In this video, This Old House general contractor Tom Silva saves a crumbling mortared-rock foundation wall.
Steps for repointing a stone foundation
- Fill a plant mister with water and spray the mortar to keep down the dust.
- Use a mason's hammer and pointed trowel to chip and scratch out the old, loose mortar from between the stones. Remove at least 2 inches of the old mortar.
- Pull out all fiberglass insulation and loose debris found along the top of the foundation wall.
- Use a soft-bristle brush to sweep all dust and dirt from the raked-out mortar joints.
- Dump a bag of Type S masonry mortar onto a mixing tarp. Hydrate the mortar with a little water and one-half gallon of bonding agent.
- With the aid of a helper, grab the handles on the tarp and slowly roll the mortar back and forth until it's thoroughly mixed.
- Scoop some mortar onto a mortar hawk. Use a pointed trowel to scrape the mortar off the hawk and into the joints between the stones. Force in the mortar, filling the joints completely.
- If a space between foundation stones is wider than 3 inches or so, fill it with mortar and then press a small rock firmly into the mortar to fill the space.
- To replace large stones that have fallen from the wall, lay down a thick bed of mortar and set the stone in place. Pack more mortar around the stone.
- After filling all the joints, let the mortar set up for about an hour. Then, lightly mist the mortar with water and use a soft-bristle brush to smooth the mortar flush against each stone.