Think Oamaru. Think Limestone

2019-01-22T00:00:00

Heather Bauchop photo - Heritage Advisor






By Heather Bauchop, Heritage Advisor 

The proximity of good building stone and the scarcity of timber has created Oamaru’s architectural identity – Whitestone City. Limestone also provided the mortar for these same buildings – smaller pieces of stone were burnt in kilns to produce quicklime. Quicklime was mixed with local sands and slaked (combined with water to produce calcium hydroxide) to make mortar.

Limestone walls breathe. Porous walls such as these exchange air with the atmosphere as temperature and air pressure change. The wall warms up during the day, the air within it expanding the wall. At the end of the day the wall cools down, and the air within it shrinks. Air and some water vapour are drawn into the wall.  

When stone walls were built in the past it was accepted that a certain amount of dampness would penetrate the masonry and that this would evaporate naturally. Traditional mortars, plasters, renders and finishes such as limewash were permeable and allowed the walls to breathe. Maintenance was mainly replacing deteriorated mortar, render and limewash, especially at the base of a wall.

Looking after limestone means letting it breathe: repairs should be in permeable lime mortars, plasters and renders. Any coatings should be permeable. 

Contemporary materials such as Portland cement or acrylic paints are relatively impermeable and prevent water getting in, but they also stop it getting out. You can tell water is not getting out when paint bubbles or blisters, and when render fails or cracks. Locking in the damp means it rises up the walls. Along with making sure there is good drainage, using traditional finishes help keep the stone in good condition. 

Here are some things to avoid:
  • Don’t change the pattern of the masonry – repairs should match the original work
  • Don’t sandblast or use high pressure water jets – either use chemical stripper or low pressure water
  • Don’t replace timber floors with poured concrete – this forces the dampness into the walls
  • Don’t use cement when repairing lime mortars, plasters and renders
  • Don’t seal walls with water-repellant coatings or use acrylic or other impermeable paints

This information has been drawn from the City of Fremantle’s Heritage Building Conservation Technical Advice Sheet 3. 

Useful link: find more information about Conservation of privately-owned heritage buildings. 

limestone building
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Page reviewed: 12 Mar 2019 3:15pm