Old stone foundations under Illinois houses

Before concrete block and poured walls were everyday materials, a house foundation was usually stone. Builders used what they could get: rounded fieldstone picked out of fields, or limestone cut into rough blocks. The stones were laid up in courses, the gaps packed with smaller stones, and everything bedded in a mortar made mostly of lime and sand.

Many of these walls are thick, sometimes much thicker than a modern wall, and many sit on little or no footing. They were built to carry the house by sheer mass.

Lime mortar is soft and it breathes. Water that gets into the wall can dry back out through the joints. That is a good thing, because it lets the wall shed moisture without trapping it behind the stone.

The tradeoff is that lime mortar slowly dissolves and washes out. Over many years the joints get deeper and emptier, until the stones are sitting on each other with nothing holding them in place. That is when stone foundations start to fail.

Water is behind most of it. On flat ground like ours, roof water dumped at the corners and soil that stays wet for weeks in spring feed a steady supply of water into the wall. Each winter that water freezes in the joints and against the outside face, and the wall gets pushed and pried a little more.

The result is a familiar list: sandy joints, stones that rock, piles of grit along the floor, a wall that bellies inward, and damp patches that come and go with the weather.

• Mortar washed out of the joints
• Stones loose or fallen out
• Inward bowing from soil and frost
• Damp walls and white salt deposits
• Settling where there was no real footing

You can usually tell which you have at a glance. Fieldstone is rounded and irregular, in all sizes and colors, with wide joints and lots of small stones filling the gaps. Cut limestone is gray or buff, laid in rougher rectangular blocks with more regular courses.

Both behave about the same as a foundation. Limestone is softer and can weather on the surface where it stays wet. Fieldstone is hard, but the wide joints between round stones mean the wall depends heavily on its mortar. When the mortar goes, a fieldstone wall loosens faster.

The most common mistake on an old stone foundation is repointing it with hard modern cement mortar, or smearing cement over the whole inside face. Hard mortar does not breathe and does not give. Water that gets into the wall can no longer dry out through the joints, so it stays in the stone and the old mortar behind the new, and frost damage speeds up.

When a stone foundation is repointed, the new mortar should be softer than the stone and suited to the old work. A lime based mortar is the usual choice.

A stone foundation that is straight and solid, with joints that are worn but stones that do not move, is a good candidate for repair. Rake out the failed mortar, repoint with the right mortar, reset loose stones, and deal with the water outside. Done right, that wall can keep going.

A stone foundation that is bowing badly, coming apart along its length, or letting the house settle is usually better replaced. In Brian's words, he does complete replacement of the foundation with modern materials, which means a new footing below the frost line and a solid wall under the house.

If you are looking at an older house on a stone foundation, go into the basement with a flashlight before you fall in love with the kitchen. Look down each wall for bowing, press on the stones, scratch the joints, and look for water stains. A stone foundation in good shape is nothing to fear. One that is failing is a cost you want to know about before you make an offer.