"Understanding Your Home" by Building Inspector Mark Visser
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Rubble stone foundation

should be used when replacing rubble stone foundation mortar joints. Today's mortar is NOT compatable with the old lime-based mortar.
Rubble Stone Foundation
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Rubble stone foundation. Lime based mortar. Weeping tiles.

Foundation materials
: Concrete block, poured concrete, rubble, brick and treated wood. The most common used materials today are concrete blocks and poured concrete.
Rubble stone foundations
. Over time, moisture migration through a wall, leeches the lime and cement out of the mortar, turning it back to sand. So, If the old mortar is crumbling and falling out, joints should be scraped clean and filled with a lime-based mortar that is compatible with the mortar used before. The mortar should be mixed with an acrylic bonding and waterproofing agent instead of water.
The interior side of the rubble foundation should NOT be parged. Parging may trap moisture inside the wall which will accelerate mortar failure. If newly applied mortar keeps falling out, poor drainage outside the foundation walls could be the culprit.
If moisture is a problem the exterior grading should be improved, downspouts extended and eavestroughs checked for leaks.
In severe cases, the exterior will have to be excavated, weeping tiles installed, mortar joints checked and/or repaired, walls parged and water proofed.

Rubble stone foundation
This would also be an excellent opportunity to insulate the foundation walls.
Weeping tiles were not used prior to the mid 1940s.

Severely bowed foundation walls can be reinforced by adding an inside foundation wall complete with proper footings. 'Add on' walls can be anywhere from 4 to 7 ft. high (1.2 to 2.1 m) and up to 36" wide (900 mm)
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