How to Repair an Interior Mortar for a Flagstone Hearth

Remove all the flagstones from the surface of the hearth so you can repair the entire mortar job at one time. Use a flat-head screwdriver or a chisel with a hammer to break away the mortar from between the flagstones, starting at a front corner of the hearth until each flagstone comes free. Take out all the mortar around each flagstone and then tap the screwdriver or chisel edge beneath the flagstone.

Scrape the large loose or broken mortar that remains on the surface of the flagstone hearth, using a paint scraper. Continue until you have removed all the mortar or until the surface of the hearth sits roughly even, in the case of a hearth built entirely from a mortar base. Go over the surface again with a wire brush to break off any small pieces that the paint scraper missed.

Mix mortar in a bucket, using the amount of water recommended on the product packaging. Scoop the mortar up with a notched trowel and spread it over the surface of the hearth. Create a new bed at least an inch thick on the front of the hearth, keeping the layer as even as possible on the hearth surface.

Press the flagstones you removed from the hearth back into their places on the hearth face. Push the flagstones down into the mortar so the face of each flagstone sits just slightly higher than the mortar on the surface of the hearth. Let the mortar dry on the hearth face for approximately 72 hours before touching the flagstones or attempting to use the hearth.