Washing Old Brick Without Hurting the Mortar
On an older brick house the mortar is meant to be the soft part of the wall. Why that matters for cleaning, what the National Park Service says about lime mortar, and when a wall should not be washed yet.
The short answer
On older brick, the mortar joints are the part most easily damaged by cleaning. Historic mortar was often lime and sand, and the National Park Service says mortar should be softer than the brick it holds. High pressure water, abrasive blasting and muriatic acid can wash or eat it out. Check that the joints are sound before any water cleaning.
The mortar is supposed to be softer than the brick
The National Park Service’s guide to repointing historic mortar explains that traditional mortar was made from lime putty and local sand, often about one part lime to three parts sand by volume. Portland cement, patented in Britain in 1824, came into wide use later, first as an additive and then as the main binder (NPS Preservation Brief 2).
The brief is clear about which part of the wall should give: mortar for repointing should be softer or more permeable than the masonry units and no harder than the historic mortar. It calls it a common error to assume a harder mortar is better. In other words, on a historic wall the joints are designed to be the sacrificial part.
Why that matters for cleaning
Soft joints are the first thing harsh cleaning removes. The National Park Service’s guide to cleaning historic masonry warns that (NPS Preservation Brief 1):
- Mortar joints, especially lime mortar, can be eroded by abrasive or mechanical cleaning, and joints make up a large share of a brick wall’s surface.
- Water at too high a pressure is very abrasive and can etch some types of brick.
- Hydrochloric (muriatic) acid should not be used on historic masonry because it can dissolve lime based mortar.
Check the wall before it gets wet
Brief 1 says that before any water cleaning, the mortar joints should be sound and the building watertight, or water can seep through to the interior. Brief 2 names the obvious signs that a wall needs repointing: disintegrating mortar, cracks in mortar joints, loose bricks or stones, damp walls, or damaged plasterwork. Any of those means a mason first, then cleaning.
How we wash brick
Brick is on the list of surfaces our house wash covers, and our house wash is a soft wash: low pressure and a professional grade cleaning solution, given time to work, then rinsed. Brief 1 recommends starting historic masonry washing at 100 psi or below, even a garden hose, and moving up only as needed. The idea is the same: the solution and time do the work, not force.
For the full picture on older houses, see cleaning an older Louisville house.
What cleaning will not fix
If mortar is crumbling, missing or recessed, the wall needs repointing by a mason before it is washed, or water can get into the wall. Repointing is mason work. Cleaning also cannot undo damage from past sandblasting or hard cement patches.
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