Common in old houses, stone foundations can be a source of trouble ... and need regular maintenance.
Mortar has fallen out of this neglected stone foundation.
Stone foundations were standard fare in houses built before World War I. Today, they're a source of frequent worry to buyers and owners of old homes. 
There's good reason for this concern: If an older house suffers from sloping floors and cracked plaster, it's reasonable to suspect foundation failure as the likely cause. But even if the floors in your house are level and the walls crack-free, you shouldn't ignore the foundation down below. Over the long term, neglect invites trouble.
A brief visual inspection will quickly disclose bulging, bowing, shifting or settlement of a stone foundation. If you find any one these conditions, you should call in the services of an experienced mason.
However, if the stones are exposed, and appear generally to be where the original builder placed them, you can probably handle the repairs and maintenance yourself. Determined do-it-yourselfers can perform much of the routine restoration and maintenance that will make their home's foundation last for future generations.
Re-Pointing
Most stone foundations have, or had at some time, a mortar coating on their interior. The purpose of this coating was to help hold the stones in place. This mortar coating will inevitably flake off from moisture migration, revealing the surface of the stones. As this coating continues to erode, the soft, sandy mortar in between the stones begins to fall out. When this occurs, re-pointing is needed as soon as possible to refill the voids where the old mortar fell out.
When repointing the exterior face of an old-house foundation, mortar needs to be softer than the surrounding stones or bricks. For this reason, you should usually avoid pre-packaged mortar mixes -- which contain a heavy dose of "hard" Portland cement -- for exterior old-house work.
Inside, it's a different story: Pre-packaged mortar mix can be used for the repointing and also the recoating. Simply follow the mixing instructions on the bag of mortar, troweling the mix between the stones and finishing with a complete coating.
To avoid perpetual repointing, you will need to finish with a complete top coating. This top coat does not have to look like a stone artisan's creation: It merely has to serve the purpose of keeping the old mortar in place.
Moisture Penetration
Don't simply repoint and recoat without correcting underlying moisture problems.
Moisture not only erodes the mortar, but in excess, can also lead to other serious conditions like pressure against the foundation and frost heaving in cold winter climates.
To avoid these conditions, you need to maintain proper drainage around the perimeter of the house:
What About Waterproofing Systems?
In my opinion, "water proofing" systems, especially those on the exterior of a building, should be used only as a last resort. Not only are they very costly, but they can cause other foundation problems.
Many water proofing systems involve water collection and discharge. When used on the exterior of the building, the soil around the foundation is usually excavated. Problems can arise because this well compacted soil has been holding the old soft, sandy mortar firmly between the exterior of the foundation stones. Remove this soil, and it is often necessary to repoint on the exterior. In some cases stones may even be loosened and will have to be reset by a mason.
Interior water collection systems don't involve disturbing the outside soil, but still, their discharge can wash away fine soil and sand particles and eventually undermine the foundation.
Of all the components of an old home that need either restoration or maintenance, the area buried deep in the ground is often the most neglected. By taking these steps to keep the mortar in between the stones, we can return to the projects exposed to daylight.
*William Kibbel III is a home inspector and restoration consultant specializing in historic residential and commercial buildings. He is vice president of Tri-County Inspection Company, serving Southeastern Pennsylvania and Central New Jersey.
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