What Is the Best Time of Year to Replace a Roof in Ontario and the Inland Empire?

Why the Usual Answer Does Not Fit Ontario

Search this question and almost every page says the same thing. Fall is best. Spring is second. Winter is bad because shingles get cold and will not seal. That advice is written for New Jersey, New York and Texas, and some of those pages say so in the title.

The cold weather rule is real. Owens Corning says its shingles use a heat activated sealant strip, and it calls for extra care when air and roof temperatures are at or below 40 degrees (Owens Corning cold weather bulletin). Below that, the shingles may not seal until warmer days arrive.

Now look at Ontario. NOAA's 1991 to 2020 normals for Ontario International Airport put the average December high at 66.9 degrees and the average January high at 67.7 degrees (NOAA NCEI climate normals). The coldest monthly normal low is 43.6 degrees, and that is at night when nobody is on a roof. Our daytime roofs are almost never cold enough to stop a shingle from sealing.

So the snow state logic falls apart here. In the Inland Empire the calendar is set by three other things. Rain, heat and wind.

Rain Is the Real Deadline

Ontario gets about 11.6 inches of rain in a normal year, using the same NOAA normals. The catch is when it falls. December through March brings about 9.2 of those inches, or roughly 79% of the year. February alone averages 3.07 inches. June averages 0.02 inches.

NOAA also counts days with at least a tenth of an inch of rain. Ontario averages about 20 of them a year, and 14 fall from December through March. From May through October there are only about 2 such days in the whole stretch.

That is why the first winter storm is the date to plan around. A tear off opens the roof down to the wood. On a dry June day that is no risk at all. In February a crew has to watch every forecast, open only what it can dry in by evening, and keep tarps ready. It can be done, and we do it on emergency jobs, but it is slower and it costs you more stress.

If your roof inspection says the roof will not make it through another wet season, the best move is to book the job before December. Do not wait for the first leak to make the call for you.

Summer Heat, Shingles and Tile

Summer in Ontario is dry, which is good for roofing. It is also hot. NOAA shows an average of 22.7 days at 90 degrees or more in July, 23.8 in August and 17.5 in September. That is about 75 days from June through September.

Heat changes how a shingle roof goes on. The Asphalt Roofing Manufacturers Association says shingles get softer in hot weather, sealant gets more aggressive, and foot traffic can scuff the surface (ARMA hot weather guidance). Its advice is to install early in the day, do the west and south slopes in the morning, keep bundles out of direct sun and wear soft soled shoes. A careful crew follows that. A careless one leaves scuff marks you will see from the street.

Concrete tile is different. The tile does not care about heat. What matters on a tile re-lay is the underlayment under it. ARMA notes that self adhered membranes get more aggressive in heat, so they need more care to lay flat and straight. Ask how your crew handles that on an August afternoon.

The heat is also a safety rule. Cal/OSHA requires shade for outdoor workers when it is over 80 degrees and high heat procedures at 95 degrees and up (Cal/OSHA heat illness prevention). Ontario's normal July and August highs are 93.8 and 94.9 degrees, right at that line. Expect a summer crew to start at dawn and slow down in the afternoon. That is a sign of a crew that does it right, not a slow one.

Our take. Summer is a fine time for a tile re-lay or a flat roof. For shingles it works too, as long as the crew works mornings. If you have a choice, May and June give you dry weather without the 95 degree days.

Santa Ana Winds Run Through the Winter

Most national pages never mention wind. Here it matters. Two climate researchers wrote in a 2014 American Meteorological Society paper that Santa Ana season usually starts in September and ends in May, with the most frequent winds in winter, especially December (Hailey and Gershunov, AMS 2014). A National Weather Service meteorologist in San Diego told Cal OES in October 2023 that the winds do not really peak until December (Cal OES).

Wind hits a roof job two ways. Loose underlayment and stacked tile can lift during a tear off. And a roof that is old and brittle loses ridge caps, tiles and shingle tabs in the first big blow. Fontana, north Rancho Cucamonga and north Upland feel it first.

October and November are still a good window, but book around the wind forecast. A good roofer keeps open sections small on windy days and pauses tear offs during a Red Flag Warning. If the wind already took pieces off your roof, call for an emergency tarp and get on the schedule before the rain.

Permits, Scheduling and Lead Time

Most full roof replacements need a city permit. The City of Ontario now runs detached single family reroof permits through an automated platform with Symbium, which the city calls a streamlined process (City of Ontario reroof and solar permits). Rancho Cucamonga, Upland, Chino, Fontana and the other cities each run their own building departments, so the roofer should pull the permit and tell you the inspection steps.

Plan for more than the permit. Tile color has to match what your HOA approved. Special order tile or trim can take time to arrive. And every roofer in the region gets the same rush of calls after the first storm of the season. It happens every year.

So the simple plan is this. Get your estimate in late summer or early fall. Pick your material and get HOA approval if you need it. Then book the work for October or early November, or for spring if your roof can safely last one more winter. Use the checklist below to stay on track.

When Not to Wait for the Best Month

A good month is a nice to have. A dry house is a must. If water is coming in, if tiles or shingles are missing, or if you can see daylight in the attic, do not wait for May.

Stop the water first with a proper tarp. Then get a written plan for the full job. A tarp buys you time to plan the replacement right. Waiting a season on an open roof costs far more than any savings from a slow month.

If you want a free written estimate on a roof replacement before the rain comes, call Ontario Roofing at (909) 404-9464.