Your fireplace hearth, repaired and measured in Stoneybrook Hills

How thick a masonry hearth has to be

Section R1001.9.1 of the state's residential building code sets the minimum thickness of a masonry fireplace hearth at 4 inches. That is the solid floor under the fire, built of concrete or masonry and carried on noncombustible support.

Thickness matters on a repair because an old hearth is sometimes chipped down, ground flat or patched over the years. When we open a damaged area, we look at what remains under the finish and tell you whether the base still has the depth the code expects, or whether it has to be built back up before anything decorative goes on top.

Hearths in front of factory-built fireplaces follow the listing

A factory-built fireplace plays by different rules. Section R1004.2 says its hearth extension is installed in accordance with the listing of the fireplace, so the manual that came with the unit sets the size and the materials.

One maker's wood-burning model, for example, calls for an extension reaching at least 20 inches in front of the opening and 12 inches beyond each side, and another model can differ. We find the model on its rating plate before measuring, so the repair follows the right document.

• Model found on the rating plate
• Manual read for hearth size and layers
• Repair matched to that document

Hearths for see-through fireplaces

A see-through fireplace opens into two rooms, and each opening sits in front of its own floor. The protection in front of one side does nothing for the other.

We measure each opening and each extension separately and write them as two items on the estimate. Repairs often differ between the sides as well, since one side usually faces the busier room.

• Each opening measured
• Each extension measured
• Two lines on the estimate

How a new finish can change which rule applies

The thin-extension exception depends on the firebox opening sitting at least 8 inches above the top of the hearth extension. Raising the extension with a thicker finish can shrink that height below 8 inches.

When that happens, the extension has to meet the full thickness rule again. We measure the finished height before choosing materials, so a new tile or stone thickness leaves the requirements where they were.

• Firebox floor height measured
• Finished extension height planned
• Thickness rule confirmed before ordering