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How to Repoint a Stone Foundation

This Old House general contractor Tom Silva saves a crumbling mortared-rock foundation wall

In this video, This Old House general contractor Tom Silva saves a crumbling mortared-rock foundation wall.

Steps for repointing a stone foundation

  1. Fill a plant mister with water and spray the mortar to keep down the dust.
  2. 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.
  3. Pull out all fiberglass insulation and loose debris found along the top of the foundation wall.
  4. Use a soft-bristle brush to sweep all dust and dirt from the raked-out mortar joints.
  5. 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.
  6. 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.
  7. 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.
  8. 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.
  9. 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.
  10. 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.

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