What Causes Asphalt to Bubble or Blister?

Most bubbles and blisters on a driveway come down to heat pushing on something trapped underneath. On a sealcoated driveway, it's usually moisture or air caught under a coat that went on too thick, too hot, or over damp pavement. On new paving or an overlay, it's typically water or vapor sealed between the new layer and the surface below, where the two didn't bond completely. When the sun heats the pavement, that trapped moisture expands and lifts a dome. The blisters often appear on hot afternoons and may shrink back overnight. Less often, a raised bump is caused by tree roots, frost, or a fuel spill softening the surface. The fix depends on which of these you're dealing with.

Driveway sealer peeling → Rain on new asphalt →

Bubbles in Sealcoat

This is the version most homeowners see. Small, round bubbles appear in fresh sealer, sometimes within a day or two, especially on the first hot, sunny afternoon.

Common causes:

• Coats applied too heavily. A thick film skins over on top while the layer underneath is still wet, trapping water and air.
• Sealing in blazing midday heat. The surface skins almost instantly, so vapor rising from the pavement has nowhere to go.
• Damp pavement. Sealing too soon after rain or washing traps moisture in the pores.
• Porous, dry asphalt. Air in the surface voids expands as the pavement warms.

What to do:

• Many small bubbles flatten as the sealer finishes curing. Give it a few warm days.
• Bubbles that pop leave tiny pinholes. These can be touched up with a thin second coat once the first is fully cured.
• Large blisters or areas that lift and flake should be scraped back, cleaned, dried, and resealed thinly.

Next time, split the job into a pair of light coats instead of a single thick one, work in the morning or late afternoon rather than peak heat, and make sure the surface is bone-dry. If flakes are already lifting, the notes on why sealer peels explain what's going on.

Blisters in New Asphalt or an Overlay

Blisters in the pavement itself are less common but more concerning. They show up as domes, sometimes a few inches across, sometimes much larger, usually on hot days in the first season or two.

A 2024 research review in Applied Sciences on pavement blistering describes two main conditions that need to be present: a hot surface and an imperfect bond between the top layer and the layer below, or a top layer with low strength. Researchers group blisters into those that appear right after paving, often from water trapped during construction, and those that appear during later hot spells.

Typical construction-related causes:

• Paving over a wet base or damp existing surface
• Missing or uneven tack coat, so the new layer isn't glued down
• Dirt or dust on the old surface before an overlay
• Very dense, low-permeability mix that traps vapor underneath

One documented case involved a hot-mix overlay on a military airfield runway in South Carolina, where blisters appeared every summer, starting about a year after construction, and seemed to grow in number over the years, according to a record in the Transportation Research Board's database. Other researchers have explored less obvious causes, including gas produced by microorganisms and slow upward buckling from repeated heating and cooling.

What to do:

• Call your installer if blisters appear on new work. It's often a warranty issue.
• Don't cut or puncture blisters yourself. Contractors decide whether to vent, reheat, patch, or leave them, depending on size and cause.
• Watch whether they grow. Small blisters that stay stable may be monitored. Growing or cracking blisters usually need repair.

Other Causes of Bumps That Look Like Bubbles

• Tree roots. A long, ridge-like bump near a tree is usually roots lifting the pavement. See the page on tree root damage.
• Frost heave. Bumps that appear in winter and settle in spring point to frozen, wet soil underneath.
• Fuel or solvent spills. Gasoline, oil, and solvents can soften asphalt, leaving a puffy or mushy spot.
• Sinking and swelling soils. Clay under the base can swell when wet.

A ridge or wave where cars brake or turn is more likely shoving than a blister; see asphalt shoving causes and repair.

What Causes Asphalt to Bubble: Quick Diagnosis

Where and when	Most likely cause
Fresh sealcoat, first hot days	Thick coat, heat, or moisture under sealer
New paving, hot afternoons	Trapped moisture, poor bond between layers
Long ridge near a tree	Roots
Winter bumps that settle in spring	Frost heave
Soft, spongy spot near where a car parks	Fuel or oil spill

Shiny, sticky patches without a raised bump are usually bleeding instead, covered in asphalt bleeding on a driveway.

Preventing Blisters

For sealcoat: thin coats, dry pavement, mild temperatures, and time between coats.

For paving: a dry, clean base or surface, proper tack coat, good compaction, and no paving right after rain. Our article on rain and fresh asphalt covers weather timing.

Bottom Line on Asphalt Bubbling and Blisters

So what causes asphalt to bubble? Usually heat expanding moisture or air trapped under sealcoat or under a new layer that didn't fully bond. Sealcoat bubbles often settle and can be touched up with thin coats. Blisters in new paving deserve a call to the installer. Rule out roots, frost, and spills if the bump doesn't fit the pattern.