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Cut a bag of thinset mortar in half using your trowel (Image 1). Half of an 80 lb. bag is enough for the hearth, legs, and header of one fireplace. If you are setting two fireplaces, mix the entire bag.
Add about a cup of clean water to a clean five-gallon bucket. This will prevent the mortar from sticking to the bottom of the bucket.
Add the thinset slowly into the bucket. Also add water in small amounts at a time – you don’t want to end up with a watery mix. Use a trowel or power mixer to stir the mixture until there are no lumps and it has a consistency of toothpaste (Image 2).
Set the mixture aside for fifteen minutes to allow it to “slake,” then remix.
The thinset mortar is now ready to be used. Once mixed, thinset remains in a workable state for up to eight hours. When it begins to dry out, do not use it, and do not add water – it will only weaken the mortar’s bond.
Place the hearth stone on two 2x4s to make it easier to move into place when you are ready to apply the thinset. The hearth stone is the largest and heaviest stone, which makes it easy to break. Handle it very carefully.
Use a hand trowel to apply a thin layer of mortar to the hearth base (Image 1).
With a helper, lift and set the granite into place (Image 2).
If the hearth is recessed into the floor, set the back edge in first, move your fingers out of harm’s way, then use a hand trowel placed under the front edge to lower the stone into place.
Tap the hearth stone gently with a rubber mallet to remove trapped air bubbles and ensure that the stone adheres to the thinset (Image 3).
Check that the stone is level, front to back and side to side (Image 4).
Begin by cutting four 2x4s to use as braces for the stone legs and header. You will need one brace for each leg and two for the header stone. Cut the 2x4 ends at 45-degree angles. The leg braces should be cut to a length that will support the stone about two-thirds of the way up.
Apply three large dollops, or a thin even layer, of thinset to the back (saw-cut) side of the first leg section (Image 1).
Set this leg into place, making sure that the polished edge is exposed (Image 2).
Adjust the piece until it is vertically plumb and level (Image 3).
Place the 2x4 support against the leg, and use a concrete block to brace the support. Repeat this step with the second leg (Image 4).
Place a level across the top of the two legs to be sure they are equal in height. This will determine whether the stone header that spans between them is level.
Place four large dollops (or one thin layer) of thinset on the back side of the header. Be sure the polished edge is facing down and exposed. Set the stone in place, using a medium amount of pressure to ensure that it adheres to the thinset (Image 1).
Check that the header is plumb and level, and brace it using two 2x4s (Image 2).
Use damp rags to clean any mortar from the edges or face of the stone sections, tools and the surrounding area before it dries hard. Allow the stone to sit for 12-24 hours before removing the braces.