In this video, This Old House landscape contractor Roger Cook shows how to affix loose flagstones.
1. Lift up and remove loosened flagstones.
2. Use wire brush to scrub clean the concrete surface below the flagstone.
3. Don safety glasses and use air compessor with air nozzle to blow dirt and debris from the concrete base.
4. Apply concrete-block adhesive to the concrete.
5. Use flat trowel to evenly spread out the adhesive.
6. Apply bead of mortar-joint repair caulk around the perimeter edges of the flagstone.
7. Set the flagstone into place and firmly press it down.
8. Wipe the surface of the flagstone clean using a damp sponge.
9. Fill any voids around the perimeter of the flagstone with mortar-joint repair caulk.
10. Sprinkle mixture of sand and stone dust over wet caulk.
11. Allow repair to cure overnight, then use a garden hose to rinse off the excess stone dust/sand mixture.
12. Stay off the repaired flagstone for four or five days.