Last edited 29 Mar 2021


The word ‘porous’ refers to materials that include voids through which liquids or gasses may pass. Porosity is the volume of such voids relative to the overall volume of the material.

The SuDS Manual (C753), published by CIRIA in 2015 defines porosity as ‘The percentage of the bulk volume of a rock or soil that is occupied by voids, whether isolated or connected.’

A porous surface is: ‘A surface that infiltrates water to the sub-base across the entire surface of the material forming the surface, for example grass and gravel surfaces, porous concrete and porous asphalt.’

Porous asphalt is: ‘An asphalt material used to make pavement layers pervious, with open voids to allow water to pass through (previously known as pervious macadam).’

A porous pavement is: ‘A permeable surface that allows water to infiltrate across the entire surface material through voids that are integral to the pavement.’

NB Technical paper 35: Moisture measurement in the historic environment, published by Historic Environment Scotland in 2021, defines porosity as: ‘A measure of the total amount of void space in a material, usually expressed as a percentage (%).’

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