Replacing a surface
What is a mill and overlay?
A mill and overlay grinds off the top layer of an asphalt surface and lays new asphalt in its place, so the lot gets a new surface without getting any taller.
The numbers
- Milling removes a set depth, normally the top inch or two.
- The ground-off material is recycled rather than landfilled.
- Finished height stays the same, so curbs, drains and door thresholds still work.
- It keeps the base, so it only works where the base is sound.
Why height is the whole reason
A plain overlay adds an inch or two to the top of a lot. On open ground nobody notices. Against a curb it buries the curb face. At a catch basin the grate ends up below the new surface, so the lot drains into a dish around the drain instead of over it. At an overhead door the threshold stops working.
Milling first takes the same depth off that the new asphalt puts back, so every one of those still lines up when the job is done.
What it does not solve
Milling is a surface operation. It removes the top of the asphalt and nothing under it, so a base that has failed under an area is exactly as failed afterward. Those areas get cut out full depth and rebuilt as part of the job.
The same goes for drainage. A lot that ponds will pond in the same spot after a mill and overlay unless the grade or the structure that caused it is fixed first.
Where it fits on a maintenance plan
Mill and overlay is what a lot gets when sealing and patching have run their course but the ground underneath is still good. It resets the surface clock, and then the same cycle starts again: crack fill it, keep the drains working, and seal it.
It is also the work most worth doing in one piece rather than in halves. Milling and paving half a lot leaves a seam down the middle at the join, and a seam is where water gets in. Where a lot has to be done in stages, the line between stages is worth putting somewhere it can be sealed and watched.
The striping goes back on afterward, on a clean surface, which is the one time a lot layout can be changed without paying twice for the paint.