Centers for Environmental Information (NCEI). Hail and wind reports come from NCEI’s Storm Events Database, which NCEI describes as the National Weather Service’s record of weather intense enough to cause deaths, injuries, significant property damage or disruption to commerce. The database runs through June 2026.

Freeze-thaw days

Water expands when it freezes. The Great Lakes Integrated Sciences and Assessments program (GLISA) at the University of Michigan says repeated freezing, thawing and refreezing can damage roads, sidewalks and other outdoor surfaces. Researchers count freeze-thaw cycles in more than one way, and GLISA lists a common definition as a day when the high is above 32°F and the low is below it. This guide counts a freeze-thaw day as one where the low reached 32°F or colder and the high rose above 32°F. A winter runs from July through June.

At the airport, freeze-thaw days have become less common over the 86 winters on record.

The fewest in the whole record came in 2015-16, with 39. Last winter, 2025-26, had 70, the most since 2018-19, and its December brought 28, the most of any month in the record. The article on December’s record has the details.

The airport likely undercounts what a house near Conshohocken goes through. In the 32 winters since 1952-53 when Norristown is missing no more than three days between October and April, the Norristown station averaged 84.0 freeze-thaw days against 71.3 at the airport in the same winters. In 2022-23, the most recent of those winters, Norristown counted 88 and the airport 50. Norristown’s readings are taken in the morning, so its days end at a different hour than the airport’s, which run from midnight to midnight.

The National Park Service’s guide to slate roofs says slate weathers mainly when impurities in the stone react during wet and dry, hot and cold cycles, and that slate’s low porosity limits the damage frost does to the stone itself. The Metal Roofing Alliance says concrete tile roofs can crack and warp in the freeze-thaw cycle of northern climates. Conshohocken’s re-roofing addendum requires ice and water shield, a waterproof underlayment, to run at least 24 inches inside the exterior wall line.

Rain

Precipitation, which counts rain and the water in melted snow, averaged 44.1 inches a year at the airport from 1991 through 2020. From 1941 through 1970 it averaged 40.1 inches. Days with an inch or more averaged 11.0 a year in the later period and 9.8 in the earlier one. Days with 2 inches or more went from 1.6 a year to 2.2. The wettest single day in the airport’s record since 1941 brought 8.02 inches, in 2013.

NRCA, the National Roofing Contractors Association, tells owners to pay attention to valleys, gutters and roof drains, since debris collects there and slows drainage. The Park Service brief tells owners of leaking slate roofs to check gutters, valleys and flashings before assuming the slates are the source.

Wind

The airport has recorded each day’s fastest 5-second wind speed since January 1996, and this guide calls those readings gusts. From 1996 through 2025 it averaged 22.7 days a year with a gust of 40 mph or more and 3.7 days with a gust of 50 mph or more. The strongest gust in those 30 years was 74.9 mph, in 2010. 2025 had the most days at both thresholds in those 30 years, with 47 days at 40 mph or more and 12 at 50 mph or more.

The Storm Events Database lists 516 thunderstorm wind reports in Montgomery County on 207 days from 1996 through 2025. Of those, 351 reports on 91 days came from 2016 through 2025.

Hail

The database lists 116 hail reports in Montgomery County from 1996 through 2025, on 65 separate days. Sixty of those reports fell on 23 days from 2016 through 2025. The reports closest to Conshohocken in the list include 0.88-inch hail at Plymouth Meeting on June 11, 2007 and 1.75-inch hail at Bala Cynwyd on June 19, 2002.

These are counts of reports, so they depend on someone seeing the hail or the wind damage and reporting it. A change from one decade to the next can come from more people reporting as well as from more storms.

After a storm

NRCA recommends inspecting a roof after severe weather such as a hailstorm, in addition to its twice-yearly spring and fall inspections. It does not recommend that homeowners walk on a roof unless they have the experience and equipment to do it safely. The roof inspection checklist lists what to look for from the ground. For insurance after storm damage, see the guide to storm damage claims.