Converting a Gravel Driveway or Farm Lane to Asphalt

Start with the rock you already have

An old gravel driveway is not a blank slate. Under the loose top layer there is usually rock that years of tires have pressed firm. That rock can often become part of the base for new asphalt, which saves digging and hauling. Not every drive qualifies, though.

The catch is that good driving gravel and good base rock do different jobs. The FHWA Gravel Roads Construction and Maintenance Guide explains that surface gravel is meant to carry extra fines and some clay-like binder so it packs hard. Pave over that kind of material and it can trap water, turn soft and fail early. Base rock under asphalt has to carry the load and let water drain out.

Before anyone prices the job, the existing rock should be checked:

• Depth: a few test holes or a steel probe show how much rock is really there. A thin skin of gravel over mud is not a base.
• Cleanliness: rock that has been churned into clay or silt for years drains poorly. It may need new crushed stone on top, or partial removal.
• Soft spots: a common check is a proof roll. A loaded truck drives the graded base while the crew watches for ruts or ground that pumps. Weak areas get dug out and replaced with stone before paving.
• Edges: grass and topsoil that crept onto the drive have to be cut back. A driveway best-practices brief from the Colorado Asphalt Pavement Association, hosted by the Asphalt Institute, says the subgrade should be free of organic material.

Build the base, then the asphalt

Most conversions regrade the old rock, add new crushed stone to reach the right depth and shape, and compact it with rollers. The Colorado brief says 6 to 8 inches of crushed aggregate base is usually adequate for a home driveway. On top of that, residential driveways commonly get 2 to 3 inches of compacted hot-mix asphalt. For best results, the same brief suggests more: a 2-inch bottom course covered by a 1.5-inch surface course, or 3.5 inches in all.

The brief also advises paving only in good weather, not on cold, rainy days. Wait a few days before driving on new asphalt, and hold off on sealcoat until it is at least several months old.

Why drainage decides how long it lasts here

Water does most of the damage to pavement in this region. NOAA's 1991 to 2020 climate normals for St. Louis Lambert airport show about 41.7 inches of precipitation a year, with around 11 days that bring an inch or more. The same normals count about 85 nights a year at or below freezing, but only about 19 days that stay at or below freezing all day. That adds up to a lot of freezing and thawing. Water trapped in the base can freeze, expand and lift the pavement, then leave a soft spot when it thaws.

The ground raises the stakes. A lot of the ridgetop and hillside ground near the Missouri and Mississippi rivers is loess, a wind-blown silt. The USDA's Menfro soil series, for example, formed in thick loess on ridgetops and slopes along the Missouri and Mississippi rivers in Missouri, and along the Mississippi in west-central and southwestern Illinois. Silty soils lose strength when they stay wet, so the goal is to keep water from getting under the asphalt at all.

• Slope the surface at least 2 percent, about 1/4 inch per foot. The Washington Asphalt Pavement Association notes that flatter slopes are hard to build and may not keep water from pooling.
• Crown a long lane so water sheds to both sides, or tilt the whole surface one way toward a ditch or swale.
• Finish the asphalt slightly above the ground beside it, so the edges shed water instead of holding it.
• If the drive runs downhill toward a garage or barn, plan a swale or drain to catch water before it gets there.

Ditches, culverts and where the lane meets the road

On a farm lane or long rural drive, the ditch is the drainage system. The FHWA gravel roads guide calls the roadside ditch the most important drainage feature on a rural road, and a paved lane still needs one. Paving is the right time to clean and reshape ditches.

Culverts carry water under the lane where it crosses a ditch, swale or low spot. The same guide points to setting each pipe right on the ditch flow line so water does not pond at either end. A crushed, rusted-through or badly set pipe should be replaced before paving, because digging it out later means cutting new asphalt.

Where your lane meets a public road, the road owner usually has a say:

• Missouri state routes: MoDOT requires a permit to build a driveway on its right-of-way and to modify an existing one, including a change in surface type. Paving the gravel inside the right-of-way counts.
• Illinois state routes: IDOT requires a permit for a new drive or a change to one in the right-of-way. Where the drive crosses a highway ditch, the permit holder must install a culvert sized for the flow and at least 15 inches inside diameter. In the Metro East, IDOT District 8 treats resurfacing an existing entrance as a permit request and calls for new concrete or corrugated metal pipe (not plastic), at least 3 inches of asphalt on 8 inches of crushed stone for a paved home entrance, and a surety bond.
• County roads and city streets: many counties and cities run their own driveway permits. Jefferson County, for example, requires a new permit to upgrade an existing entrance on a county road. Its inspector approves the pipe size, and the code calls for at least a foot of compacted cover over the pipe.
• Before any digging for a culvert or ditch, call 811 so buried utilities get marked (Missouri One Call in Missouri, JULIE in Illinois).

