Stone Foundation Repair in Ann Arbor

Repointing replaces the mortar between the stones, not the stones. The National Park Service's Preservation Brief 2 is the standard reference here, and its first rule is that new mortar must be softer and more vapor-permeable than the masonry units, and no harder than the historic mortar. The mortar is meant to be the sacrificial part. When it is harder than the stone, stresses from frost, moisture and settlement crack the stone instead of the joint.

The city's guidelines say the same in local terms: using mortar high in Portland cement where the historic mortar is soft is listed as not appropriate, and the goal is to duplicate the old mortar's composition, color, texture and joint size.

Some numbers from the brief that are worth knowing before a mason shows up:

• Rake depth. Old mortar comes out to at least 2 to 2 1/2 times the joint's width. In stone walls with wide joints, that can mean several inches.
• Mortar strength. ASTM mortar types run from Type M at about 2,500 psi down through S, N and O to Type K at about 75 psi. Historic stone usually calls for the soft end.
• Lime. Hydrated lime for repointing should meet ASTM C207, Type S or SA.

Where the mortar has washed out completely, stones loosen and eventually fall. Resetting them means shoring above if the area is large, removing loose stones, cleaning the bed, and relaying them in the same lime mortar.

Michigan's masonry chapter sets one structural rule for rubble stone that a rebuild should respect: walls 24 inches thick or less need bonder units, stones that pass through the wall to tie its two faces together, spaced no more than 3 feet apart vertically and horizontally. Our view: a rebuilt patch of face stones with loose rubble behind isn't tied to anything. Ask how the contractor will bond the new section into the old.

Brick foundations fail in similar ways: soft joints erode, and bricks spall where water and frost get in. HomeGuide lists repointing and replacing damaged bricks as standard repairs for brick foundations (October 2025).

The same mortar rule applies, with sharper consequences. Preservation Brief 2 shows soft brick with its faces popped off because the hard portland cement mortar around it trapped moisture. For most brick joints, the old mortar comes out to roughly 1/2 to 1 inch deep. The brief also warns that power saws on thin brick joints almost always damage the brick edges, so ask how joints will be cut out: by hand chisel, or by a grinder in practiced hands.

A stone wall that sheds sand and grit into the basement can be parged on the inside face. Our view: use a breathable lime-based coat for the same reason as the mortar. A dense cement skin traps damp in the wall, and the brief's moisture warning applies to it too.

Where a stone wall has settled or has no adequate footing, it can be underpinned. New concrete is placed beneath it in short, alternating sections so the wall is never unsupported along its length, the same sequence described on the basement lowering page. Piers are the other route for a settled corner (see helical piers). Either way it's structural work on an old wall, so an engineer should see it first (foundation inspection), and a permit applies (see basement wall replacement).