DEAR TIM: I have an existing concrete fireplace hearth. Is it possible to cover this fireside hearth with slate? If so, is that a difficult task? Instead of slate, should I just install a hearth pad? I am really confused about what to do. Marianne C., Millis, MA
DEAR MARIANNE: You can cover the concrete hearth with slate, tile, stone or just about any other non-flammable building product. If you had x-ray vision, like some super heroes, you would see concrete just below many of the hearths in older and even modern homes. Builders like myself install these concrete hearths as an extension of the concrete pad that the fireplace is built upon.
Usually the biggest challenge is planning ahead so the finished floor in the main part of the room transitions cleanly with the finished material on the hearth. Sometimes this is handled by creating a raised hearth where both the hearth and the firebox are higher than the finished floor in the room.
I happen to prefer that the top of the finished hearth is at the same elevation as the underside of hardwood flooring that is in the room. This way a beveled strip of hardwood flooring can overlap the finished hearth for a very professional look. To achieve this look, all you have to do is pour the concrete lower than the wood subfloor of the house. The distance you drop the concrete is a function of the thickness of the material you are placing on the concrete and the thin layer of adhesive you will use to attach it to the concrete.
Slate will bond easily to the concrete hearth you have. I would use thinset adhesive, which is just a blend of Portland cement, silica sand and often some chemical bonding agents. The thinset is also fireproof.
Perhaps your biggest challenge in working with slate is that all cuts need to be made with a diamond wet saw or by hand using a carbide rod saw that fits into a standard hacksaw. You can't use a standard ceramic-tile snapper cutter with slate as the slate will fracture along an uneven line. It is possible to layout all of the slate and pre-mark all of the pieces to cut. Sometimes the slate store has a saw they will use to cut the pieces you bring back. Not all stores offer this great service.
Slate is a great material to use for a fireplace hearth for a number of reasons. It is a very durable natural stone, it is attractive and looks fantastic next to a burning fire, it is easy to clean and is available in a variety of mottled natural tones including red.
If you try to solve your dilemma with a hearth pad, I feel you may have a problem finding one the right size. Hearth pads are pre-made thin platforms that you just set on the floor. Some come covered with tile and I have seen some made with slate. Many of the hearth pads I have seen come in a few popular sizes. It may be very hard to locate one that covers your existing concrete hearth, and is also proportioned correctly for the room.
You can always resort to building your own custom hearth pad to cover the concrete already in place, but I feel all this effort should be concentrated on just installing the slate you want in the first place.
If you have any trepidation about installing slate, I would propose you making your own test hearth pad in your garage. Purchase a small piece of cement board that is perhaps 30 x 36 inches. Get some discount slate and just apply it to the cement board. Forget about making cuts unless you want to do it by hand with the carbide rod saw. My guess is that you will discover in short order that the slate goes down with little effort.
When working with slate or tile for a fireplace hearth, it is important to think about how the material will look against the other flooring materials that are adjacent to the hearth. If the slate butts up against hardwood flooring, a grout line between the slate and wood flooring often looks best. If the slate is surrounded by carpet, eliminate this grout line.
Be sure to seal the slate with a grout-release sealer before grouting the slate. This clear chemical helps minimize grout messes with slate. It is hard to grout slate because the surface of slate is uneven. The thin layers of minerals in the slate that create its gorgeous texture also are the bane of tile setters who struggle to clean all grout from the surface of slate.
The slate surface will be ready to walk on about 48 hours after the grout has been installed. It is best to keep the grout damp for this wait period. The presence of water will help strengthen the grout so it can be as durable as the slate.