Micro Milling Asphalt: Fine-Tooth Drums and Thin Overlays
Micro milling is cold milling with a much finer drum. Instead of the usual teeth spaced about 5/8 inch apart, a micro milling drum packs teeth roughly 5/16 inch apart or closer. The surface it leaves has shallow, tight ridges rather than deep grooves, which makes it a good base for thin overlays and surface treatments.
Asphalt millings calculator →Standard, fine and micro milling
| Drum type | Typical tooth spacing | Surface left behind |
|---|---|---|
| --- | --- | --- |
| Standard | About 15 mm (5/8 in) | Deep, coarse grooves |
| Fine | About 8 to 10 mm | Moderate texture |
| Micro | About 8 mm (5/16 in) or less | Shallow, fine texture |
Exact spacings and names vary by manufacturer, so agencies often specify the result they want, such as a maximum ridge-to-valley depth, rather than a drum model.
When micro milling makes sense
- Thin overlays of 1 inch or less, where deep grooves would show through or leave the overlay too thin over the ridges.
- Chip seals and microsurfacing, which follow the surface closely and need an even base.
- Removing markings or a thin, raveled layer without cutting deep.
- Profile correction for small bumps or wheel ruts.
- Traffic running on the milled surface for days before paving.
How it compares on cost
Micro milling is usually slower than standard milling because the drum has more teeth and the cuts are shallower. Tooth wear is also higher. That means a higher price per square yard in many markets, partly offset by the ability to use a thinner overlay. For conventional milling costs see asphalt milling cost.
Tips for a good result
- Specify texture. Set a maximum texture depth or a straightedge tolerance for the milled surface.
- Run steady. Consistent machine speed and drum speed keep the pattern even.
- Keep teeth in good shape. Worn or missing teeth leave streaks that show through thin overlays.
- Sweep thoroughly. Fine milled surfaces trap dust that ruins bond.
- Tack generously enough. Milled surfaces have more area to cover than smooth ones; see tack coat.
Planning quantities
Micro milling generates fine millings, often reused as RAP. Example with made-up numbers: removing 3/4 inch from a 24-foot by 1,000-foot section is 24 × 1,000 × 0.0625 = 1,500 cubic feet, about 55.6 cubic yards in place. At roughly 1.9 tons per in-place cubic yard of old asphalt, that is around 105 tons of millings. The asphalt millings calculator helps with the reuse side.
For broader context see road milling machine and mill and overlay.
Questions people ask
What is micro milling?
Milling with a drum that carries many more teeth set much closer together than a standard drum, which leaves a fine, shallow texture instead of deep grooves.
How close are the teeth on a micro milling drum?
Tooth spacing is commonly around 8 mm (about 5/16 inch) or less, compared with about 15 mm (about 5/8 inch) on a standard drum.
When is micro milling used?
Before thin overlays, chip seals and microsurfacing, to remove a thin layer of distressed surface, correct small profile problems or remove markings.
Is micro milling slower than standard milling?
Usually yes. The finer drum and shallower cuts mean lower production per hour, so it is reserved for jobs that need the finer texture.
Can you drive on a micro milled surface?
Yes. The fine texture is much smoother than conventional milling, which is one reason it is used when traffic must run on the milled surface for a while.
How deep can micro milling go?
It is usually used for shallow cuts, often well under 2 inches, although machines can go deeper at lower production.
Related guides
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