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By Bill Kibbel
Stone foundations, the most common type below buildings
built before 1915, seem to frequently be a concern for owners of
homes. With their sloping floors and cracked plaster, it's only natural to
suspect foundation failure to be the primary cause. A brief visual inspection
will quickly disclose bulging, bowing, shifting or settlement that may require
an experienced mason to repair. If there is a crumbly mortar coating, some
minor gaps in the joints but the stones appear to be generally where the
original builder placed them, then most smart do-it-yourselfers can restore and
maintain their home's foundation.
Most stone foundations in my area have (or originally had
at one time) a mortar coating on the interior. This mortar coating will
inevitably flake off from moisture migration and expose 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. If
enough of the old mortar falls out, stones will start shifting around and the
walls start to bulge inward. To avoid perpetual re-pointing, it is best to
maintain the foundation by patching the interior mortar coating when
repointing is needed, it is very rare that the entire foundation needs
repointing, just where it has eroded back or fallen out between the
stones. Contractors inexperienced with stone foundations often
assume the old mortar, being gritty, sandy and loose needs to be
entirely replaced. This is because they assume it started out like
modern mortar and should 'glue' the stones together. The original
mortar was used only to bed the stones and fill the irregular voids
There's alot of information about using "soft" mortar, that doesn't
contain modern Portland cement, for historic masonry repairs. I see many, many
stone foundations repointed and coated with the now-common pre-mixed hard
mortar and honestly, it's never shown any sign of causing problems, when
applied on the interior. If doing repointing or repairs on the
exterior, lime-based mortar would be the better choice, particularly if
the stones are of the softer, sedimentary type.
Bill Kibbel is a historic building
inspector & consultant.
Cross section of typical fieldstone or rubble
|Moisture penetration through
foundations not only erodes the mortar, but in excess, can cause
against the foundation and frost heaving in cold winter climates. The
mortar coating applied on the interior is not going to stop water
penetration and most stone foundations are going to allow
drainage around the perimeter of the building is important to avoid
serious issues caused by water. The ground and impervious surfaces,
like patios adjacent to the
foundation, should have a positive slope away from the structure. Roof
should be collected in a well maintained gutter system with downspouts
discharging well away from the foundation walls. Sump pump discharge
should also be extended well away from the house.
Adding sub-grade drainage systems should only be used as a last resort. These
systems are not only very costly, but many of them require disturbing the soil
against the foundation, which has been well compacted over a very long time.
Even worse are "waterproofing" systems, that are installed around the
bottom of interior walls and drain into a sump pit. Many of these end up
sucking the earth out from under the foundation.
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