When to Pave Asphalt in St. Louis and the Metro East

The short answer for St. Louis

Around St. Louis, the practical season for hot-mix asphalt usually runs from about April into mid-November. The most dependable stretch is May through early October, when days are warm, nights stay well above freezing and the ground has dried out from winter. Paving at the edges of that window can work on the right day, with less margin for error.

The weather record explains why. NOAA's 1991 to 2020 climate normals for St. Louis Lambert International Airport show an average high of 68.0°F in April, 77.1°F in May and 81.1°F in September, falling to 69.2°F in October and 55.5°F in November. Even around November 15, the normal daily high is still about 56°F. January averages a high of just 40.4°F and a low of 23.8°F. Outlying towns and low river-valley spots can run a few degrees colder on clear, calm nights, so treat these numbers as a guide.

Why temperature matters: the compaction window

Hot mix is stone, sand and liquid asphalt binder, heated at the plant and hauled to your job hot. Rolling squeezes out air and locks the stone together, and that density keeps water out and the surface from raveling. Once the mix cools past a certain point it gets too stiff, and more rolling does little good. Pavement Interactive puts that cutoff at about 175°F for dense-graded hot mix. The minutes between laydown and that temperature are the compaction window.

That window depends on layer thickness, mix temperature, air temperature, wind, sun and the temperature of the base underneath. The base is the one people forget: cold ground pulls heat out of the bottom of the mat while cool air and wind pull it from the top. Minnesota DOT offers a tool called PaveCool that was built for exactly this problem. It estimates how long a lift takes to cool from its delivery temperature to 175°F.

Modeled examples show how tight it gets. In sample MultiCool runs published by Pavement Interactive (a clear 10 a.m. start, a 10 mph wind, and air and base at the same temperature), a 2-inch lift delivered at 250°F had about 16 minutes to reach 175°F at 60°F, and about 12 minutes at 25°F. A 4.2-inch lift at the same mix temperature had about 39 to 54 minutes. A 1-inch lift had only 7 to 9 minutes, even delivered hotter at 300°F. Driveway and parking lot surface layers sit at the thin end, so a cold, windy, cloudy day hits them hardest.

What MoDOT and IDOT require

State highway specifications are a useful floor. MoDOT's standard specifications for asphalt pavement (Section 403) say no mix goes down on a wet or frozen surface, or when the air or the surface being paved is below 40°F.

MoDOT is stricter about chip seal, which its specifications call a seal coat: air and pavement must both be above 60°F, and none goes down when the National Weather Service forecast calls for temperatures below 40°F within 24 hours.

On the Illinois side, IDOT's inspector checklist for hot-mix binder and surface courses asks for an air temperature in the shade of at least 40°F and rising for binder courses and 45°F and rising for surface courses.

These are minimums written for highway projects with inspectors on site. A thin driveway surface on a cold morning has less room for error, so it makes sense to build in more cushion: a forecast in the 50s or warmer and rising, a dry base and light wind.

How hot-mix plants run through the year

Contractors buy hot mix from asphalt plants, and in regions with real winters, plants typically slow down or shut down for the coldest months. Tennessee DOT, for example, says asphalt plants there are closed during the winter months and hot asphalt is not available until spring, so its crews typically patch potholes with cold mix in the meantime.

Plan around the same general pattern here. With a January normal high of 40.4°F and state specs that stop paving below 40°F, midwinter hot-mix work rarely makes sense. Plants typically ramp up as spring road work starts. Exact dates vary by plant and by year, and a mild spell can extend the season on either end.

Cold patch is a stopgap for potholes until hot mix is available again. TDOT notes cold mix is not always as durable, and that patching material does not stick as well when conditions are cold or wet.

Rain, wet ground and the base under your asphalt

Temperature is only half of it. Hot mix should not go on a wet surface, and a soaked base will not compact the way it should. Spring is the wettest stretch of the year here. At Lambert, May averages 4.82 inches of rain and April 4.73 inches, and May sees measurable rain on about 13 days. September is the driest month of the paving season at 2.96 inches, with measurable rain on about 7 days, the fewest of any month.

Your asphalt is only as good as the base under it. After heavy rain, base rock and the soil beneath it, clay especially, can stay soft for days. A careful crew checks for soft spots first, often by running a loaded truck over the base and watching for movement. Waiting a couple of dry days costs far less than paving over a wet spot that settles next spring.

• Wait if water is standing on the base or the old pavement.
• Wait if base rock ruts or pumps under a loaded truck.
• Wait if rain is forecast during laydown and rolling.
• Wait if there is frost, or the air or surface is below 40°F.

Month by month around St. Louis

How the year typically plays out, using NOAA's 1991 to 2020 normals for Lambert:

• December to February: normal highs of about 40 to 46°F and roughly 19 to 24 freezing nights a month. A good time for estimates and planning.
• March: normal highs near 57°F, lows near 37°F and about 10 freezing nights. The ground is often cold and wet, so thin surface paving is a gamble.
• April: normal highs near 68°F. The median last spring freeze is about April 2, and in about 1 year in 10 the last freeze comes on or after April 17. Paving picks up on dry, mild stretches.
• May and June: normal highs near 77 and 86°F. Good paving temperatures, but May is the wettest month of the year (4.82 inches), so expect some rain delays.
• July and August: normal highs near 90 and 88°F, with about 16 and 13 days of 90°F or hotter. Long compaction windows.
• September and October: normal highs near 81 and 69°F, and September has the fewest rainy days of the year. Often the best paving weather, so fall calendars tend to fill.
• November: normal highs near 56°F and lows near 37°F. The median first fall freeze is about November 4, with a 1 in 10 chance of one by October 23 and a 9 in 10 chance by November 18. Thin surface work gets riskier each week.

Paving in summer heat, and caring for new asphalt

Heat is not a reason to avoid paving. It gives the crew a long compaction window. The tradeoff is that fresh asphalt stays soft longer. MoDOT's specifications keep traffic off new asphalt until the surface cools to 140°F or below, and a dark surface in July sun can take a while to get there.

For a new driveway, keep cars off for typically about 3 days, longer in a heat wave, and keep heavy trucks off longer still. Turn the wheel only while the car is rolling. Through the first summer, set plywood under kickstands, trailer jacks and dumpster rails. Sealcoat comes later: new asphalt usually cures for months, not days, before it is sealed.

Planning and booking: work backward from your date

Paving schedules are usually set weeks ahead, so plan backward from your date. For spring or early summer paving, get estimates in late winter, around February or March. For fall paving, call by mid to late summer. The estimate visit should cover drainage, the base, truck access and how the new surface meets your garage and the street.

Some cities and counties require a permit for a driveway approach or work in the public right-of-way, and rules vary by town, so check early. HOA approval can add weeks. For a parking lot, phase the work around business hours and plan the restriping. ADA.gov says a business or state or local government that restripes a parking lot must provide accessible spaces as required by the 2010 ADA Standards, with the count figured separately for each parking facility, not for the whole site.

Chip seal and sealcoat need the warmest weather of all. MoDOT requires air and pavement above 60°F for chip seal, and sealcoat products typically call for dry pavement and at least 50°F and rising (check the label). Schedule those for late spring through early fall.

When you are ready, Chesterfield Asphalt Paving offers free estimates across St. Louis and the Metro East. Call (866) 490-7680 or request one online, and we will talk through timing for your driveway, parking lot or private road.

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