Historic Old North Knoxville - Restoration Tips

Historic House Restoration:

Tuck Pointing Historic Masonry

By: Kim Davis (Old North Knoxville webmaster)
August 30, 2003

Here is a good example of a brick wall badly in need of a repointing job!

On August 9, 2003, Knox Heritage Preservation Network presented, "Tuck Pointing Brick," by Shawn Gallant of WASCO, Inc. Conducted at the Time Warp Tea Room on North Central St., Shawn discussed various methods of restoring brick through tuck pointing. WASCO, with offices on Dutch Valley Road, has been in business since 1963. More recently, they have been involved with the restoration of the U.S. Post Office, the Tennessee Theater, and the Church Street United Methodist Church, all located in downtown Knoxville.

Tuck pointing (or "repointing") is often required in historic masonry because the original mortar mix was only designed to last 25 years, while the bricks can easily have a lifespan of over 100 years. Shawn emphasized the four steps to a successful repointing job:

Wall preparation

Creating a balanced mix

Workmanship, requiring knowledge, skill and patience

Adequate protection of the brick from sunlight, wind and freezing temperatures during the curing process while keeping the bricks moist

For thousands of years, mortar consisted of lime putty and sand, with three parts sand to one part putty. These lime mortars had low compressive strength compared to modern mortars (between 50 and 300 pounds per square inch, or psi) and were slow to set. However, lime putty was more appropriate for bonding older "masonry units" (bricks) since it was softer and more flexible, and provided the needed permeability to wick moisture away from the bricks. This "first line of defense" provided by the mortar is critical to keep the bricks from undergoing the strain of movement and moisture build-up.

In the 1870s, Portland cement was added to the mortar mix for the first time, to speed up the setting process. However, lime putty was still the primary mortar ingredient until the 1930s. After 1950, mortar consists of two primary ingredients: pre-bagged masonry cement, a premixed, bagged mortar which is a combination of portland cement and ground limestone, and hydrated lime, machine-slaked lime that eliminated the time-consuming slaking of quicklime into putty on-site. The modern mortar mixture is much harder, with compressive strengths of 750 - 2500 psi. This sets up in a couple of hours, compared to 72 hours for lime putty mixtures. Most importantly, though, modern mortar mixtures should not be used with historic masonry due to the possibility of damage to the older, softer bricks.

Five mortar types, each with a corresponding recommended mix, have been established by ASTM to distinguish high strength mortar from soft flexible mortars. The ASTM designated them in decreasing order of approximate general strength as Type M (2,500 psi), Type S (1,800 psi), Type N (750 psi), Type O (350 psi) and Type K (75 psi). Type O is what is recommended for the mortar mix in the Old North Knoxville Historic Guidelines, and has a mixture of:

1 part Portland cement
2 parts hydrogenated lime and
9 parts sand (use river sand rather than builder's sand to obtain the proper color).

(By comparison, cement-lime-sand proportions of Type S is 2:1:9, Type N is 1:1:6, and Type K is 1:4:15.)

When mixing up the mortar, the sand should be measured while it is damp; otherwise, the mixture may end up having too much sand since sand expands in volume when water is added.

When determining the proper mix for matching the color of the new mortar to the existing mortar, you can use either pre-colored cements and/or different colored sands (e.g., Nolichucky River sand is red). It is best to wait at least a week on the test mixtures to see what the true color will be for the cured mortar.

This picture shows how WASCO removed the old mortar from the bed joint at the historic Hermitage Hotel in Nashville.

The proper removal of existing mortar is a crucial step, to avoid damaging the brick. Shawn uses a very thin diamond blade grinder to cut the center of the bed joint (the horizontal seam), then chips away the rest of the mortar with a hand chisel and hammer. As a rule of thumb, he recommends removing all mortar to a ¾" - 1" depth (no more than 1/3 width of the brick).

During the tuck pointing process (which should never occur outside of the temperature range of 40 - 90º F), the wall should be kept wet and the mortar shouldn't be soupy, but dry enough to roll the mortar into a ball in your hand. The sand should be angular (no round edges) and clean, and water should be free of acids, alkalines and salts.

This mortar is the proper consistency for repointing historic brick.

Finally, here is a rundown of the tools Shawn says are needed:

Hammer
Chisel
Nylon brush (for cleaning the repointed masonry)
Plaster's Hawk
Tuck pointer
Trowel (2" margin)
Pointed trowel
Jointer
Safety glasses
Spray bottle
Dust mask
Gloves

Or, you can call WASCO, Inc. for their expert help, at 865 281-9196. They do jobs ranging from $500 - $500,000. Figure on $8-15 per square foot for a turnkey restoration job.

Another good website to read about the procedures discussed above is at http://www2.cr.nps.gov/tps/briefs/brief02.htm#Properties%20of%20Mortar .