Width, turnarounds and heavy loads

Gravel drives tend to spread wider over the years as tires wander. Paving is the chance to set the width on purpose. As a general guide, a single-lane driveway is typically 10 to 12 feet wide, and room for two cars side by side usually takes 18 to 24 feet. A farm lane should fit the widest truck or equipment that uses it, with extra room on curves. At the road, local rules may set the entrance width: Jefferson County allows 12 to 28 feet for a single-lot home entrance, and IDOT District 8 calls for 12 to 24 feet for a non-commercial entrance, with corner radii of at least 15 feet.

On a long lane, nobody wants to back out a few hundred feet. A paved turnaround near the house, in a hammerhead or Y shape or as a wider parking pad, lets cars, delivery vans and a propane or grain truck turn without cutting across the yard. Long lanes, or lanes that serve more than one home, may also have to meet your fire protection district's access rules, so ask before the layout is final.

Thickness should match the heaviest regular visitor, not the family car. The FHWA guide cites the Asphalt Institute's recommendation of at least 4 inches of full-depth asphalt, or 3 inches of asphalt over a suitable granular base, even for low-volume roads. It adds that 4 inches of hot mix carries about 10 times as much traffic as 2 inches over a thin base. Grain trucks, concrete trucks and loaded trailers are why a working farm lane usually needs more base and asphalt than a house driveway.

Budget options: chip seal and recycled asphalt

Hot mix is not the only step up from gravel. Two surfaces that usually cost less can work on the right lane, as long as you know what you are getting.

Chip seal, often called tar and chip, is liquid asphalt sprayed on the base with stone chips rolled into it, sometimes in two layers. The FHWA guide treats a surface under about 2 inches thick as a surface treatment: it sheds water and protects the base but carries little load on its own, so it needs excellent drainage and steady upkeep. Expect some loose stone at first and a rougher, stony texture. One cost example in the guide estimated about 6 years of life for a double chip seal against 12 for 3 inches of hot mix, while noting that actual life varies. It can suit long rural lanes where cost per foot matters more than a smooth finish.

Recycled asphalt, often called millings or RAP, is old pavement ground up for reuse. FHWA's recycled materials guidelines list residential driveways and gravel road rehabilitation among its uses as a granular base, and note good drainage and durability when it is properly processed. They also note that bearing strength drops as the share of RAP goes up. As a top surface, rolled millings usually give a firmer, less dusty drive than loose gravel, but not the sealed, smooth finish of hot mix.

Either one can be a first stage. The FHWA guide describes even a chip seal as a first step toward a load-bearing pavement, and a well-built base under chip seal or millings can often take hot mix later.

What the estimate should spell out

Two quotes to pave the same gravel drive can describe very different jobs. The Colorado brief says to get more than one written quote and to have the contract spell out dates, dimensions, material types and thicknesses, drainage, utilities, permits, extra work and damage to nearby areas. For a gravel conversion, look for these lines:

• Area, in square feet or square yards, with the length and width.
• Asphalt thickness after compaction, not loose, and whether it goes down in one lift or two.
• Base work: how the old rock is regraded, how much new crushed stone is added, and how deep.
• Soft spots: how they are found, and how undercutting and extra stone are handled if the proof roll turns them up.
• Drainage: the slope or crown, any ditch work, and each culvert's size, material and length.
• Permits: who applies for the entrance permit and who covers any bond or escrow.
• Edges and tie-ins: how the edges are finished and backfilled, and how the new asphalt meets the road, garage slab or walks.
• Weed control, cleanup, and when you can drive on it.

Get a free estimate for your drive or lane

Chesterfield Asphalt Paving offers free estimates across greater St. Louis, in Missouri and the Metro East. We can look at your rock, drainage and entrance with you and lay out hot mix, chip seal and recycled asphalt side by side. Call (866) 490-7680 or request your free estimate online.

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