What Is Road Metal?

Mary McMahon
Mary McMahon

Road metal is crushed rock for use in paving. This term references the early origins of the word “metal,” which involves materials generally mined or quarried, like rock, and was originally used to discuss all hard quarried materials. To avoid confusion, terms like “gravel” are used to refer to this material in most regions of the world. People in some locales, like New Zealand, continue to refer to aggregate rock used in road construction as road metal.

Gravel may be used to fill holes in roads temporarily.
Gravel may be used to fill holes in roads temporarily.

This product is made by quarrying rock and crushing it. Crushing equipment is capable of controlling the size of the pieces, which can be run through sorting drums. Pieces above a certain size can be pulled out successively to create a series of rock piles containing pieces of a roughly similar size. When contractors order road metal, they can specify the size range or ranges they need for given projects.

Kiwis continue to refer to aggregate rock used in road construction as road metal.
Kiwis continue to refer to aggregate rock used in road construction as road metal.

Plain road metal can be used for rural and lightly trafficked roads. Work crews may use spreaders to scatter the crushed rock over the surface of a prepared road bed. They can follow with compression devices like rollers to push the rock together, compacting the surface. This can improve both driving conditions and drainage, two key concerns on finished roads. Gravel fill can also be used to fill in holes in a roadway, although eventually the entire road may need to be regraded.

In the case of roads to be paved with a tarmac, road metal can be used to prepare the underlying surface, known as the road bed. This needs to promote even drainage to reduce damage to the upper surface. It also suppresses plant growth to limit problems caused by large roots and weeds trying to work up through the paving materials. Several layers of road metal may be placed to provide adequate drainage and protection before the final layer of paving is applied.

Paths and walkways can also be prepared with road metal. A finer grade may be used to make the surface more aesthetically pleasing, and crews may simply rake the gravel into place and leave it, or compress it with a roller for more durability. Delivery is typically available for loads over a certain size, and it may be possible to contract workers to spread the gravel on arrival. For small projects, it is usually necessary to pick supplies up at a construction company or quarry facility because of delivery minimums.

Macadam was a variety of roads constructed in the 19th century using three layers of stone with gravel packed on top.
Macadam was a variety of roads constructed in the 19th century using three layers of stone with gravel packed on top.
Mary McMahon
Mary McMahon

Ever since she began contributing to the site several years ago, Mary has embraced the exciting challenge of being a wiseGEEK researcher and writer. Mary has a liberal arts degree from Goddard College and spends her free time reading, cooking, and exploring the great outdoors.

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