Cleaning brick and stone without damaging them

The wrong cleaning or coating is one of the main causes of damage to older brick and stone. Masonry often gets cleaned because cleaning seems like an improvement, but the goal is to remove the dirt, stains or growth in the gentlest way that works. High-pressure water and blasting can etch soft stone and some brick for good. Clean only what you can reach from the ground. For anything higher, have a professional with experience on older masonry do it.

What you need

• A garden hose
• A natural or synthetic bristle brush, never a wire brush
• A non-ionic detergent, for oily dirt
• A bucket

Steps

1. 1

   Decide whether to clean at all

   Good reasons to clean are dirt or growth that looks bad or is damaging the masonry, or a clean surface for matching new mortar. Find out what you're removing, since soot, oil, metal stains, mold and mildew, and old ivy tendrils each need a different approach. If the wall is painted, think about why before removing it. Paint sometimes covers a mismatched repair or protects soft brick.

   5 min

2. 2

   Check the wall first

   Make sure the mortar joints are sound and the wall is watertight. Water forced through a leaky wall can rust metal anchors inside it and ruin plaster. Badly deteriorated masonry may not take water cleaning at all.

   10 min

3. 3

   Pick the right weather

   Don't wash masonry in cold weather or when frost is possible. Water inside the masonry can freeze and crack or spall it. A wall can take more than a week to dry, so stop washing several days before the first average frost date.

   1 min

4. 4

   Test a small patch

   Start with a small, out-of-the-way area, and let it dry completely before you judge it. Test each type of masonry and finish separately. Enlarge the test area as you settle on what works, eventually to a square yard or more that takes in several bricks and joints.

   30 min

5. 5

   Wash with low-pressure water

   Start at very low pressure, 100 psi or less, from a garden hose. Scrub especially dirty areas with the bristle brush. A metal brush scratches the surface and leaves bits of metal that stain.

   60 min

6. 6

   Add detergent for oily dirt

   For oily soil, add a non-ionic detergent to the water and scrub. Unlike most household detergents, it leaves no visible residue. Rinse with plain water after.

   15 min

What to avoid

• Power washing and water blasting. Water at too high a pressure is very abrasive and can easily etch marble, other soft stone and some brick. A professional going above a garden hose generally stays at 300 to 400 psi or less.
• Sandblasting and other abrasive blasting. Abrasive methods shouldn't be used to clean older masonry, and never on a whole building.
• Grinders and sanding disks. These grind away the surface.
• The wrong chemical. Acid cleaners etch or dissolve marble, limestone and some sandstone. Chemical cleaning is a job for a professional, after testing.
• A water-repellent coating on a sound wall. Most watertight walls don't need one. A coating can slow moisture leaving the wall, and salts that build up behind it can make the surface spall.

Keeping it clean

Simple cleaning with low-pressure water and a bristle scrub brush can be very effective if it's done regularly. Commercial sprays are also available to remove biological growth such as mold and mildew.

For the mortar between the bricks, see Mortar joints and repointing brick.