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Caution! Adding Mulch: Too Much of a Good Thing?

This is the time of year when I see many homeowners improving their yards with beautiful new plants and landscaping. I am also an enthusiastic gardener and love to see improvements to the gardens of our neighborhoods. Unfortunately, I often see homes where gardeners and landscapers got carried away with applying mulch to their new planting beds.

For many of our homes, year after year of treating our gardens to a fresh bed of mulch has raised the level of the garden above our foundations and this can cause severe damage to the masonry of our homes. While our gardens look great and are well cared for, we are harming one of the most valuable assets of our bungalows: the brick.

I visit many homes to discuss potential renovation work and as part of a tour of their home, I often observe deteriorated masonry in the basement interior. This generally occurs at the first few bottom courses of brick above the foundation along with the white, powdering salts left on the surface known as efflorescence. In these cases, I can predict that the soil or adjacent concrete walkway is above the foundation line and against the brick at this same location on the exterior of the home. Any material that can remain damp, such as mulches, dirt or concrete cannot be placed next to the brick. (See examples in Images 1 & 2)

For good measure, all landscaping materials should be kept at least 1 inch below the level of the foundation as shown in the “Correct” Image 3:

Even soil that covers as little as one course of brick at the bottom of the wall can cause damage to the masonry above it by a process know as “rising damp.” You can see that the mortar and even some of the brick is beginning to deteriorate on this home where as little as a couple of inches of dirt cover the brick. (See image 4)

A landscape designer once stated to one of my clients that because they had large eaves and the landscaping was sloped away from the home, there was no reason for concern that the mulch and soil were entirely covering the bottom few courses of the brick. Water was not a problem this designer said, because water cannot flow uphill. For any of us familiar with capillary action, we know this is not true. The force that allows trees to pull water up to the height of a couple of stories is a force that is not just confined to plant life. Any material that is porous can and will pull water up even a vertical surface. Anyone that drinks tea using a tea bag is familiar with the cotton cord leaving a trace of tea on their cup above the height of the liquid tea. Place a sponge upright in a dish of water and over time the entire sponge becomes wet. Colored water traveling through papers towels to combine into new colors has fascinated kids at day-camps for years. (If you are a parent looking for something fun to do with your kids and aren’t familiar with this, I highly encourage you to look up ‘walking water experiments’).

Brick is a highly porous material and the historic brick on our bungalows is especially susceptible to soaking up moisture. Soil only needs to be damp for the dampness to be absorbed by the adjacent brick. The moisture in the damp soil adheres to the particles of the brick and via cohesion, the water is pulled into the pours of the brick in the same way that a paper towel absorbs water. (See image 5)

The next force working against the longevity of our old brick is evaporation. Water always moves toward drier air because of evaporation. In this case, the dampness absorbed by our brick will be evaporated to the drier air in our basements or to the drier air above the damp soil at the exterior. The evaporating moisture causes more dampness to be pulled into the vacated pours of the brick and up the height of the wall until the water in the adjacent soil is entirely dry, thus the term ‘rising damp.’ As long as the soil or mulch outside our home is damp, the adjacent masonry will remain damp. This can damage the brick over time as weather cycles cause the mortar and brick to undergo freeze/thaw cycles. Water that has been absorbed by our bricks will expand during cold seasons causing the mortar to fail and the face of the brick to spall. Eventually the bottom of the load bearing masonry walls of our home will become too deteriorated to support the loads above and once the deterioration has occurred, the only solution is to have the damaged brick and mortar replaced.

It is critical for the care of our masonry homes to be a bit restrained while tending to our gardens. Mulch can help keep our plants from becoming parched during dry seasons and keep the weeds in check but it can cause long term and severe damage to the longevity of our brick exteriors and load bearing walls so be careful of applying too much of a good thing.


Julie Liska, RA, AIA, LEED AP, is the founding Principal of Liska Architects.

4044 N Lincoln Ave, #253, Chicago, IL 60618

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