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Installing Cultured Stone On A Fire Place

Cultured stone is a manufactured product designed to look like real stone, but with a flat back for easy installation. It may be installed over cement block, plywood, rigid backwall, stucco or metal. This instruction will detail installing cultured stone for a fireplace with a ¾” plywood frame.

Tools: Hammer or staple gun, wheelbarrow and hoe, trowel, masonry cement, circular table saw with carborundum or diamond blade, wide mouth nippers or hatchet, safety glasses, level, metal jointing tool or wood stick, grout bag, whisk broom, stencil brush sold in craft stores.

Materials: Mortar mix, mortar color, weather resistant barrier (water proof paper or asphalt saturated rag felt or ASTM D226, Type 1 No 15 felt) Note: Weather resistant barrier must be used on all exterior and interior mortar applications except those over masonry, concrete, or stucco.
Metal diamond mesh lath (minimum 2.5Lb), galvanized nails or staples, concrete nails.

Build your frame to support your fireplace and stone according to the fireplace manufacturer’s instructions. If you are installing a gas fireplace, it may be best to have the dealer install it if you are unfamiliar or uncomfortable with gas lines. When building your frame, be sure to take into account the width of the stone (2 to 3”) and also add ¼” to 3/8” for mortar.

Cover your plywood frame with a breather type weather resistant barrier, overlapping the joints 4”. You may use a staple gun to install the barrier.

Next install the metal lath over the entire surface using galvanized nails 6” on center vertically. Studs must be penetrated a minimum of 1”
Lay out your stone on a template of your fireplace front. You can use newspaper as a template. Keep in mind this will just be preliminary plan. No matter how exact you think you are, the rocks will never fit exactly like they do on the template. Be sure and vary size, color, and texture of the stone. You may have to cut or chip some stones to fit. Leave your joints evenly spaced and place them fairly close together. (1/2”) You may think you are placing the stones close together, but will find they are farther apart than you imagined they would be, so be very aware of spacing as you go. You can break up one stone into small pieces to use as fillers for tight spots. Keep an open mind while you are installing the stone. If a piece doesn’t fit like you thought it would, you may have to change your plan.

Mix your mortar to a consistency that is like cake batter. Too thick will dry and crumble and too thin will be a mess. You may have to experiment a little to find the right consistency. When mixing your mortar, use the same recipe for all batches. It may be one pail of water to 3 scoops of mortar and 1 cup of mortar color. Whatever amount you decide on, do the same for all batches, especially if you are using a mortar color. Mix the mortar thoroughly so there are no lumps.

Using a mason’s trowel, apply ½” mortar to a small section of lathe. (enough for 2 to 3 stones) We suggest starting at the top and working down because this is a messy job and mortar will fall on your previously applied stone. Be very careful not to get mortar on the stone, as it discolors it.

Butter the flat back of the stone with a thin layer of mortar and apply it to the mortared lath. Apply pressure firmly enough to squeeze some mortar out around the stones edges. Make sure pressure is applied to the entire surface of the stone to ensure a good bond. Hold the stone in place for several minutes. We used 3” screws screwed in with a power drill at the bottom of each stone to help hold it in place. We used one or two screws for each piece, and that really helped to keep the holding time down to a few minutes for each stone. The screws were removed after the mortar dried and before the grout was put in. Continue to apply mortar to the lath a few feet at a time and install the stones. We set corner pieces in first and worked from the ends in. Use nippers to cut small pieces between joints, or just use a hammer to break one stone into several pieces of varying size and shape for fillers. When using cut stone, try to position the cut edges up when they are above eye level and down when they are below eye level.

This is a big job and you will probably want to do it in several sections over a few days. If you start making mistakes, you are getting too tired and it is time to quit for the day. After you are certain the mortar is dry and the stones are all set, remove the support screws.

Now it is time for the grout. Mix your grout as you did the mortar, using the same materials. (mortar and mortar color with water) You may have to mix this a little thinner, or it will plug up the grout bag. A grout bag is a bag with a large opening on one end and a very small one on the other. It is similar to a baker’s icing bag for decorating cakes. Fill the grout bag with mortar and start filling all the gaps between the stones. It is OK if it is a little thin, as long as it does not drip onto the applied stone. Be very careful not to get any mortar on the stone. If you do, remove the grout only after is has become crumbly with a whisk broom or dry bristle brush. Never use a wet brush or a wire brush.

When the mortar has become firm or thumb print dry, the joints should be pointed up with a wood stick or metal jointing tool. Rake out the excess mortar with the tool, maintaining even surfaces. Compact and seal the edges around the stone, avoiding any cracks or open areas. A stencil brush does a good job of compacting the grout. This is just a circular stiff bristle brush about 1” in diameter sold at most craft stores.

Cleanup is done with a whisk broom, lightly brushing over the joints after they have sufficiently set up. This will remove any loose mortar. Never use a wet brush to clean the mortar joints. Never use acid or acid based products to clean the stone or mortar joints.


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