How to repair fire brick mortar joints in your fireplace to prevent fire damage to the home.
One of the most important components of your masonry fireplace is the fire brick inside the fire box. After years of use fireplace mortar can begin to crack, crumble, and possibly fall out of place. Open mortar joints leave the bricks more vulnerable to damage and can expose combustible materials to high temperatures, soot, and creosote. The fire brick and mortar joints must be in good condition to limit the chance of structural fires in the home. Before you build a fire this season you should examine the condition of the mortar in the firebox and take an hour or two to replace any that has begun to deteriorate.
Some tools you will need are a carbide-tipped scoring tool that is typically used for cutting tile cement board, a brick trowel, and a tuck-pointing trowel narrow enough to fit in the brick joints, a pointing tool called a jointer, and a few other tools like an old paint brush, awl, and vacuum. You will also need a dry-mix refractory mortar such as Rutland Dry Mix 211, Heat Stop II, or AW Perkins 245 which is specially formulated to withstand a wood fire's intense heat. Make sure that the type of refractory mortar you use meets NFPA 211 specifications which is the Standard for Chimneys, Fireplaces, Vents, and Solid Fuel-Burning Appliances.
Cracked fire brick and loose mortar.
Fire bricks come in different thicknesses, colors, shapes, and sizes.
Visually inspect you firebox, if you notice serious damage to any of the fire bricks, bulging, or water damage, contact a licensed professional to inspect the fireplace and chimney before performing any repairs to the fire brick and mortar.
Sweep out and vacuum the firebox and fire brick.
1. Remove loose mortar
Carefully check for loose mortar by probing the joints with an awl or the scoring tool. Rake out the joint with the scoring tool or a grout rake. Dig out the damaged mortar until the joint is about ½ to 3/4 inch deep and the brick surfaces on both sides of the joint are clean.
2. Clean the joint
To clean the brick joints, use an old paintbrush to sweep out the joints and a vacuum until there is no loose debris or dust. Apply water to the joints and onto the brick faces with the brush. The mortar will not cure or adhere to dry brick, but always read the instructions for the refractory mortar you are using.
3. Pointing the Fire Brick
Mix up a small batch of mortar in a bucket of clean, room-temperature water. Be careful not to add more water than necessary. Load mortar on the edge of the brick trowel, place it against the brick in line with the joint, and push the mortar into the joint with a tuck-pointing trowel. Pack the joint about halfway full, then press down firmly as you pull the pointing trowel, handle-first, along the joint a couple of times. Repeat until the joint is filled.
Scrape the excess mortar off the brick faces with the edge of a trowel and then wipe them down immediately with a damp sponge without touching the joint with the sponge. Now strike the mortar by running the jointer over the soft mortar to make it harden properly. In 30 to 90 minutes, when the mortar is firm to the touch, go over everything with a stiff-bristle brush.
Follow the recommendations of the mortar manufacturer concerning curing and drying times. Typically you can use the fire place the next day, but it is a good idea to start with a small fire at first.
If you mortar is in fairly good shape, you can opt to caulk the joints that have minor chips or cracks with a refractory caulk.