Glossary

A measure of the rate at which a liquid or vapour passes through a solid material, usually expressed as grams of the liquid or vapour transmitted through a given area of the solid material in 24 hours. Pores must be interlinked (cf porosity).

Ratio of the volume of pore space to total volume of a solid material, usually expressed as a percentage. Pores may or may not be interlinked (cf permability).

A material containing reactive silica and alumina that can be used to impart a hydraulic set to a non-hydraulic lime mix, or to confer a faster set upon a naturally hydraulic lime or Portland cement. Typical pozzolanic materials are volcanic ashes and lightly-fired clays, crushed and ground bricks.

One of the larger rafters that, if present, forms the upper part of a timber roof truss. It carries the purlins, which, in turn, support common rafters.

A member running along the length of a roof frame to provide support for common rafters at their mid-spans or for profiled sheeting. Carried by trusses or cross-walls.