How to Repoint and Prevent Further Damage to a Fieldstone Foundation
Ask This Old House mason Mark McCullough repoints a crumbling fieldstone foundation and shares an important lesson about water management on the outside of the house
- Start by identifying the cause of the foundation damage. Go outside to the corresponding location near the damage in the basement. Check to see if there is proper drainage, grading, and a functional gutter system. If water is able to pool anywhere near the area in question, the foundation damage will return.
- Address any water issues on the outside of the house to prevent additional water from coming in.
- Dip a masonry brush in a bucket of water and dampen the mortar and stones that are going to be repointed to keep the dust down.
- Use a pickax or another sharp tool to pull the existing mortar from between the stones. If small stones fall out, set them aside to be put back in place during repointing.
- Once all the mortar is removed, dip the masonry brush back in the water and clean out all the joints.
- Using a separate bucket, mix the mortar, water, and the bonding agent.
- Holding the cement finishing trowel upside down, plop some mortar on its blade. Use a margin trowel to push the mortar off the cement finishing trowel and deep into the joints of the fieldstone foundation. Put the small stones back in where there are really large areas to fill in. Do this until all the joints are filled with mortar and small stones.
- Using a small, wet paintbrush, smooth over all the joints to expose the aggregate. Check underneath all the stones and makes sure no sagging has happened between the mortar joints and the stones. If that’s happened, use the paintbrush to push the mortar back up against the stones.
Mark emphasizes that if you suspect water damage in your basement, it’s just as important to identify and eliminate the cause of the water as it is to repoint the basement. He explains that having a good gutter and overall drainage system is essential to preventing further foundation damage to a house. Gutters and downspouts can be found at home centers.
To repoint the fieldstone foundation, Mark used a Type N mortar, manufactured by Quikrete. In general, Type S mortar is used for structural or foundation work, but Mark finds that the Type N works better in this application because it's softer and will accommodate some movement in the foundation. To help the mortar bond better with the stones, Mark also added C-21 All Acrylic cement modifier admix, which is manufactured by Silpro.
The other tools Mark used to repoint the foundation, including the buckets and trowels, can all be found at home centers and masonry supply stores.
Expert assistance with this segment was provided by MJM Masonry.