Repointing a Stone Foundation

Fix the cause first

Crumbling mortar, loose stones, and damp walls are reasons to repoint, but repointing alone won’t fix what caused them. Leaking gutters, settling, and rising damp have to be dealt with first ¹, which is why lesson 1 comes before this one. Repointing is also one of the basic ways to keep exterior water out of an old building ².

Softer than the stone

A house built in 1905 likely has lime mortar in its foundation. Traditional mortar was lime putty and sand, about 1 part lime to 3 parts sand; portland cement, patented in 1824 and made in the U.S. from 1872, came into general use later ¹.

The rule for new mortar ¹:

• Match the sand; color and texture usually follow.
• Make it softer and more vapor-permeable than the stone or brick.
• Make it as soft and as permeable as the old mortar, or more so.

The reason: when a wall moves or gets wet, you want the joint to take the strain and let moisture out, not the stone. Hard cement mortar forces the damage into the masonry instead.

ASTM mortar types run from hard to soft: M about 2,500 psi, S 1,800, N 750, O 350, and K 75; L is straight lime and sand ¹. The soft, lime-rich end is where an old foundation usually belongs; the right mix depends on your stone and old mortar, and a mortar analysis or a preservation mason can tell you.

Rake it out

Remove old mortar to a depth of at least 2 to 2 1/2 times the joint’s width, plus anything loose beyond that. For brick that’s about 1/2 to 1 inch; for wide stone joints it can be several inches ¹.

Use hand chisels and a mash hammer. It’s slow, but it’s the least likely to damage the masonry. Power saws and grinders in unskilled hands chip edges and overcut joints ¹.

Rinse the joints to clear dust. Fill them damp but with no standing water; very absorbent masonry can be misted for a few hours beforehand ¹.

Fill it back in

1. Mix dry ingredients by volume, add water sparingly, and use the mortar within about 30 minutes. Don’t add more water to stiffening mortar ¹.
2. Fill deep areas first, in layers about 1/4 inch thick, packed well into the back of the joint ¹.
3. Let each layer reach thumbprint hardness before the next. Layering keeps shrinkage down ¹.
4. Tool the final layer when it’s thumbprint-hard, to match the old joints ¹.

Try a 3-foot-square test panel on a hidden part of the wall first; it sets the standard for the rest ¹.

Try it

Scratch the old mortar in a joint with a key or nail. Lime mortar usually scratches and crumbles easily; very hard, gray mortar that resists is probably a later cement repair. Knowing which you have tells you how soft the new mortar needs to be.