sealants for masonry

by Marthe Sandvik, CSI, CCPR, ChemRex, Inc.


the two most important elements for a good joint are joint design and the installer

Whenever I am asked to write or speak about sealants, I always get a sinking feeling. Will anyone read this? Does anyone care? Well, I have found out over the years, that while sealants might not be the most exciting topic, it is an important one. And if you've ever had problems with a sealant joint, you suddenly join the group that cares.

Before we go on, we have to understand what a sealant is. In short, it is simply a material that has minimum movement capabilities of plus or minus 25%. It is usually a urethane or silicone rubber, but other materials are also used. A sealant differs significantly from a caulk, which accepts movement of plus or minus 12-1/2% or less.

There is a noticeable gap between these two products - one tolerates movement of at least 25%, the other no more than half that amount. Interestingly enough; there are no commercial grade products that I am aware of that falls in between. One way to look at it is that caulks are void fillers, while sealants handle movement and are serve as a weatherseal. Caulks are used in interior joints where little movement is expected; sealants are used outside or in interior moving joints. Unfortunately, these terms are often used interchangeably, but there is a difference.

sealants for masonry

The most common product used in masonry control joints is urethane. There are many reasons for that, but one of the most important is color availability. Urethanes can be tinted to any color, and it can be done on-site.

When speaking of performance once the sealant is in the joint, that is a point of controversy.

Being from a company that produces urethane sealants, I am a firm believer that urethanes stick better to a wider variety of substrates than any other material on the market. Urethane sealant manufacturers have done a lot of testing to prove that point. Also, urethanes generally do not require primers to stick to masonry, where other products might. (I could write a whole article on that topic alone!)


The first step in surface preparation is removing mortar from in the inner face of the joint. This is done to square up the shoulders of the joint, giving the sealant a sound, smooth surface on which to bond. The installer then brushes out the joint, removing all dust.

The next step is installing the backer rod. This is a round foam that is place in the joint to help control the depth of the sealant, and to help provide proper joint cross section. Once the rod is properly placed, the sealant is installed. Generally speaking, the installer will use a bulk loading gun to install the sealant.

The last step is tooling. Tooling forces the sealant against the shoulders of the joint to ensure proper adhesion. A well-tooled joint also provides an aesthetically pleasing appearance. The tool used is a butter knife (the same as the one in your kitchen) that has been rounded off to produce a slightly concave surface. This sounds like it would be both easy and fun, but I dare you to give it a try! It is a messy job for those of us who don't possess the finesse to do it right.

key elements for a good sealant joint

When people ask me what is most in getting a good sealant job, I first tell them to get an experienced installer that specializes in sealant applications. Second, I tell them to make sure their joint designs are correct.

Joint location is critical in construction of all exterior assemblies. Each material expands and contracts differently in response to changes in temperature and moisture, and proper joint design will take this into consideration. The first element of joint design is proper spacing. The Masonry Institute recommends a joint every 30 feet of running wall. Each joint must be wide enough to allow expected movement in adjacent sections of the structure. With joint spacing of 30 feet, the Masonry Institute recommends a 3/8 inch minimum joint width. The ratio of depth to width ratio is also important. A rule of thumb for standard applications requires the depth of the sealant to be half the width. For example, if you have a joint that is one inch wide, the depth of the sealant - measured in the middle of the joint - should be 1/2 inch. There are some exceptions to that rule, so be sure to read the manufacturer's recommendations.

Marthe is the Midwest Strategic Accounts Manager for ChemRex, Inc. She also represents Sonneborn, Thoro, Hydrozo, ThoRoc, and Master Builders. 

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