Identify any loose mortar around the outside of the stones. Because of the heat cycles the stone and mortar experiences, you may find loose mortar between the stones and some may even fall out.
Have a professional examine the state of the chimney. Consider placing a chimney liner in while you do the repairs. Often, house fires happen when a chimney falls in disrepair and a liner will assure years of trouble free and safe service of your stone fireplace.
Remove loose stones and mortar and clean all surfaces of any loose debris. Use a stiff wire brush to make sure you have all loose mortar cleaned out.
Re-mortar the stones with mortar of the same or as close to the same expansion rating as the stones you repair. This is called the Thermal Expansion Coefficient.
Seal with an appropriate stone and mortar sealant suitable for use on your type of stone and mortar for the heat that such stone fireplaces will experience.
- If your stone fireplace is farther gone than just a few loose stones and some chipping mortar, replace the entire system. Remove all the stones and mortar and put in a manufactured firebox with a new chimney liner. Use the stone to decorate the outside of the firebox. This gives you the great look of a stone fireplace with the extra safety of contained flames.